oa A Shallow Seismic Reflection Study Of The Water Table

A shallow seismic reflection survey was conducted<br>approximately one mile southeast of Newark, Delaware to determine<br>the depth and geometry of the water table. The site is located<br>0.25 miles from a landfill where this method may be used in a<br>pollution remediation effort. Of particular interest in this<br>study are the optimum in-field seismograph recording parameters<br>and geophone array spacings, the adequacy of a hammer/aluminum<br>plate seismic source in this environment, the vertical and<br>horizontal resolution of the water table reflector given an<br>unsaturated overburden, the seismic facies characteristics of the<br>water table reflector (reflector frequency, amplitude, coherency,<br>interval velocity, and configuration), and the three dimensional<br>geometry of the water table surface. A series of walkaway noise<br>tests were performed to (a) determine the feasibility of<br>accurately recording a reflection from the water table, (b)<br>define the ,low cut filter which maximizes the water table<br>reflection signal while simultaneously suppressing ground roll<br>effects, (c) to ascertain the nature of all coherent and<br>incoherent seismic noise (ground roll, ground coupled air wave,<br>multiple reflections, wind, and electrical), Cd) to observe<br>possible phase and amplitude distortions in the water table<br>reflector as predicted by the Zoeppritz equations, and (e) to<br>compare calculated reflection and refraction depth and velocity results.<br>All data were recorded with severe, low cut pre-emphasis<br>filters (350 Hz) to attenuate the high amplitude, low frequency<br>ground roll component which dominated the early portions of the<br>record. Reflection signals from the water table interface<br>exhibited dominant frequencies of loo-150 Hz and stacking<br>velocities of 1420 feet per second, leading to a theoretical<br>vertical resolution of 2.4 feet (based on l/4 wavelength<br>criterion). Reflection depths and velocities were 34.8 feet and<br>1420 feet per second as compared to refraction depths and<br>velocities of 35.5 feet and 1434 feet per second.<br>Two parallel six-fold common depth point seismic sections<br>and 5 common offset seismic sections comprised a three<br>dimensional seismic grid from which the geometry of the water<br>table was determined. The CDP seismic sections exhibited a<br>coherent, high amplitude reflection at 50 ms. Using a stacking<br>velocity of 1420 feet per second, a reflector depth of 35 feet<br>was calculated. Later drilling confirmed the water table depth at<br>35 feet.