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Detecting fracture intersections is crucial for understanding rock hydraulic properties. For this purpose, we study the dynamic SV‐wave signatures of fluid‐saturated porous rocks containing intersecting fractures. A theoretical model is derived using the dynamic Biot's poroelasticity equations. Using this model, we analyze the features of SV‐waves and compare to those of previously studied P‐waves. The results show that the dispersion and attenuation of SV‐waves caused by elastic scattering and FF‐WIFF (Fracture‐Fracture Wave‐induced Fluid Flow) have a similar dependence on properties of intersecting fractures and fluid as those of P‐waves. However, the FB‐WIFF (Fracture‐Background Wave‐induced Fluid Flow) causes much smaller dispersion and attenuation for SV‐waves than for P‐waves. In particular, such dispersion and attenuation of SV‐waves are negligibly small for the orthogonally intersecting fractures regardless of wave incidence angles. In addition to the dispersion and attenuation, the WIFF and elastic scattering also greatly affect the anisotropy properties, which gives rise to frequency‐dependent velocity and attenuation anisotropies for both SV‐ and P‐ waves. Such anisotropy properties are sensitive to fracture intersection angles and are quite different between SV‐ and P‐ waves. These complementary features of SV‐ and P‐ waves provide the basis for fracture intersection detection using the combined features of these two waves. By comparing our model to the known results for the limiting cases, we validate our model. Plain Language Summary: Detecting intersecting open fractures can help us understand rock hydraulic properties. Because rock elastic properties are sensitive to fractures, the seismic exploration and sonic logging are widely used for the detection of fractures. To detect the fracture intersection, its effects on rock elastic properties need to be studied. However, previous studies show that fracture intersections have little effects on rock static elastic properties. Hence, studying the effects of fracture intersections on rock dynamic elastic properties is of great importance. For this purpose, we study the dynamic SV‐wave signatures of intersecting fractures in fluid‐saturated porous rocks and compare the results to those of previously studied P‐waves. We find that the dispersion and attenuation caused by elastic scattering and FF‐WIFF (Fracture‐Fracture Wave‐induced Fluid Flow) have similar characteristics between SV‐ and P‐ waves. However, those caused by FB‐WIFF (Fracture‐Background Wave‐induced Fluid Flow) are quite different. In addition, the WIFF and elastic scattering also cause distinctly different frequency‐dependent anisotropy between SV‐ and P‐waves. These complementary features of SV‐ and P‐ waves indicate that the fracture intersections can be detected by combining the seismic or sonic logging data of these two waves. Key Points: Dynamic SV‐wave signatures of fluid‐saturated porous rocks containing intersecting fractures are studied and compared to P‐wavesFracture‐Background Wave‐induced Fluid Flow causes much smaller dispersion and attenuation for SV‐waves than for P‐wavesSV‐wave anisotropy is greatly affected by Wave‐induced Fluid Flow and elastic scattering, which has distinctly different features compared to P‐waves [ABSTRACT FROM AUTHOR]

Despite public concerns, only a few CO2 injections into saline aquifers have reported microseismicity. We analyze passive seismic monitoring of a small (15,000 tonnes and 0.15 MPa pressure) injection of supercritical CO2‐rich mixture for Stage 2C of the CO2CRC Otway Project (Victoria, Australia), which induced 19 detectable events with maximum moment magnitude MW‐0.5. The locations and dynamic parameters of the triggered events indicate a reactivation of a small fault patch where CO2 flowed through the fault. Time‐lapse seismic images of the plume and reservoir simulations show that the reactivation occurred when the CO2 plume reached this fault. This might be indicative of a fault weakening by the plume that enabled subsequent reactivation by pressure variations. Our observations suggest that a leakage from a commercial‐scale storage may trigger felt seismicity in the overburden without strong overpressure, thus, the de‐risking workflows should involve a detailed study of small faults. Plain Language Summary: Geological carbon storage is one of the key technologies for reducing the greenhouse gas effects. The storage projects may reactivate subsurface faults and lead to felt earthquakes and even surface deformations. We present a new case study, Stage 2C of the CO2CRC Otway Project (Victoria, Australia), where a small leakage‐like injection triggered seismicity that could be detected by high‐sensitivity sensors but is way below the "felt" levels. Typically, pre‐existing faults and fractures are triggered by pressure build‐up from an injection. Timing and locations of the triggered micro‐earthquakes at the Otway Project may suggest that the injected gas might have weakened the rocks filling the fault gouge thus making the fault more prone to reactivation. Our study implies that even a small leakage into such a reservoir, driven mainly by buoyancy, has seismogenic potential and thus this risk should be carefully considered by site operators for geological carbon storage projects. Key Points: A low‐pressure injection of 3,000 tonnes of supercritical CO2 into a high‐quality sandstone aquifer induced detectable seismicityAll of the seismic events are located on a small fault undetected in the seismic images prior to the injectionThe seismic monitoring data suggest that a fluid‐rock interaction enabled the fault's reactivation by the increased pore pressure [ABSTRACT FROM AUTHOR]

