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For four decades, the use of the drill-bit seismic source has been evaluated extensively and investigated for seismic-while-drilling (SWD) purposes. After an initial diffusion phase, however, the routine application on drill-bit reverse vertical seismic profiling (RVSP) has declined because of issues related to variability in the signal-to-noise ratio of drill-bit signals obtained with different drilling conditions. Consequently, additional efforts have been dedicated to improving the recording of reference signals. Recent technology developments enable direct ground-force pilot measurements at downhole positions close to the bit, which enable reliable estimations of the far-field drill-bit-radiated signature. The knowledge of the ground force below the source in the near-field is a key aspect not only for the exploitation of the drill-bit source but also for conventional seismic vibrator sources. The SWD deconvolved results obtained by the ground force are different from those obtained by recording only motion vibration, such as acceleration. The ground force and particle velocity are linked by a complex impedance in the near-field, and together they provide dual measurements. In addition, the ground force provides signals with effective coupling conditions at the bit-rock interface in the presence of the drill-bit displacement action with penetration during rock fracturation. The analysis of the impedance at the bit source includes the drillstring reflections, which are observed in the reference drillstring pilot signal and are simulated by using an appropriate mechanical model. The processing of real seismic signals with different approaches by deconvolution and correlation using ground-force and motion signals produces improvements in the extraction of the source signal, with effective removal of the undesired pilot wavefields, and with cleaner estimations of the drill-bit seismic wavefields in a crosswell survey.

This paper deals with the design, acquisition and processing of vertical seismic profiling data collected in 2016 at the directional Thonex‐01 geothermal well, drilled in 1993 in the Geneva Canton (Switzerland). The aim of the study was to obtain additional information from existing‐abandoned well for downhole geophysical characterization of potential geothermal reservoirs in the Geneva area. The main results obtained by this study allowed an improved subsurface image by multi‐offset acquisitions near and around the deviated well. Specifically we were able to (1) improve the velocity model of the Geneva Basin, which, at present, is only constrained by one unique deep exploration well, located at the south‐west of the basin, far away from the study area; (2) image the transition between the Tertiary Molasse siliciclastic sediments and the Mesozoic carbonates; (3) define an acquisition strategy for future vertical seismic profiling surveys in support of the geothermal exploration campaigns in the area. This study has demonstrated the value of vertical seismic profiling method for the refinement of the subsurface geology at local scale, and its usefulness to assist the planning of second well associated with geothermal doublets.

Seismic propagation in the upper part of the crust, where geothermal reservoirs are located, shows generally strong velocity dispersion and attenuation due to varying permeability and saturation conditions and is affected by the brittleness and/or ductility of the rocks, including zones of partial melting. From the elastic-plastic aspect, the seismic properties (seismic velocity, quality factor and density) depend on effective pressure and temperature. We describe the related effects with a Burgers mechanical element for the shear modulus of the dry-rock frame. The Arrhenius equation combined to the octahedral stress criterion define the Burgers viscosity responsible of the brittle-ductile behaviour. The effects of permeability, partial saturation, varying porosity and mineral composition on the seismic properties is described by a generalization of the White mesoscopic-loss model to the case of a distribution of heterogeneities of those properties. White model involves the wave-induced fluid flow attenuation mechanism, by which seismic waves propagating through small-scale heterogeneities, induce pressure gradients between regions of dissimilar properties, where part of the energy of the fast P-wave is converted to slow P (Biot)-wave. We consider a range of variations of the radius and size of the patches and thin layers whose probability density function is defined by different distributions. The White models used here are that of spherical patches (for partial saturation) and thin layers (for permeability heterogeneities). The complex bulk modulus of the composite medium is obtained with the Voigt-Reuss-Hill average. Effective pressure effects are taken into account by using exponential functions. We then solve the 3D equation of motion in the space-time domain, by approximating the White complex bulk modulus with that of a set of Zener elements connected in series. The Burgers and generalized Zener models allows us to solve the equations with a direct grid method by the introduction of memory variables. The algorithm uses the Fourier pseudospectral method to compute the spatial derivatives. It is tested against an analytical solution obtained with the correspondence principle. We consider two main cases, namely the same rock frame (uniform porosity and permeability) saturated with water and a distribution of steam patches, and water-saturated background medium with thin layers of dissimilar permeability. Our model indicates how seismic properties change with the geothermal reservoir temperature and pressure, showing that both seismic velocity and attenuation can be used as a diagnostic tool to estimate the in situ conditions.

