Using Elastic Wave Seismic Data to Image an Ultra-shallow Buried Paleo- channel

Previously we conducted a set of compressional wave seismic investigations at a groundwater contamination site (Hill Air Force Base, Ogden, UT), in order to better characterize the subsurface environment determined from well data. The seismic target is the bottom of a paleo-channel eroded in a clay layer at a depth of 10-15 m that is overlain by a mix of sands, clays, and gravels with different compaction and water saturation. The channel acts as contaminant trap for dense non-aqueous phase liquids (DNAPLs), therefore a detailed map of its geometry and depth is crucial to the remediation effort. At the same site, we recently performed another experiment using elastic-wave sources, in order to determine subsurface elastic parameters, to complement the P-wave data already acquired and processed, and to exploit the lack of sensitivity of the shear modulus to fluid saturation. The seismic data were acquired along two profiles using 60 groups of three single-component 40 Hz geophones mounted in a Galperin configuration. The length of each profile is 29.5 m. The transverse component was extracted and processed in order to obtain SH- wave images of the subsurface. The SH-wave depth migrated sections show geometry of and depth to the clay layer that are remarkably similar to those derived from well data. Moreover, the sections can be compared to the velocity model previously obtained from P-wave waveform tomography: If one assumes that the 1100 m/s P-wave velocity contour the top of the clay in the waveform tomography, the difference between the channel imaged by the SH- wave data and waveform tomography is insignificant. We have also obtained P-wave and S-wave tomography velocity models along the two lines. By combining the Vp and Vs information, a map of the Vp/Vs ratio and the Poisson's ratio was reconstructed. These parameters can be related to lithology changes in the subsurface. Low Vp/Vs and Poisson's ratio values normally relate to stiff materials (i.e. sands), while high value are generally interpreted to be less rigid materials (i.e. saturated clays). Our study shows that it is possible to image and delineate the geometry of the channel, with fairly high resolution and accuracy, using shear wave sources, as well as the using compressional-wave sources. The images made using the different seismic methods are in good accordance with each other and the well data, and allow a cross-validation of the final results.