Ocean‐generated seismic waves are omnipresent in passive seismic records around the world and present both a challenge for earthquake observations and an input signal for interferometric methods for characterization of the Earth's interior. Understanding of these waves requires the knowledge of the depth dependence of the oceanic noise at the transition into the continent. To this end, we examine 80 days of continuous acquisition with distributed acoustic sensor (DAS) system deployed in two deep boreholes near the south‐eastern coast of Australia. The iDASv3™ system deployed in a deep borehole at the CO2CRC Otway Project site provides sufficiently high sensitivity and low instrumentation noise for frequencies from 100 mHz to 20 Hz. Analysis of the seismograms and correlation with wave climate allows decomposing the DAS response into microseisms generated by swell from remote storms (∼ $\sim $0.15 Hz) and local winds (between 0.2 and 2 Hz), and strong body wave energy from large surf breaks at the coast (from 2 to 20 Hz). The depth dependence of the microseisms provides useful insights into the energy partition between the Rayleigh wave modes and may augment conventional kinematic analysis of the sparse surface seismological arrays. Overall, ocean‐generated signals at each channel along the borehole are strongly related to the wave climate, so that—with sufficient amount of training data—the passive seismic records on several downhole DAS sensors has a potential for high‐precision monitoring of formations surrounding the borehole as well as remote storms in the ocean. Plain Language Summary: This study attempts to decipher the complex ambient seismic wavefield in coastal regions. The microseisms have been under scrutiny for a long time: first, as a noise in earthquake monitoring systems and, later, as a target signal used for characterization of the upper Earth crust. The study relies on a unique data set acquired in a deep borehole using the most advanced distributed acoustic sensing (DAS) system deployed at the coast of Victoria (Australia). Compared with the conventional seismological networks, this new measurement technology adds a new dimension to the data–depth variation in the broad frequency range of the parameters of the seismic waves. Using just one borehole, we identified and quantitatively characterized the multitude of ambient seismic sources at the Victorian coast: remote storms, local winds, and surf breaks. Our measurements show a reasonable agreement with the established nonlinear models that predicts the statistical characteristics of seismic wavefield ocean waves. We found that the link between the amplitudes on DAS along the borehole and wave climate is so stable that with sufficient amount of training data, the passive seismic records may be used for high‐precision monitoring of both—formations surrounding the borehole and remote storms in the ocean. Key Points: A DAS system in a single borehole to identify and quantitatively characterize the multitude of ambient seismic signals at the Australian coastSeismic measurements are in a good agreement with theoretical predictions from the ocean waves spectraThe link between the DAS response and wave climate enables monitoring both the formations surrounding the borehole and remote storms in the ocean [ABSTRACT FROM AUTHOR]

Time‐lapse seismic reservoir monitoring is used as a way to gain insight into subsurface processes. Yet, the application of standard 4D technology onshore faces challenges, such as high cost, significant environmental footprint and, consequently, relatively infrequent surveys. As part of the Otway Project Stage 3 CO2 injection study, continuous automated borehole‐based monitoring using distributed acoustic sensing has been paired with permanently deployed surface sources, referred to as surface orbital vibrators, as a way to monitor the spreading CO2 plume. The injection of 15,000 tonnes of CO2 in a saline reservoir at a depth of 1550 m is monitored using five boreholes instrumented with enhanced sensitivity fibre optic cables and nine surface orbital vibrators, creating an array of 45 well–source pairs. The data are processed with an offset vertical seismic profiling processing workflow developed to address key challenges of the continuous distributed acoustic sensing acquisition using surface orbital vibrators. The processing flow includes deconvolution with a source sweep recorded by a pilot geophone installed below the surface orbital vibrators. A second deconvolution with a wavelet estimated from direct arrivals compensates for the difference between distributed acoustic sensing measurements and the pilot geophone as well as near‐surface variations. Image quality is noted to be best for short offsets and decreases with increasing offsets and well deviations. As surface orbital vibrators generate unique sweeps in two rotation directions, further processing is applied to stack these rotation signals together, which further improves the images. The resulting 2D transects of each well–source pair visually provide good illumination of the subsurface, suggesting continuous monitoring of the spreading CO2 plume should be possible with some further tuning of the processing workflow for time‐lapse repeatability. [ABSTRACT FROM AUTHOR]

Elastic properties of sandstones can be highly pressure dependent. However, the pressure‐induced deformation of intact sandstones in X‐ray micro‐Computed Tomography (micro‐CT) images are below the resolving power of conventional image analysis methods. Also, most digital rock physics studies of the elastic properties operate with micro‐CT images acquired at ambient pressure. Here, we perform a comprehensive analysis of an intact sandstone sample with pronounced stress‐sensitivity. A purpose‐built X‐ray‐transparent pressure cell enables scanning micro‐CT images at confining pressures of up to 36 MPa. To detect the small pressure‐induced changes from these images, we design an overall deformation detection workflow with slice‐searching technique. We apply this workflow to a set of micro‐CT images of a 2 × 2 × 2 mm sample scanned at several pressures with a voxel size length of ∼1 μm. The results give the static modulus of 2.78 GPa, which is ∼0.5 times of the dynamic modulus derived from ultrasonic measurements. This ratio is consistent with literature data, showing the accuracy of the deformation detection workflow. Images scanned at different pressures are then segmented into two‐phase labels. The porosity change detected from segmented labels is consistent with the value derived from static modulus using poroelastic theory. The two‐phase segmented images are utilized to simulate the elastic properties with a finite element method. Most of the pressure dependency of elastic moduli shown in ultrasonic measurements is caused by closure of compliant pores at grain contacts, which cannot be resolved by the micro‐CT. The accuracy improvement of the estimated elastic properties from images scanned at higher pressures is negligible. Plain Language Summary: X‐ray micro‐Computed Tomography (micro‐CT) technique is the main tool for imaging three‐dimensional rock microstructure. Often, researchers attempt to substitute the real laboratory measurements on rock samples ‐ time‐consuming and expensive ‐ by numerical simulations of diverse physical processes in these images. However, such an approach ignores the effect of stress on the rock properties, because micro‐CT images are usually scanned at ambient pressures. This study estimates the stress‐induced changes on the images of a porous sandstone. To this end, we acquired a set of images of a sandstone sample at different confining pressures with a purpose‐built X‐ray‐transparent pressure cell and accompanied the images with a comprehensive laboratory data set. Standard algorithms for time‐lapse analysis of rock images failed at our data set, because the deformations are at the limit of the algorithms' sensitivity, hence, we designed a novel deformation detection workflow. The subsequent numerical elastic simulations using these images, ultrasonic measurements and petrophysical experiments suggest that the main stress effects are concentrated in thin intergranular contact that may not be resolved directly by the modern CT‐machines. Hence, the effect of stress should be estimated using indirectly from the analysis of multiphysics measurements and the images of sandstones may be taken at ambient pressure. Key Points: Micro‐Computed Tomography (micro‐CT) images of a sandstone are scanned in a X‐ray transparent pressure cell at confining pressures of up to 36 MPaAn overall deformation detection workflow is designed to track the sandstone deformation from micro‐CT imagesThe differences of the computed moduli based on scans at different pressures are demonstrated [ABSTRACT FROM AUTHOR]