Part of the crust shows generally brittle behaviour while areas of high temperature and/or high pore pressure, including the mantle, may present ductile behaviour. For instance, the potential heat source of geothermal fields, overpressured formations and molten rocks. Seismic waves can be used to detect these conditions on the basis of reflection and transmission events. Basically, from the elastic-plastic point of view the seismic properties (seismic velocity, quality factor and density) depend on effective pressure and temperature. Confining and pore pressures have opposite effects on these properties, and high temperatures may induce a similar behaviour by partial melting. In order to model these effects, we consider a poro-viscoelastic model based on the Burgers mechanical element and the squirt-flow model to represent the properties of the rock frame to describe ductility in which deformation takes place by shear plastic flow, and to model local and global fluid flow effects. The Burgers element allows us to model the effects of the steady-state creep flow on the dry-rock frame. The stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity, which is obtained by the Arrhenius equation and the octahedral stress criterion. Effective pressure effects are taken into account in the dry-rock moduli by using exponential functions whose parameters are obtained by fitting experimental data as a function of confining pressure. Since fluid effects are important, the density and bulk modulus of the saturating fluids (water at sub- and supercritical conditions) are modeled by using the equations provided by the NIST website. The squirt-flow model has a single free parameter represented by the aspect ratio of the grain contacts. The theory generalizes a preceding theory based on Gassmann (low-frequency) moduli to the more general case of the presence of local (squirt) flow and global (Biot) flow, which contribute with additional attenuation mechanisms to the wave propagation.

The upper part of the crust shows generally brittle behaviour while deeper zones, including the mantle, may present ductile behaviour, depending on the pressure–temperature conditions; moreover, some parts are melted. Seismic waves can be used to detect these conditions on the basis of reflection and transmission events. Basically, from the elastic–plastic point of view the seismic properties (seismic velocity and density) depend on effective pressure and temperature. Confining and pore pressures have opposite effects on these properties, such that very small effective pressures (the presence of overpressured fluids) may substantially decrease the P- and S-wave velocities, mainly the latter, by opening of cracks and weakening of grain contacts. Similarly, high temperatures induce the same effect by partial melting. To model these effects, we consider a poro-viscoelastic model based on Gassmann equations and Burgers mechanical model to represent the properties of the rock frame and describe ductility in which deformation takes place by shear plastic flow. The Burgers elements allow us to model the effects of seismic attenuation, velocity dispersion and steady-state creep flow, respectively. The stiffness components of the brittle and ductile media depend on stress and temperature through the shear viscosity, which is obtained by the Arrhenius equation and the octahedral stress criterion. Effective pressure effects are taken into account in the dry-rock moduli using exponential functions whose parameters are obtained by fitting experimental data as a function of confining pressure. Since fluid effects are important, the density and bulk modulus of the saturating fluids (water and steam) are modeled using the equations provided by the NIST website, including supercritical behaviour. The theory allows us to obtain the phase velocity and quality factor as a function of depth and geological pressure and temperature as well as time frequency. We then obtain the PS and SH equations of motion recast in the velocity–stress formulation, including memory variables to avoid the computation of time convolutions. The equations correspond to isotropic anelastic and inhomogeneous media and are solved by a direct grid method based on the Runge–Kutta time stepping technique and the Fourier pseudospectral method. The algorithm is tested with success against known analytical solutions for different shear viscosities. An example shows how anomalous conditions of pressure and temperature can in principle be detected with seismic waves.

Distributed acoustic sensing (DAS) using fiber optic cables is an emerging seismic acquisition technology for the oil and gas industry, geothermal resource exploration, and underground fluid-storage monitoring. This technology offers the advantage of improving seismic acquisition by enabling massive arrays for monitoring of seismic wavefields at reduced cost with respect to conventional methods. In general, it is accepted that this method provides acoustic signals comparable with conventional seismic data, however, without the multicomponent directional information typical of geophones. We have developed a modified data extraction method and found that, as a result of the dense spatial distribution of recording points along the optic cable, DAS can provide two linked wavefield components in the axial direction, even when using a single 1D cable line. These signal pairs consist of dual components that are related to native strain rate (or strain) and particle acceleration (or velocity) fields at a given recording location. These dual signals are easily usable for wavefield separation purposes simply performing a trace-by-trace combination by appropriate scaling coefficient. The analysis performed with borehole data from linear and helically wound cables demonstrates the effectiveness of polarity recovery and dual-wavefield separation. We show real examples in which the data can be combined to provide separation of up- and downgoing wavefields. The ratio of the dual components provides information on local slowness properties in the formation.

