Agenda - 03-23-1993 - VII-A - Laserfiche WebLink

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Mr. Reginald Gillespie L2757-92 August 3, 1992 Page 3 vertical extent of a site being evaluated for a possible landfill will be approximated based on this information coupled with any geophysically mappable characteristics of the site. Aquaterra proposes using DC Resistivity to determine any traceable bedrock units for several reasons: • it is non-invasive • the relative ease of conducting the survey • it is not susceptible to aboveground interferences Seismic Refraction Techniques Another method of determining the vertical extent of a site being evaluated for a possible landfill is seismic refraction techniques. The seismic refraction technique uses information gathered of the arrival times of seismic compressional waves to determine depths to and competency of definite geologic units. Refraction lines can be conducted at selected sections to trace out a mappable bedrock unit and detect depth to ground water. If the geologic conditions between the soil and the bedrock are significantly different, the outline along the refraction lines may be approximated. This method involves using a recorder that depicts arrival times at set stations. These devices are sensitive to the movement of seismic waves. An energy source (such as a hammer-like device) is used to generate the seismic wave. The ability to trace out geologic bedrock units and the water table is the key advantage of this method. The advantages of this method are its non-invasive nature, ease to conduct, and its ability to avoid aboveground interferences. Aquaterra has used seismic refraction techniques on several sanitary landfill siting studies. One particular study for Rowan County, reduced the need to employ invasive drilling techniques to only two sites with one being the ultimate location for their present landfill. Electromagnetic Method In this method, a remote transmitter is used as an energy source with frequencies varying between 15 to 30 kilohertz. The receiver is set at a position normal to the transmitter wave direction and profile lines are conducted along this trend. The receiver measures the radio signal distortion along the profile caused by electrically conductive structures in the subsurface. Aquaterra normally uses an ABEM Wadi VLF instrument to conduct these surveys. Fractures are located based on current density distributions of an electromagnetic field. Current density is calculated by the conductivity meter and then plotted along the profile. In general, positive current densities are associated with porous fractures. Fracturing is considered a major limitation for a prospective landfill site. AQUATERRA - '