The quality and accuracy of a seismic image can be significantly affected by anisotropy. When anisotropy is significant, neglecting it can lead to smearing of the image, dispositioning of reflections and distortion of amplitudes. Thus, it is imperative to estimate anisotropy and take it into account, especially for long‐offset or wide‐azimuthal seismic data. This study estimates P‐wave anisotropy parameters at the Otway site (Victoria, Australia). A wide range of available offsets acquired within the CO2CRC Otway Project provides a unique opportunity for anisotropy estimation from 3D vertical seismic profile data. We estimate P‐wave anisotropy from direct‐wave arrivals in vertical seismic profile data acquired with geophones and distributed acoustic sensors installed in two wells. Data analysis reveals seismic anisotropy of the subsurface with a significant presence of both polar and azimuthal anisotropy. We estimate key parameters such as normal moveout velocity, azimuthal ellipticity, polar anellipticity and fast‐velocity azimuth by nonlinear fitting of the analytical travel‐time approximation to observed direct P‐wave travel times from 3D vertical seismic profile data. Obtained anisotropy parameters show that polar anellipticity remains almost constant and equal to 0.1 for the whole depth range, while azimuthal anisotropy changes significantly with depth: it is relatively weak in the shallow section then increases significantly below a depth of 600 m. The fast‐velocity azimuth coincides with the orientation of the maximum horizontal stress in the study area. The results show that despite their low sensitivity for large offsets, the distributed acoustic sensor receivers provide sufficient information on anisotropic travel times over the entire length of the well, which is critical for reflection imaging from both vertical seismic profile and surface seismic data. [ABSTRACT FROM AUTHOR]

Permanent reservoir surveillance is an invaluable monitoring tool for CO2 storage projects, because it tracks spatial‐temporal evolution of the injected plume. The frequent images of CO2 plumes will facilitate history‐matching of the reservoir simulations and increase confidence of early leakage detection. However, continuous data acquisition and real‐time interpretation require a new approach to data analysis. Here, we propose a data‐driven approach to forecasting future time‐lapse seismic images based on the observed past images and test this approach on the Otway Stage 2C data. The core component of the predictor is a convolutional neural network, which considers subsequent plume maps as color layers, similar to standard red‐green‐blue blending. Based on the extent of the past plumes, we may predict the future contour of the seismically resolvable portion of the plume. The neural networks reproduce the dynamics of CO2 migration after training on reservoir simulations for a wide range of injection scenarios and subsurface models. Extensive testing shows that realistic plumes for Stage 2C are too complicated and the neural network should be pretrained on simpler reservoir simulations that include only one or two geological features, such as: faults, spill‐points, etc. Such staged training can be seen as a gradual descent of the neural network optimization to a global minimum. The approach is practical, because each CO2 storage project requires extensive preinjection reservoir simulations. Once the predictor has been trained, it can forecast plume evolution near real‐time and adapt efficiently to changing dynamics of CO2 migration. Plain Language Summary: All CO2 storage projects are legally obliged to prove that (1) the injection conforms with the existing predictive models of the CO2 migration through subsurface and (2) the absence of unwanted migration from the designated formation. The most informative monitoring tool, time‐lapse seismic, is expensive and invasive, hence a site operator may afford only few images of the injection in decades. Permanent seismic monitoring provides a cost‐effective solution to the problem, but this method requires automation of the big data processing and interpretation for immediate use by reservoir engineers. Here, we developed is a neural network, which considers subsequent plume maps as color layers. Based on the spatial distribution of these "colors," we may predict the future contour of the seismically visible part of the plume. The neural network learns from the abundance of reservoir simulations that a site operator produces for de‐risking the injection. Extensive testing shows that realistic plumes for a real injection are too complicated and the neural network should be pretrained on simpler reservoir simulations that include only one or two geological features, such as: faults, spill‐point, etc. But after training is done, the network can effectively adapt to observed migration and detect a potential CO2 leakage early. Key Points: Permanent seismic monitoring enables accurate early detection of risks to CO2 containmentSuch a detector may rely on rapid forecasting of a CO2 plume evolution by a fully convolutional neural networkThe network predicts the future plumes based on the patterns observed in a combination of conceptual and realistic reservoir simulations [ABSTRACT FROM AUTHOR]

Low‐frequency forced‐oscillation methods applied to a reservoir sandstone allowed determination of the Young's modulus and Poisson's ratio (from axial loading), bulk modulus (by oscillation of the confining pressure) and shear modulus (from torsional‐forced oscillations) for comparison with conventional ultrasonic data. All tests were performed on a common sandstone core sample from an oil reservoir offshore West Africa. The results show a steady increase in ultrasonic velocities and shear modulus of the dry specimen as functions of pressure, which suggests a progressive closure of the inter‐granular contacts. An increase of bulk and Young's moduli and Poisson's ratio is observed on decane saturation of the sample when tested with a sufficiently small dead volume. This observation, consistent with Gassmann's theory, suggests that such measurements probe undrained (saturated isobaric) conditions. Diminution or absence of such fluid‐related stiffening for low‐frequency measurements with dead volumes comparable with the pore volume of the specimen indicates partially drained conditions and highlights the critical role of experimental boundary conditions. Directly measured bulk and shear moduli are consistent with those derived from Young's modulus and Poisson's ratio. These results of the inter‐laboratory testing using different measurement devices are consistent in terms of the effect of frequency and fluid saturation for the reservoir sandstone specimen. Such broad consistency illustrates the validity of forced‐oscillation techniques and constitutes an important benchmarking of laboratory testing of the elastic properties of a porous medium. [ABSTRACT FROM AUTHOR]

Numerical computation from high‐resolution 3‐D microtomographic images of rocks (known as digital rock physics) has the potential to predict elastic properties more accurately. However, successful examples are limited to samples with simple structure and mineralogy. The physical size of sample is often too small to present heterogeneities at a larger scale and the image resolution is insufficient to characterize the details of rocks. Also, the grayscale values of different minerals in microtomographic images are often similar, and previous attempts to segment them as separate phases are not very successful. Here, we propose a practical digital rock physics workflow for somewhat more complex and ubiquitous rocks, namely, sandstones that contain mostly quartz and a small fraction of dispersed clay (known as arenites). Based on a set of images, we obtain a suite of postcomputation corrections to compensate for the effects of sample size and resolution of the microtomographic images. Furthermore, we build a segmentation workflow that effectively detects feldspar and clay minerals, despite their grayscale similarity to quartz. A moduli‐porosity trend is derived from the subsamples of the original digital images. Bulk moduli agree well with the ultrasonic measurements on the dry samples at 40 MPa. Shear moduli remain overestimated, which is likely caused by poor knowledge of the mineral stiffness. We compensate for this effect using a heuristic correction to the matrix moduli. The final version of the workflow provides accurate elastic moduli trends with porosity and clay content based on only two samples of Bentheimer sandstone. Key Points: Effects of microtomographic image scanning parameters on computed effective moduli are investigated and correctedDispersive clay and feldspar are segmented as separate phasesA rock physics template is built with images from only two Bentheimer sandstone miniplugs [ABSTRACT FROM AUTHOR]