We present a modified interferometry method based on local tangent-phase analysis, which corrects the cross-correlated data before summation. The approach makes it possible to synthesize virtual signals usually vanishing in the conventional seismic interferometry summation. For a given pair of receivers and a set of different source positions, a plurality of virtual traces is obtained at new stationary projected points located along the signal wavefronts passing through the real reference receiver. The position of the projected points is estimated by minimizing travel times using wavefront constraint and correlation-signal tangent information. The method uses mixed processing, which is partially based on velocity-model knowledge and on data-based blind interferometry. The approach can be used for selected events, including reflections with different stationary conditions and projected points with respect to those of the direct arrivals, to extend the interferometry representation in seismic exploration data where conventional illumination coverage is not sufficient to obtain the stationary-phase condition. We discuss possible applications in crosswell geometry with a velocity anomaly and a time lapse.

The earth's crust presents two dissimilar rheological behaviors depending on the in situ stress-temperature conditions. The upper, cooler part is brittle, while deeper zones are ductile. Seismic waves may reveal the presence of the transition but a proper characterization is required. We first obtain a stress–strain relation, including the effects of shear seismic attenuation and ductility due to shear deformations and plastic flow. The anelastic behavior is based on the Burgers mechanical model to describe the effects of seismic attenuation and steady-state creep flow. The shear Lamé constant of the brittle and ductile media depends on the in situ stress and temperature through the shear viscosity, which is obtained by the Arrhenius equation and the octahedral stress criterion. The P and S wave velocities decrease as depth and temperature increase due to the geothermal gradient, an effect which is more pronounced for shear waves. We then obtain the P−S and SH equations of motion recast in the velocity-stress formulation, including memory variables to avoid the computation of time convolutions. The equations correspond to isotropic anelastic and inhomogeneous media and are solved by a direct grid method based on the Runge–Kutta time stepping technique and the Fourier pseudospectral method. The algorithm is tested with success against known analytical solutions for different shear viscosities. A realistic example illustrates the computation of surface and reverse-VSP synthetic seismograms in the presence of an abrupt brittle–ductile transition.

Seismic interferometry based on cross-correlation of traces recorded at different positions makes it possible to recover new Green's functions and to obtain signals as if sources were placed at the receiver positions. This case history describes results obtained using sea-bottom hydrophone signals recorded in shallow water below the ice plate in an Alaska seismic survey. The aim was to simulate new shots at the sea-bottom receiver positions and to redatum the surface-generated seismic signals below the ice layer. The application was performed and the method investigated to attenuate the strong noise due to dispersive wave modes (mainly flexural waves) which propagate in the ice layer.

We present the results of a seismic interferometry experiment in a shallow cased borehole. The experiment is an initial study for subsequent borehole seismic surveys in an instrumented well site, where we plan to test other surface/borehole seismic techniques. The purpose of this application is to improve the knowledge of the reflectivity sequence and to verify the potential of the seismic interferometry approach to retrieve high-frequency signals in the single well geometry, overcoming the loss and attenuation effects introduced by the overburden. We used a walkaway vertical seismic profile (VSP) geometry with a seismic vibrator to generate polarized vertical and horizontal components along a surface seismic line and an array of 3C geophones cemented outside the casing. The recorded traces are processed to obtain virtual sources in the borehole and to simulate single-well gathers with a variable sourcereceiver offset in the vertical array. We compare the results obtained by processing the field data with synthetic signals calculated by numerical simulation and analyse the signal bandwidth and amplitude versus offset to evaluate near-field effects in the virtual signals. The application provides direct and reflected signals with improved bandwidth after vibrator signal deconvolution. Clear reflections are detected in the virtual seismic sections in agreement with the geology and other surface and borehole seismic data recorded with conventional seismic exploration techniques.

In this work we investigate the use of seismic interferometry based on a generalized-phase concept using natural and model-based information. We present a generalized-phase interferometry method based on tangent analysis, which corrects the correlation data before summation. The approach makes it possible to synthesize virtual signals usually vanishing in the conventional seismic interferometry summation. New traces are obtained at new stationary projected points for different source positions along the signal wavefronts passing through the real reference receiver. The position of the projected points is estimated by minimization of traveltimes using wavefront constraint and tangent information. The method, using a mixed processing, partially based on velocity-model knowledge and partially on interferometry, can be used for selected events, including reflections with different stationary conditions and projection points, to extend the interferometry to seismic exploration data where conventional illumination coverage is not sufficient to obtain stationary-phase condition.

We present the results of a seismic-while-drilling (SWD) survey performed using the working drill-bit as a seismic source to support drilling from a geophysical point of view in a well drilled for geothermal purposes. In this application we perform revers

In this work we present the results of a seismic while drilling (SWD) application using the drill-bit as a seismic source in geothermal wells. The survey was performed in a well drilled in the Nevada desert with the aim of providing geophysical information while drilling and images of the geological structures around the well. We present a summary description of the drill-bit seismic technology adapted for geothermal purposes, and the SWD results obtained along 2D profiles passing through the well. The good-quality seismic-while-drilling results have been subsequently used to obtain images of the well area in the direction normal to the main fault system, with SWD reverse vertical seismic profiling (RVSP) data migration, using seismic interferometry to extend laterally the coverage of the SWD RVSP images. The migrated seismic interferometry results confirm the main trends of the 2D geological model in the geothermal area.

