CORRESPONDENCE_2005-2006 - Laserfiche WebLink

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• separation between dipoles increases, so does the depth of investigation. Once the maximum separation is reached,the current dipole is moved along the profile one dipole length and the entire procedure is repeated. ' For each reading,a value is calculated that represents the apparent resistivity of the volume of earth g P pp Y that the current flows through.The term,apparent,is used because the value represents the resistivity ' of a volume rather than an individual layer.The apparent resistivity values are then"pseudo" in cross- section and contoured to form what is referred to as a"pseudo-section". The term pseudo is used ' because the vertical scale is not scalar but is proportional to the dipole separation. In addition, the resistivities are apparent rather than true. However,the pseudo-section can be inverted to generate a 2-D model showing the depth and true resistivity of subsurface layers. ' Instrumentation ' Apparent resistivity data is typically acquired using a SuperSting R1 Resistivity meter with the Swift automatic multi-electrode system. Both systems are manufactured by Advanced Geosciences Incorporated(AGI). The Sting is a self-contained unit that transmits current at outputs ranging from 1 to 500 milliAmps(mA). The unit also measures the potential drop and converts the data to values of apparent resistivity for a number of electrode arrays. The data are stored in internal memory and can be downloaded to a computer for processing. The Swift consists of an electrode interface ' console, four cables, and 56 stainless steel electrodes. Each cable has 14 individual take-outs that can be connected to electrodes at intervals up to 10 meters. Depending on the objective of the survey, the Swift can operate using 28 to 56 electrodes. ' Data Acquisition ' ER surveys using the Sting/Swift resistivity system are initiated by laying out the cables,end-to-end, along each profile. The Swift console is then connected between the two cables and to the Sting ER ' meter. At each take-out in the cable, stainless steel electrodes are driven into the ground and then fastened to the respective take-out. To begin the survey,the ER meter tests the contact resistance of each electrode.If any of the values are abnormally high,the electrode plant as well as the connection ' between the electrode and the switch is inspected, and if necessary, improved. The survey is begun once all of the electrode contacts tested satisfactory. To start out,readings are taken with the dipoles separated by their common length and moved along the length of the array. For example, if the ' length between two electrodes (referred to as a dipole) is 10 meters, then the distance between the current and potential dipoles(two electrodes each)will also be 10 meters. Since each of the switches are individually addressable by the Sting,the instrument is able to move this configuration down the array by turning the appropriate switches on and off, as necessary, to switch from one dipole to another. Subsequent readings are then taken by increasing the distance between dipoles,up to eight times the dipole separation,along the array. It then repeats the entire procedure using dipole lengths ' typically two to three times the length of the initial dipole. For example, if the initial dipole was 10 meters, then the Sting/Swift system repeats the process using dipole lengths of 20 and 30 meters. A-5