Seismic anisotropy is an important attribute for fracture detection and characterization. If fracture size is comparable to the wavelength or fluid diffusion length, the P wave anisotropy is frequency dependent. By solving the Biot's equations of dynamic poroelasticity and using the Foldy approximation, we investigate the frequency‐dependent P wave anisotropy in a fluid‐saturated porous rock with a set of aligned penny‐shaped fractures. This approach includes the effects of both wave‐induced fluid flow (WIFF) and elastic scattering on P wave anisotropy. The results show that the velocity and attenuation anisotropy in the WIFF and elastic scattering regimes have quite different features and are affected by various factors. In particular, the background permeability and fluid viscosity primarily affect the frequency‐dependent anisotropy caused by WIFF. The fracture geometry (radius and thickness) and fracture density have significant effects on both WIFF and elastic scattering‐induced frequency‐dependent anisotropy. The fluid bulk modulus also affects the frequency‐dependent anisotropy. With the vanishing fluid bulk modulus, the frequency dependency vanishes in the WIFF regime but is the largest in the elastic scattering regime. Predictions of the model agree with previous models in all known limiting cases. Comparison with experimental data shows that WIFF and elastic scattering are important causes for frequency‐dependent P wave anisotropy, and our model is capable to account for these effects. Key Points: An integrated model for frequency‐dependent P wave anisotropy considering both WIFF and elastic scattering is developedVarious influencing factors on frequency‐dependent P wave anisotropy are investigatedPrevious models are special cases of our model, and experimental data show reasonable agreements with our model [ABSTRACT FROM AUTHOR]

Laboratory experiments of ultrasonic velocities of fluid‐saturated rocks are often much higher than the predictions of the Gassmann theory. This difference is usually attributed to the velocity dispersion caused by fluid pressure relaxation between pores of different shapes and orientation. This paper proposes a simple model to characterize pressure and frequency effects on the elastic moduli of fluid‐saturated rocks in a broad frequency range. The proposed model incorporates micromechanics of wave‐induced fluid pressure relaxation at grain contacts (between crack‐like contacts and stiff pores) into pressure dependency of elastic moduli. Previously, the pressure dependency of the velocities or elastic moduli was ascribed to the progressive closure of cracks with the increasing effective pressure and expressed as an integral of crack compliance over the range of aspect ratios. For isolated cracks, this compliance is a function of crack geometry only. For cracks hydraulically connected to stiff pores, this crack compliance can be replaced by a frequency‐dependent solution of the micromechanical problem of fluid pressure relaxation between a single crack and surrounding pores. The resulting equation expresses the bulk and shear moduli of the fluid‐saturated rock as functions of both pressure and frequency. Furthermore, if pressure‐dependent moduli of both dry and fluid‐saturated moduli are known, the aspect ratio distribution can be obtained from the pressure dependency of the dry moduli, and then the saturated moduli can be computed with no adjustable parameters. The model predictions show reasonable agreement with laboratory data measured using ultrasonic and forced oscillation techniques. Key Points: A micromechanical model of wave‐induced fluid pressure relaxation is incorporated into pressure dependency of elastic moduli of porous rocksThe resulting equation expresses elastic wave dispersion and attenuation in porous rock as functions of effective pressureThe model predictions show reasonable agreement with laboratory data measured using ultrasonic and forced oscillation techniques [ABSTRACT FROM AUTHOR]

Fracture‐background wave‐induced fluid flow (FB‐WIFF) and elastic scattering are considered two important mechanisms for P‐wave dispersion and attenuation in the fractured and porous fluid‐saturated rock. While numerous theoretical models have been proposed to study these two mechanisms, their coupling effects are seldom studied. Hence, in this work, we investigate the coupling between these two mechanisms theoretically based on the Biot equations of dynamic poroelasticity. The P wave is assumed to propagate perpendicular to the fracture plane in the fluid‐saturated porous rock with aligned penny‐shaped fractures. The general solutions are first obtained from the governing equations, then the expressions for the coefficients of the general solutions are derived using the boundary conditions. To illustrate the influencing factors on the coupling between FB‐WIFF and elastic scattering, a numerical example is given. The results show that the coupling strength between FB‐WIFF and elastic scattering are greatly influenced by the ratio of fluid viscosity to background permeability. With the decrease of this ratio, the coupling strength becomes stronger. Furthermore, the coupling effects are also affected by the fracture radius and thickness. However, the fracture density and fluid bulk modulus have negligible influence on the coupling effects. This new model shows good agreement with other theories. Furthermore, comparison of this new model with the linear superposition of FB‐WIFF and elastic scattering indicates the strong nonlinear coupling between these two mechanisms when their characteristic frequencies are close. Finally, the extension of this model and implications of our results for seismic exploration and sonic logging are discussed. Key Points: Coupling between FB‐WIFF and elastic scattering are studied theoreticallyBackground permeability and fracture properties have significant influence on coupling between FB‐WIFF and elastic scatteringEffects of nonlinear coupling between FB‐WIFF and elastic scattering are notable when their characteristic frequencies are close [ABSTRACT FROM AUTHOR]