Virtual seismic signals are synthesized by composing traces recorded by a plurality of sources and receivers. Seismic interferometry typically uses cross-correlation to compose the recorded traces, and to get virtual sources at the position of receivers.

We performed an interferometry experiment to simulate seismic signals in a single well. A seismic vibrator source was used to generate vertical and horizontal components of ground motion along a surface seismic line, and an array of 3C seismic receivers to record the signals in the borehole. The recorded traces are processed to obtain virtual sources in the borehole, and to simulate single well imaging (SWI) gathers with variable offset in the array. We compare results obtained by processing the real data with synthetic signals calculated by numerical simulation, and analyze the signal bandwidth and amplitude versus offset to evaluate near-field effects. The analysis shows that the method provides an approximation of the direct and reflected SWI signals, with results depending on the available coverage for interferometry. Good quality reflections are detectable in the simulated SWI sections.

Seismic methods contribute to the exploration of geothermal areas and characterization of existing geothermal resources. Seismic velocity and attenuation depend on the pressure and temperature conditions of the geothermal systems, which are closely related to the properties of the rock frame and geothermal fluids. We calculate the seismic velocities and attenuation in terms of the subsurface distribution of the confining and pore pressures and temperature, assuming that the heat transfer from below is convective or conductive. The pore pressure is assumed hydrostatic. In hydrothermal systems the temperature is calculated assuming the boiling point condition at the specific pore pressure down to the reservoir. Beneath the reservoir it is assumed constant in convectively heated systems and following a constant temperature gradient in conductively heated systems. In Enhanced Geothermal Systems (EGS) conductive heat transfer and constant temperature gradient are assumed. We present three application examples, considering simplified subsurface models to describe the geothermal systems beneath the production wells. The seismic wave properties are calculated using the rock's mechanical Burgers model and the Arrhenius equation to take into account rock-properties-variability with temperature and the Gassmann model for fluid saturating the porous rocks.

Geophysical characterization plays a key role for the definition of the deep structures of geothermal reservoirs and the consequent assessment and validation of the geothermal conceptual model. Seismic methods may provide a valuable contribution for this purpose. This involves a deep and reliable understanding of the sensitivity of seismic-wave propagation to physical and temperature variations, with complex interactions. We present the theory and sensitivity analysis based on rock's mechanical Burgers model including Arrhenius temperature equations, integrated with Gassmann model for fluid saturated porous rocks, pressure effects for bulk and shear moduli, as well as permeability and squirt flow effects. Assuming a temperature gradient model, the analysis applied at low seismic frequencies compares the interpretation of the sensitivity effects for different typical seismic elastic quantities, showing the different performance in relation to physical effects, including melting, supercritical conditions, and observability obtained in different temperature regions. With a quantification of the physical properties, the results of the study show that in deeper zones the main expected contributions in terms of variations in seismic velocity, moduli and seismic attenuation due to temperature come from melting transition, while in shallower porous fluid-saturated formations the trends are governed by pressure effects, with minor contributions of permeability and possible effects related to soft porosity. The new calculated elastic moduli are complex-valued and frequency-dependent, and temperature dependent through the fluid properties. In this complex scenario, not always the increments in the velocity and elastic wave moduli correspond to an increment in the temperature. Moreover, with mobility decreasing as a function of depth, the analysis shows that the shear quality factor is sensitive to permeability, which introduces moderate effects for velocity and attenuation of shear waves. The analysis applies to active exploration seismic and passive seismology.

Seismic interferometry by crosscorrelation is a known representation method used in seismic exploration to reconstruct the Green's function between receivers and to remove the overburden effects when buried receivers are used to record the seismic signals. An issue in conventional seismic interferometry applications for exploration purposes may be the lack of illumination coverage by real sources with a given receiver geometry for natural, i.e., data-based, Kirchhoff-Helmholtz representation of crosscorrelation type for the propagating wavefields. This shortcoming may be overcome by modified representation based on tangent analysis for selected events, and using partial model knowledge to locate new projected points where to calculate virtual signals. This interferometry method uses a generalized-phase operator in a modified representation integral. We investigate the sensitivity of the generalized phase interferometry (GPI) method to velocity variation, and the use of this approach for time-lapse purposes. We present crosswell examples where the acquisition geometry has no complete coverage for the illumination of a buried velocity lens by conventional seismic interferometry application. The results show that the GPI provides a set of new seismogram gathers for each couple of receivers, sensitive to time-lapse variations.