High-Resolution P- and S-wave Seismic Reflection Investigation of a Shallow Stratigraphic Sequence
U.S. Army Corps of Engineers, Huntington, West Virginia, USA
The effectiveness of P- and S-wave reflection surveys for mapping a shallow stratigraphic sequence (flat-lying unsaturated and saturated overburden materials above consolidated units) was evaluated through the acquisition and analysis of high-resolution multicomponent data. The combined P- and S-wave common-mode reflection information allowed the near-surface sequence to be imaged more effectively than using solely the P- or S-wave information. S-wave reflections from the bedrock and overburden interface were consistently measured in both the XX component (inline-inline, or SV-SV) and the YY component (crossline-crossline, or SH-SH) field data. However, surface wave noise resulted in the optimum reflection window of XX component data being relatively narrow, and stacked YY component data had a higher signal-to-noise ratio and better imaged the top-of-bedrock. P-wave reflections from the unsaturated and saturated overburden interface were recorded in ZZ component (vertical-vertical, or P-P) field data, but S-wave reflections from this interface were not observed. P-wave events from deeper contrasts in impedance could not be resolved in field data due to surface wave and air wave noise, a high P-wave reflection coefficient at the top of the saturated overburden, low P-wave reflection coefficients at deeper interfaces, and interference effects and poor resolution. Calculations based on P- and S-wave velocities (Vp and Vs) and dominant wavelengths suggest that the vertical resolution of S-waves in the study area dry overburden was more than 1.7 times the resolution of P-waves, while the resolution of S-waves in the saturated overburden was more than 4.7 times that of P-waves. The potential for determining detailed variations in Poissonís ratio (n) using Vp/Vs ratios was found to be limited due to the small number of reflection events and the fact that P- and S-wave reflections did not correlate to similar interfaces; however, representative lithology values of n were able to be estimated using measurements and reasonable assumptions. Although P-wave reflection data have traditionally been acquired during shallow reflection surveys, results of this study demonstrate that due to differences in P- and S-wave propagation, media compressional and shear impedance contrasts, and variations in receiver sensitivity (as a function of orientation), it is necessary to consider the probable usefulness of different data components/wave-type reflections prior to conducting a shallow reflection survey. Near-surface imaging and characterization may be best accomplished through the acquisition and analysis of one particular, or multiple data components/wave-type reflections.
Keywords: Seismic Reflection, Multicomponent, Imaging, Characterization, P-waves, S-waves
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