Quantifying the effects of pore‐filling materials on elastic properties of porous rocks is of considerable interest in geophysical practice. For rocks saturated with fluids, the Gassmann equation is proved effective in estimating the exact change in seismic velocity or rock moduli upon the changes in properties of pore infill. For solid substance or viscoelastic materials, however, the Gassmann theory is not applicable as the rigidity of the pore fill (either elastic or viscoelastic) prevents pressure communication in the pore space, which is a key assumption of the Gassmann equation. In this paper, we explored the elastic properties of a sandstone sample saturated with fluid and solid substance under different confining pressures. This sandstone sample is saturated with octadecane, which is a hydrocarbon with a melting point of 28°C, making it convenient to use in the lab in both solid and fluid forms. Ultrasonically measured velocities of the dry rock exhibit strong pressure dependency, which is largely reduced for the filling of solid octadecane. Predictions by the Gassmann theory for the elastic moduli of the sandstone saturated with liquid octadecane are consistent with ultrasonic measurements, but underestimate the elastic moduli of the sandstone saturated with solid octadecane. Our analysis shows that the difference between the elastic moduli of the dry and solid‐octadecane‐saturated sandstone is controlled by the squirt flow between stiff, compliant, and the so‐called intermediate pores (with an aspect ratio larger than that of compliant pore but much less than that of stiff pores). Therefore, we developed a triple porosity model to quantify the combined squirt flow effects of compliant and intermediate pores saturated with solid or viscoelastic infill. Full saturation of remaining stiff pores with solid or viscoelastic materials is then considered by the lower embedded bound theory. The proposed model gave a reasonable fit to the ultrasonic measurements of the elastic moduli of the sandstone saturated with liquid or solid octadecane. Comparison of the predictions by the new model to other solid substitution schemes implied that accounting for the combined effects of compliant and intermediate pores is necessary to explain the solid squirt effects. [ABSTRACT FROM AUTHOR]

Shales play an important role in many engineering applications such as nuclear waste, CO2 storage and oil or gas production. Shales are often utilized as an impermeable seal or an unconventional reservoir. For both situations, shales are often studied using seismic waves. Elastic properties of shales strongly depend on their hydration, which can lead to substantial structural changes. Thus, in order to explore shaly formations with seismic methods, it is necessary to understand the dependency of shale elastic properties on variations in hydration. In this work, we investigate structural changes in Opalinus shale at different hydration states using laboratory measurements and X‐ray micro‐computed tomography. We show that the shale swells with hydration and shrinks with drying with no visible damage. The pore space of the shale deforms, exhibiting a reduction in the total porosity with drying and an increase in the total porosity with hydration. We study the elastic properties of the shale at different hydration states using ultrasonic velocities measurements. The elastic moduli of the shale show substantial changes with variations in hydration, which cannot be explained with a single driving mechanism. We suggest that changes of the elastic moduli with variations in hydration are driven by multiple competing factors: (1) variations in total porosity, (2) substitution of pore‐filling fluid, (3) change in stiffness of contacts between clay particles and (4) chemical hardening/softening of clay particles. We qualitatively and quantitatively analyse and discuss the influence of each of these factors on the elastic moduli. We conclude that depending on the microstructure and composition of a particular shale, some of the factors dominate over the others, resulting in different dependencies of the elastic moduli on hydration. [ABSTRACT FROM AUTHOR]

The elastic properties of crystalline rocks can be estimated from ultrasonic measurements on the powders of crushed rocks produced by the drilling process. To determine the elastic properties of grains from properties of powder packs, we study the dependence of their ultrasonic wave velocities on pressure. From this dependency, using the Hertz–Mindlin theory, we can calculate the effective ratio of the grain shear modulus to one minus the Poisson ratio. The Hertz–Mindlin theory requires the knowledge of grain coordination number as a function of pressure, which can be obtained using an empirical relation based on published numerical simulations. Previous work has shown that this approach gives an accurate prediction of the effective bulk modulus of glass beads but produces a significant discrepancy for sand. This discrepancy may be attributed to the angularity of sand grains. To overcome this problem, we introduce a shape factor into the empirical relation for the coordination number. This new shape factor allows us to reconcile the rock physics model with laboratory measurements. We show that the shape factor varies from 1 to 2.5 for different grain shape angularity and sorting (grain size distributions). The modified theory allows us to estimate a combination of elasticity parameters of the grains from the measured dependence of P‐wave velocity in the pack on the pressure. [ABSTRACT FROM AUTHOR]

A number of studies show that adsorption of small amount of water causes significant reduction of elastic moduli and increase of elastic wave attenuation in sandstones. This effect is of practical importance for near‐surface seismic studies and for applications of conventional rock physics theories, which require measurements of elastic properties of dry rock. Additionally, adsorption of water causes deformation of porous materials and rocks, which can be explained by the concept of varying surface stress, or change in the pressure of fluid adsorbed in pores. In this work we suggest that the adsorption‐induced elastic weakening of sandstones is caused by changing fluid pressure in compliant pores on grain contacts. In order to validate this concept, we conduct simultaneous measurements of the elastic moduli and deformation of the Bentheim sandstone with adsorption and desorption of water and observe ~20% change in the bulk and shear moduli accompanied with the strain of the order of 10−4. Then, we estimate that the fluid pressure change should be of the order of several megapascals to cause such deformation. Comparison of the measured variations in the elastic moduli related to the estimated change in the fluid pressure with the stress dependency of the bulk and shear moduli in Bentheim sandstone shows a broad agreement of the two sets of measurements. A reasonable magnitude of the estimated fluid pressure change is consistent with the suggested hypothesis. Key Points: Sorption of water in Bentheim sandstone causes ~20% variation in elastic moduli and deformation of the order of 10−4These effects are explained by changes in fluid pressure resulting from adsorption of water in compliant pores at grain contactsThe change in moduli caused by change in fluid pressure is consistent with the change in moduli with stress measured in a triaxial test [ABSTRACT FROM AUTHOR]

Abstract: We estimate the seismic attenuation and velocity dispersion induced by fluid pressure diffusion in a saturated porous medium containing a sparse distribution of aligned slit fractures. This is done by solving the scattering problem for a single crack and using Waterman‐Truell scattering approximation for a distribution of cracks. This approach gives a numerical solution for the entire frequency range and analytical solutions for the low‐ and high‐frequency limits. Analysis of the results shows that the characteristics of the seismic dispersion and attenuation for the slit cracks are similar to known results for penny‐shaped crack. At high frequencies, the attenuation and dispersion are controlled by the crack density regardless of the crack geometry. At low frequencies, due to the different geometries of the slit and penny‐shaped cracks, the characteristics of seismic dispersion and attenuation are different. For both the slit and penny‐shaped crack case, the slope of the frequency dependence attenuation of factor (inverse quality factor) Q−1 tends to be 1. Yet the low‐frequency asymptotic solutions for these two cases are somewhat different. For penny‐shaped cracks, the Q−1(ω) on the bilogarithmic scale has a straight line asymptote with slope 1, so that the attenuation factor at low frequencies is proportional to the frequency ω. However, for slit cracks, the asymptotic low‐frequency solution contains a logarithmic function of ω, and no such asymptote exists. At the same time, similarly to the penny‐shaped crack case, the solution for compressional velocity in the low‐frequency limit is consistent with the anisotropic Gassmann theory. [ABSTRACT FROM AUTHOR]

