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

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Mr. Reginald Gillespie L2757-92 August 3, 1992 Page 2 Near surface geologic and manmade structures significantly influence the total magnetic field. The closer the magnetic sensor is to a near surface structure, the greater effect that structure has on changing the total magnetic field where the object is located. Magnetic susceptibility is the degree to which a substance may be magnetized. For example, formations that contain a high percentage of ferrous material (i.e., diabase dikes) would have a relatively high susceptibility whereas a formation that contains little or no ferrous materials would have a low susceptibility. Based on these two factors, magnetics could be used to determine whether these formations are present at prospective sites. Typically, these investigations involve the use of a gradiometer to determine the amount of ferrous material in the underlying formations. The earth's vertical magnetic gradient is particularly sensitive to near surface variations as they are typically very localized especially in the cases of diabase dikes. The effects of the near surface structures on the magnetic field rapidly decrease as the distance between the object and sensor increases. This method offers four key advantages: • it is non-invasive with no disturbance of the tract (i.e., trees, road cutting, drilling) • the relative quickness for conducting a survey (the site can be covered in 1 to 3 days, depending on the size of the area and level of detail of the survey) • anomalous structures can be located in the field • the minimal cost for the procedure Aquaterra has used the magnetic survey techniques for mapping diabase dikes for both water supply and ground water remediation. Although the diabase dikes are relatively impermeable, when they are injected through the older rocks, they form very permeable zones on either side of the dike which can readily transmit ground water and contaminants. Their presence is considered a major limitation for a prospective landfill site. 'Aquaterra has mapped for two clients in the Research Triangle Park over 1,500 acres of land, locating nearly 4 miles of dikes. The magnetic survey results were used to place 5 extraction wells and numerous monitoring wells for ground water remediation and one well for water supply. Electrical Resistivity Soundings Further definition of the depth to bedrock can be assessed using electrical survey methods such as DC Resistivity. Vertical electric soundings (YES) can be used to determine whether a traceable geologic bedrock unit can be found. Generally, bedrock is more resistant than soils or sediments because of the soils or sediments fluid content. If this is achieved, several lines consisting of VES will be conducted through the prospective site to determine whether this unit has continuity. The depth of the traceable unit will be determined by using a geophysical model; the AQUATERRA