We used vertical seismic profiling (VSP) data collected in shallow boreholes (about 40-60 m deep) to determine the shear-wave velocity at four sites in the Mississippi embayment in southwestern Tennessee. The source was an air-powered hammer that produces repeatable SH waves, which were recorded by source monitor geophones deployed on the surface very close to the source. Three approaches were used to determine interval velocities: an approximate zero-offset method, a layerstripping method, and a waveform-matching method. The first two methods use arrival-time picks, whereas the latter is based on the fit of synthetic VSP data to the first half-cycle (approximately) of each trace. The advantage of this method over the other two is that it uses a segment of the data, rather than a single data point. Therefore, the velocities determined using the waveform-matching method are better constrained and are not affected by picking errors, which may translate into significant spurious velocity variations. The source wavelets recorded by one of the monitor geophones and the velocity model computed with the layer-stripping method were used to generate synthetic vertical seismic profiling data for comparison with the actual data. Then the model velocities were modified interactively, one layer at a time, until a satisfactory match was achieved. This required including attenuation in the computation of the synthetic data. The four sites investigated in this study can be divided into two groups: low-velocity sites (Shelby Farms and Covington) and high-velocity sites (Brownsville and Jackson). These last two sites are at larger distances from the embayment axis than the other two, which means that the difference in velocities probably corresponds to the presence of different geologic units. Good agreements between the lithology in the boreholes and the velocity profiles were obtained for all the four sites.
We used vertical seismic profiling (VSP) data collected in four shallow boreholes (about 40 to 60 m deep) to study the shear-wave attenuation in the Mississippi embayment in southwestern Tennessee. The source was an air-powered hammer that produces repeatable SH waves that were recorded by monitor geophones deployed on the surface very close to the source. The spectral ratio method was used to estimate the shear-wave quality factor (Q S). The method assumes that the amplitudes of the seismic waves decay exponentially in the frequency domain. The spectral ratio was computed using a VSP trace at a certain depth and the corresponding monitor trace. Using the source monitor trace as reference eliminates possible artifacts that may be introduced by changes in the source-ground coupling. The slope of a least-square line fitted to the logarithm of the spectral ratio versus frequency gives the attenuation coefficient αz for that depth. Then a straight line is fitted to αz as a function of z, and the slope of this line is used to estimate an average value of Q S. For the four sites,
Two walk‐away tests were conducted at two sites in Memphis, Tennessee, USA. One site is representative of an urban setting (road median) while the other represents a rural setting (metropolitan park). Three P‐wave sources, a 7.5 kg sledgehammer, a 20 kg weight‐drop and a 12‐gauge shotgun, were tested. Analysis of the data collected indicates that the seismic data recorded from the shotgun source possess the strongest energy, the highest dominant frequency, the broadest frequency band and the least amount of ground roll energy. The source repeatability was also studied by observing the first cycle of each seismic source, showing that the shotgun can generate the most repeatable source wavelets. None of the data recorded from the three sources show significant seismic energy above 100 Hz due to seismic wave attenuation. The loess in the rural site exacerbated the attenuation and resulted in a much lower peak frequency (43.7 Hz), which is nearly half of the peak frequency recorded at the urban site (85.3 Hz). Since attenuation can be a big factor in shallow reflection surveying, we recommend that site attenuation be considered before a reflection survey is performed in the Memphis area and a reflection survey be conducted outside of the loess blanketed area when possible. Since the final goal of the survey is to search for aquitard breaches, Hagedoorns’ plus‐minus method was applied to the walk‐away data set to map the first refractor, the top of the aquitard. One depression from the obtained structure was interpreted as a paleochannel, indicating that river channel erosion may be one of the causes for the formation of aquitard breaches.
We collected P‐wave seismic reflection data to image suspected breaches in the confining unit (aquitard) above the Memphis aquifer in Memphis, Tennessee, USA, where previous studies of water quality have suggested potential contamination. A 1‐km‐long reflection line was acquired across depressions of the water table that have been interpreted to reflect a breach in the aquitard. Although raw data are dominated by surface waves, after frequency filtering, shot static correction and f−k filtering, consistent reflections can be observed. Integration of constant velocity analysis (CVS) and super gather semblance analysis was used to determine the RMS velocity field. The stack section shows a set of shallow reflectors interpreted to correspond to the top and bottom of the confining unit. The aquitard thins by about half from the north end to the south end along the reflection line. The continuity of the top and bottom of the aquitard was disrupted by a few faults interpreted along the line. None of these faults juxtapose the surficial aquifer against the Memphis aquifer but it is possible that the faults themselves may act as a hydraulic connection between the surficial aquifer and the Memphis aquifer and thus serve as pathways for a potential leakage. The refraction tomography technique was applied to the first arrival data and it revealed three depressions that are interpreted as paleochannels on the upper part of the confining unit, which is consistent with the background geology of the area. These inferred paleochannels may suggest that ancient river channel erosion may contribute to the process responsible for the formation of aquitard breaches in this area.
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