Abstract: Recent progress in extraction of unconventional hydrocarbon resources has ignited the interest in the studies of nanoporous media. Since many thermodynamic and mechanical properties of nanoscale solids and fluids differ from the analogous bulk materials, it is not obvious whether wave propagation in nanoporous media can be described using the same framework as in macroporous media. Here we test the validity of Gassmann equation using two published sets of ultrasonic measurements for a model nanoporous medium, Vycor glass, saturated with two different fluids, argon, and n‐hexane. Predictions of the Gassmann theory depend on the bulk and shear moduli of the dry samples, which are known from ultrasonic measurements and the bulk moduli of the solid and fluid constituents. The solid bulk modulus can be estimated from adsorption‐induced deformation or from elastic effective medium theory. The fluid modulus can be calculated according to the Tait‐Murnaghan equation at the solvation pressure in the pore. Substitution of these parameters into the Gassmann equation provides predictions consistent with measured data. Our findings set up a theoretical framework for investigation of fluid‐saturated nanoporous media using ultrasonic elastic wave propagation. [ABSTRACT FROM AUTHOR]

ABSTRACT Most seismic processing algorithms generally consider the sea surface as a flat reflector. However, acquisition of marine seismic data often takes place in weather conditions where this approximation is inaccurate. The distortion in the seismic wavelet introduced by the rough sea may influence (for example) deghosting results, as deghosting operators are typically recursive and sensitive to the changes in the seismic signal. In this paper, we study the effect of sea surface roughness on conventional (5-160 Hz) and ultra-high-resolution (200-3500 Hz) single-component towed-streamer data. To this end, we numerically simulate reflections from a rough sea surface using the Kirchhoff approximation. Our modelling demonstrates that for conventional seismic frequency band sea roughness can distort results of standard one-dimensional and two-dimensional deterministic deghosting. To mitigate this effect, we introduce regularisation and optimisation based on the minimum-energy criterion and show that this improves the processing output significantly. Analysis of ultra-high-resolution field data in conjunction with modelling shows that even relatively calm sea state (i.e., 15 cm wave height) introduces significant changes in the seismic signal for ultra-high-frequency band. These changes in amplitude and arrival time may degrade the results of deghosting. Using the field dataset, we show how the minimum-energy optimisation of deghosting parameters improves the processing result. [ABSTRACT FROM AUTHOR]

ABSTRACT In fractured reservoirs, seismic wave velocity and amplitude depend on frequency and incidence angle. Frequency dependence is believed to be principally caused by the wave-induced flow of pore fluid at the mesoscopic scale. In recent years, two particular phenomena, i.e., patchy saturation and flow between fractures and pores, have been identified as significant mechanisms of wave-induced flow. However, these two phenomena are studied separately. Recently, a unified model has been proposed for a porous rock with a set of aligned fractures, with pores and fractures filled with two different fluids. Existing models treat waves propagating perpendicular to the fractures. In this paper, we extend the model to all propagation angles by assuming that the flow direction is perpendicular to the layering plane and is independent of the loading direction. We first consider the limiting cases through poroelastic Backus averaging, and then we obtain the five complex and frequency-dependent stiffness values of the equivalent transversely isotropic medium as a function of the frequency. The numerical results show that, when the bulk modulus of the fracture-filling fluid is relatively large, the dispersion and attenuation of P-waves are mainly caused by fractures, and the values decrease as angles increase, almost vanishing when the incidence angle is 90° (propagation parallel to the fracture plane). While the bulk modulus of fluid in fractures is much smaller than that of matrix pores, the attenuation due to the 'partial saturation' mechanism makes the fluid flow from pores into fractures, which is almost independent of the incidence angle. [ABSTRACT FROM AUTHOR]

ABSTRACT The detection and characterisation of domains of intersecting fractures are important goals in several disciplines of current interest, including exploration and production of unconventional reservoirs, nuclear waste storage, CO2 sequestration, and groundwater hydrology, among others. The objective of this study is to propose a theoretical framework for quantifying the effects of fracture intersections on the frequency-dependent elastic properties of fluid-saturated porous and fractured rocks. Three characteristic frequency regimes for fluid pressure communication are identified. In the low-frequency limit, fractures are in full pressure communication with the embedding porous matrix and with other fractures. Conversely, in the high-frequency limit, fractures are hydraulically isolated from the matrix and from other fractures. At intermediate frequencies, fractures are hydraulically isolated from the matrix porosity but can be in hydraulic communication with each other, depending on whether fracture sets are intersecting. For each frequency regime, the effective stiffness coefficients are derived using the linear-slip theory and anisotropic Gassmann equations. Explicit mathematical expressions for the two characteristic frequencies that separate the three frequency regimes are also determined. Theoretical predictions are then applied to two synthetic 2D samples, each containing two orthogonal fracture sets: one with and another without intersections. The resulting stiffness coefficients, Thomsen-style anisotropy parameters, and the transition frequencies show good agreement with corresponding numerical simulations. The theoretical results are applicable not only to 2D but also to 3D fracture systems and are amenable to being employed in inversion schemes designed to characterise fracture systems. [ABSTRACT FROM AUTHOR]

ABSTRACT Most sedimentary rocks are anisotropic, yet it is often difficult to accurately incorporate anisotropy into seismic workflows because analysis of anisotropy requires knowledge of a number of parameters that are difficult to estimate from standard seismic data. In this study, we provide a methodology to infer azimuthal P-wave anisotropy from S-wave anisotropy calculated from log or vertical seismic profile data. This methodology involves a number of steps. First, we compute the azimuthal P-wave anisotropy in the dry medium as a function of the azimuthal S-wave anisotropy using a rock physics model, which accounts for the stress dependency of seismic wave velocities in dry isotropic elastic media subjected to triaxial compression. Once the P-wave anisotropy in the dry medium is known, we use the anisotropic Gassmann equations to estimate the anisotropy of the saturated medium. We test this workflow on the log data acquired in the North West Shelf of Australia, where azimuthal anisotropy is likely caused by large differences between minimum and maximum horizontal stresses. The obtained results are compared to azimuthal P-wave anisotropy obtained via orthorhombic tomography in the same area. In the clean sandstone layers, anisotropy parameters obtained by both methods are fairly consistent. In the shale and shaly sandstone layers, however, there is a significant discrepancy between results since the stress-induced anisotropy model we use is not applicable to rocks exhibiting intrinsic anisotropy. This methodology could be useful for building the initial anisotropic velocity model for imaging, which is to be refined through migration velocity analysis. [ABSTRACT FROM AUTHOR]

