Imaging a shallow aquitard with seismic reflection data in Memphis, Tennessee, USA. Part I: source comparison, walk‐away tests and the plus‐minus method

Abstract

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.

High resolution 3D near surface imaging of fracture corridors and cavities by combining Plus‐Minus method and refraction tomography

“…The Plus‐Minus method or conventional reciprocal method is a simple technique for interpreting in‐line seismic refraction data, consisting of forward and reverse travel times (Overmeeren ). The GRM method is actually a generalization of the Plus‐Minus method which is widely used in refraction prospecting (Ge et al ). This method assumes that first‐arrivals are only originated by critical refraction and lateral continuous refractors with relatively simple velocity distributions.…”

Determination of Contaminant Transport Parameters for a Local Aquifer by Numerical Modeling of Two Plumes: Trichloroethylene and Hexavalent Chromium

“…In Shelby County, Tennessee, the primary drinking water aquifer is the semi-confined unconsolidated Memphis aquifer [36,37]. The aquifer is threatened by the presence of localized preferential pathways (i.e., breaches) in the overlying aquitard (the upper Claiborne confining unit-UCCU), warranting concern that contaminants could easily bypass the aquitard's natural protection [38][39][40][41][42][43][44][45][46][47][48]. Very little is known about the contaminant transport properties of the Memphis aquifer.…”