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The prediction of porosity and water content of near-surface chalks is important for agricultural, environmental, hydrologic, and geologic engineering investigations. However, current methods based on laboratory measurements or crosshole ground-penetrating radar (GPR) are typically costly and applicable to only a limited area. We have combined the refraction seismic and reflection GPR data sets measured on the ground surface along a 96 m long profile line to predict the porosity and water saturation simultaneously. Using integrated rock-physics templates, we develop and apply a quantitative method that combines acoustic and electromagnetic velocities to estimate the petrophysical parameters of chalks. The seismic and GPR data are processed independently and subsequently combined to invert the porosity and water saturation. Field data tests demonstrate that the predicted results are consistent with laboratory measurements and field observations, demonstrating that the integration of seismic and GPR data facilitates efficient and reliable estimates of the porosity and saturation of chalks on scales relevant to regional field mapping. Hence, this method allows the petrophysical parameters of near-surface chalks to be evaluated efficiently at a regional scale.

This paper develops a minimum-support focusing stabilizer to perform a joint inversion of the vertical components of gravity and magnetic data using fuzzy c-means clustering (FCM) with the regularized Newton method in a space of weighted parameters. Not only does this joint inversion technology arrive at the conditionally well-posed traditional potential field inversion, but it also increases the structural correlation between multiple inverted models. The FCM and the focusing stabilizer make it possible to balance the convergence of the data space (D) and the model space (M), guiding multimodal geophysical parameters toward assigned petrophysical values, which makes the results more stable and realistic. Two model studies are presented to illustrate the method, a simple synthetic model with two rectangular bodies in a homogenous background and a realistic model of the Volcanogenic Massive Sulfide (VMS) deposits in northeastern New Brunswick, Canada. These models demonstrate that the new focusing joint inversion algorithm produces better images than traditional methods because the FCM function uses the structural correlation of density contrast and magnetic susceptibility as constraints.

The characterization of shallow subsurface formations is essential for geological mapping and interpretation, reservoir characterization, and prospecting related to mining/quarrying. To analyze elastic and electromagnetic properties, we characterize near-surface chalk formations deposited on a shallow seabed during the Late Cretaceous-Early Paleogene (Maastrichtian-Danian). Electromagnetic and elastic properties, both of which are related to mineralogy, porosity, and water saturation, are combined to characterize the physical properties of chalk formations. We also perform rock physics modeling of elastic velocities and permittivity and analyze their relationships. We then use measured ground penetrating radar and P-wave velocity field data to determine the key model parameters, which are essential for the validity of the models and can be used to evaluate the consolidation degree of the rocks. Based on the models, a scheme is developed to estimate the porosity and water saturation by combining the two rock physics templates. The predictions are consistent with previous findings. Our templates facilitate fast mapping of near-surface porosity and saturation distributions and represent an efficient and cost-effective method for near-surface hydrological, environmental, and petrophysical studies. In the current formulation, the method is only applicable to rock type (chalk) comprising a single mineral (pure calcite). It is possible to tailor the formulation to include more than one mineral; however, this will increase the uncertainty of the results.

Investigating seismic dispersion and attenuation characteristics of loosely compacted marine sandstone is essential in reconciling different geophysical measurements (surface seismic, well logging and ultrasonic) for better characterization of a shallow marine sandstone reservoir. We have experimented with a typical high-porosity and high-permeability sandstone sample, extracted from the Paleogene marine depositional setting in the Gulf of Mexico, at the low-frequency band (2–500 Hz) as well as ultrasonic point (106 Hz), to investigate the effects of varying saturation levels on a rock's elasticity. The results suggest that the Young's modulus of the measured sample with adsorbed moisture at laboratory conditions (room temperature, 60%–90% humidity) exhibits dispersive behaviours. The extensional attenuation can be as high as 0.038, and the peak frequency occurs around 60 Hz. The extensional attenuation due to moisture adsorption can be dramatically mitigated with the increase of confining pressure. For partial saturation status, extensional attenuation increases as increasing water saturation by 79% with respect to the measured frequencies. Additionally, the results show that extensional attenuation at the fully water-saturated situation is even smaller than that at adsorbed moisture conditions. The Gassmann–Wood model can overall capture the P-wave velocity-saturation trend of measured data at seismic frequencies, demonstrating that the partially saturated unconsolidated sandstone at the measured seismic frequency range is prone to be in the relaxed status. Nevertheless, the ultrasonic velocities are significantly higher than the Gassmann–Wood predictions, suggesting that the rocks are in the unrelaxed status at the ultrasonic frequency range. The poroelastic modelling results based on the patchy saturation model also indicate that the characteristic frequency of the partially saturated sample is likely beyond the measured seismic frequency range.

Characterizing physical properties of near surface chalk formations is always a challenging topic. Although acoustic and electromagnetic measurements can independently provide information on different properties, little research has combined them to characterize chalk rocks. Since both EM and acoustic properties are related to the porosity, saturation, and mineral composition of the rock, we develop rock physics templates to predict P-wave velocity (Vp), S-wave velocity (Vs), and permittivity from porosity and saturation. In this work, we first estimate the IF value of the IF model independently from field P-wave velocity and porosity measurements. Then, by combining crosshole ground-penetrating radar (GPR) velocity and porosity, we also estimate the value of the cementation index. Finally, given that the IF value and cementation index are independent of porosity and water saturation, we develop three rock physics templates to link Vp, Vs, and permittivity to porosity and saturation.

Investigations of the elastic mechanical characteristics of loosely compacted mixed sand-clay samples are essential for the characterization of shallow seafloor sedimentary reservoirs. Here, we used a series of sand-clay mixtures (0–100% clay by weight) to investigate the effects of pressure load histories (2–50 MPa) and clay fraction on porosity, P-wave velocity, S-wave velocity, and static modulus. The results indicated that, under pressure load cycles, the sand-clay mixture samples exhibited high permanent compaction, and porosity decreased rapidly with increasing clay content. However, porosity changed differently in response to pressure load as compared to pressure unload. Correspondingly, P-wave velocities differed significantly between pressure load and unload. The rock physical models implied that the sample frame evolved with the varying clay fraction. As the clay fraction increased, the sample frame gradually transitioned from primarily sand grains with dispersed pore-filling clay to primarily clay, ultimately transforming into a clay frame with dispersed sand grains. The measured data generally conformed to the theoretical model and falling within the Voigt and Reuss bounds. Moreover, for all sand-clay mixture samples, although the static bulk modulus was consistently lower than the dynamic bulk modulus, the ratio of the dynamic modulus to the static modulus was highly dependent on pressure loads and clay fraction.