ABSTRACT A major cause of attenuation in fluid-saturated media is the local fluid flow (or squirt flow) induced by a passing wave between pores of different shapes and sizes. Several squirt flow models have been derived for isotropic media. For anisotropic media however, most of the existing squirt flow models only provide the low- and high-frequency limits of the saturated elastic properties. We develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media. In this paper, we focus on media containing aligned compliant pores embedded in an isotropic background matrix. The low- and high-frequency limits of the predicted fluid-saturated elastic properties are respectively consistent with Gassmann theory and Mukerji-Mavko squirt flow model. Results are also expressed in terms of Thomsen anisotropy parameters. It turns out that the P-wave anisotropy parameter ε tends to zero in the high-frequency limit, whereas the δ parameter remains the only indicator of P-S⊥ anisotropy. The S-wave anisotropy parameter γ is not affected by the presence of fluid and remains the same for all frequency ranges. A new definition for attenuation anisotropy parameters is also proposed to quantify the attenuation anisotropy. In the most important case of liquid saturation, analytical expressions are derived for elastic properties, velocity anisotropy parameters, quality factors, and attenuation anisotropy parameters. A companion paper considers the case of cracks with an ellipsoidal distribution of orientations resulting from the application of anisotropic stress. [ABSTRACT FROM AUTHOR]

ABSTRACT A major cause of attenuation in fluid-saturated media is the local fluid flow (or squirt flow) induced by a passing wave between pores of different shapes and sizes. Several squirt flow models have been derived for isotropic media. For anisotropic media, however, most of the existing squirt flow models only provide the low- and high-frequency limits of the saturated elastic properties. We develop a new squirt flow model to account for the frequency dependence of elastic properties and thus gain some insight into velocity dispersion and attenuation in anisotropic media. In a companion paper, we focused on media containing aligned compliant pores embedded in an isotropic background matrix. In this paper, we investigate the case for which anisotropy results from the presence of cracks with an ellipsoidal distribution of orientations due to the application of anisotropic stress. The low- and high-frequency limits of the predicted fluid-saturated elastic properties are respectively consistent with Gassmann theory and Mukerji-Mavko squirt flow model. In the most important case of liquid saturation, analytical expressions are derived for elastic properties and Thomsen anisotropy parameters. The main observations drawn from this model are as follows. Crack closure perpendicular to the applied stress leads to an increase in seismic velocities as a function of stress in the direction of applied stress and a decrease in squirt-flow-induced dispersion and attenuation in this direction. The anisotropy of squirt flow dispersion engenders a decrease in the degree of anisotropy with frequency. The stress-induced anisotropy remains elliptical, even in saturated media, for all frequency ranges. [ABSTRACT FROM AUTHOR]

ABSTRACT CO2 sequestration projects benefit from quantitative assessment of saturation distribution and plume extent for field development and leakage prevention. In this work, we carry out quantitative analysis of time-lapse seismic by using rock physics and seismic modelling tools. We investigate the suitability of Gassmann's equation for a CO2 sequestration project with 1600 tons of CO2 injected into high-porosity, brine-saturated sandstone. We analyze the observed time delays and amplitude changes in a time-lapse vertical seismic profile dataset. Both reflected and transmitted waves are analyzed qualitatively and quantitatively. To interpret the changes obtained from the vertical seismic profile, we perform a 2.5D elastic, finite-difference modelling study. The results show a P-wave velocity reduction of 750 m/s in the proximity of the injection well evident by the first arrivals (travel-time delays and amplitude change) and reflected wave amplitude changes. These results do not match with our rock physics model using Gassmann's equation predictions even when taking uncertainty in CO2 saturation and grain properties into account. We find that time-lapse vertical seismic profile data integrated with other information (e.g., core and well log) can be used to constrain the velocity-saturation relation and verify the applicability of theoretical models such as Gassmann's equation with considerable certainty. The study shows that possible nonelastic factors are in play after CO2 injection (e.g., CO2-brine-rock interaction and pressure effect) as Gassmann's equation underestimated the velocity reduction in comparison with field data for all three sets of time-lapse vertical seismic profile attributes. Our work shows the importance of data integration to validate the applicability of theoretical models such as Gassmann's equation for quantitative analysis of time-lapse seismic data. [ABSTRACT FROM AUTHOR]

ABSTRACT A significant portion of the world's hydrocarbon reserves are found in carbonate reservoirs, yet analysis of the petrophysical properties of these reservoirs is associated with a number of challenges. Some of these challenges stem from physical and chemical interactions between the carbonate rock matrix and pore fluids, which can affect elastic properties of the rock. Hence, the study of the pore fluid effects on the elastic properties of carbonates is important for understanding a change of the field performance properties of а carbonate reservoir caused by fluid movements during hydrocarbon extraction in producing fields. In this laboratory study, we investigate the applicability of Gassmann's model for predictions of the elastic moduli of water- and hydrocarbon-saturated Savonnières limestone and the influence of partial water saturation on elastic and anelastic properties of the rock. We present the results of two sets of laboratory experiments on the Savonnières oolitic limestone where we: (i) evaluate the effect of full water and n-decane saturation on elastic moduli and attenuation at seismic (0.1 Hz-120 Hz) and ultrasonic (0.5 MHz) frequencies; and (ii) quantify the dependence of elastic moduli and extensional attenuation on water saturation at two seismic frequencies of 1 Hz and 10 Hz. We demonstrate that the change in the bulk modulus of limestone fully saturated either with n-decane or water is in agreement with Gassmann's fluid substitution theory, whereas the shear modulus is noticeably reduced. The measurements with partial saturation show that the bulk modulus decreases with increasing water saturation to a lesser extent than the Young's and shear moduli. Our results show that extensional attenuation in the samples with closed boundaries is insignificant under dry and fully saturated conditions but is influenced greatly by the liquid content when saturation is between 0 and 20% or 95% and 100%. [ABSTRACT FROM AUTHOR]

ABSTRACT Quantitative interpretation of time-lapse seismic signatures aims at assisting reservoir engineering and management operations. Time-lapse signatures are thought to be primarily induced by saturation and pressure changes. Core-flooding and reservoir flow simulations indicate that a change of the driving forces during dynamic fluid injection gives rise to a varying saturation scale. This saturation scale is yet another variable controlling the time-lapse seismic signal. In this work, we investigate the saturation scale effect on time-lapse seismic signatures by analysing simple modelling scenarios. We consider three characteristic saturation scales, ranging from few millimetres to metres, which may form during gas injection in an unconsolidated water-saturated reservoir. Using the random patchy saturation model, we compare the corresponding acoustic signatures, i.e., attenuation, reflectivity, and seismic gather associated with each saturation scale. The results show that the millimetre saturation scale produces minimum attenuation and the same seismic signatures with those obtained from the elastic modelling. The centimetre saturation scale produces maximum attenuation, whereas the metre saturation scale causes highest velocity dispersion. The analyses of the time shift and amplitude change indicate that ignoring a time-dependent saturation scale can result in biased estimation/discrimination of the saturation and the fluid pressure. In particular, the 4D signal can be strongly affected by the saturation-scale change when the reservoir gas saturation is low and the effective pressure is high. In the presence of an increasing (decreasing) saturation scale during injection, interpreting an observed time shift and amplitude change using the Gassmann model will lead to underestimation (overestimation) of the change in gas saturation and fluid pressure. We show that including the effects of capillarity and residual saturation into the rock physics modelling can potentially reduce the interpretation uncertainty due to the saturation-scale change. [ABSTRACT FROM AUTHOR]

ABSTRACT Although most rocks are complex multi-mineralic aggregates, quantitative interpretation workflows usually ignore this complexity and employ Gassmann equation and effective stress laws that assume a micro-homogeneous (mono-mineralic) rock. Even though the Gassmann theory and effective stress concepts have been generalized to micro-inhomogeneous rocks, they are seldom if at all used in practice because they require a greater number of parameters, which are difficult to measure or infer from data. Furthermore, the magnitude of the effect of micro-heterogeneity on fluid substitution and on effective stress coefficients is poorly understood. In particular, it is an open question whether deviations of the experimentally measurements of the effective stress coefficients for drained and undrained elastic moduli from theoretical predictions can be explained by the effect of micro-heterogeneity. In an attempt to bridge this gap, we consider an idealized model of a micro-inhomogeneous medium: a Hashin assemblage of double spherical shells. Each shell consists of a spherical pore surrounded by two concentric spherical layers of two different isotropic minerals. By analyzing the exact solution of this problem, we show that the results are exactly consistent with the equations of Brown and Korringa (which represent an extension of Gassmann's equation to micro-inhomogeneous media). We also show that the effective stress coefficients for bulk volume α, for porosity nϕ and for drained [ABSTRACT FROM AUTHOR]

ABSTRACT Hard rock seismic exploration normally has to deal with rather complex geological environments. These types of environments are usually characterized by a large number of local heterogeneity (e.g., faults, fracture zones, and steeply dipping interfaces). The seismic data from such environments often have a poor signal-to-noise ratio because of the complexity of hard rock geology. To be able to obtain reliable images of subsurface structures in such geological conditions, processing algorithms that are capable of handling seismic data with a low signal-to-noise ratio are required for a reflection seismic exploration. In this paper, we describe a modification of the 3D Kirchhoff post-stack migration algorithm that utilizes coherency attributes obtained by the diffraction imaging algorithm in 3D to steer the main Kirchhoff summation. The application to a 3D synthetic model shows the stability of the presented steered migration to the presence of high level of the random noise. A test on the 3D seismic volume, acquired on a mine site located in Western Australia, reveals the capability of the approach to image steep and sharp objects such as fracture and fault zones and lateral heterogeneity. [ABSTRACT FROM AUTHOR]

ABSTRACT Naturally fractured reservoirs are becoming increasingly important for oil and gas exploration in many areas of the world. Because fractures may control the permeability of a reservoir, it is important to be able to find and characterize fractured zones. In fractured reservoirs, the wave-induced fluid flow between pores and fractures can cause significant dispersion and attenuation of seismic waves. For waves propagating normal to the fractures, this effect has been quantified in earlier studies. Here we extend normal incidence results to oblique incidence using known expressions for the stiffness tensors in the low- and high-frequency limits. This allows us to quantify frequency-dependent anisotropy due to the wave-induced flow between pores and fractures and gives a simple recipe for computing phase velocities and attenuation factors of quasi-P and SV waves as functions of frequency and angle. These frequency and angle dependencies are concisely expressed through dimensionless velocity anisotropy and attenuation anisotropy parameters. It is found that, although at low frequencies, the medium is close to elliptical (which is to be expected as a dry medium containing a distribution of penny-shaped cracks is known to be close to elliptical); at high frequencies, the coupling between P-wave and SV-wave results in anisotropy due to the non-vanishing excess tangential compliance. [ABSTRACT FROM AUTHOR]

ABSTRACT 4D seismic is widely used to remotely monitor fluid movement in subsurface reservoirs. This technique is especially effective offshore where high survey repeatability can be achieved. It comes as no surprise that the first 4D seismic that successfully monitored the CO2 sequestration process was recorded offshore in the Sleipner field, North Sea. In the case of land projects, poor repeatability of the land seismic data due to low S/N ratio often obscures the time-lapse seismic signal. Hence for a successful on shore monitoring program improving seismic repeatability is essential. Stage 2 of the CO2CRC Otway project involves an injection of a small amount (around 15,000 tonnes) of CO2/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array can be utilised to improve signal to noise ratio and hence repeatability. A small-scale trial of such an array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3 m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed into boreholes of 1-12 m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower compared to the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30-90 Hz frequency range. The influence of burying depth on S/N ratio and repeatability shows that significant improvement in repeatability can be reached at a depth of 3 m. The level of repeatability remains relatively constant between 3 and 12 m depths. [ABSTRACT FROM AUTHOR]