4.5 Geoelectrical methods - LIAG

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80 INGELISE MØLLER, KURT I. SØRENSEN & ESBEN AUKEN Fig. 4.5.4: The CVES method. a) A sketch of a field setup for CVES and data density for a profile acquired in Wenner configuration with 5 electrode spacings. b) Photo of the CVES system in the field. The productivity in the field depends on the density of the data points in the profiles and the number of channels in the resistivity instrument. The initial electrode spacing do also count, because the longer electrode spacing the longer distances the field crew have to walk, while they roll cables out, enter electrodes into the ground, and connect them to the cables. In groundwater mapping an initial electrode spacing of 5 m is very common. When the data are collected in Wenner arrays the maximum electrode spacing is thus typically about 100–120 m with a total array length of 300–360 m. This leads to a penetration depth about 60–80 m. Other types of electrode arrays with similar maximum array lengths result in similar penetration depths. If the target is shallower, the initial electrode spacing can be decreased. This will also result in a higher resolution of particular the near surface small-scale resistivity structures. Pulled array continuous electrical sounding The PACES system was developed to meet the demands of high productivity, reliability and detailed, dense sampling (Sørensen 1996, Sørensen et al. 2005). Figure 4.5.5 shows schematics and photograph of the system. A small caterpillar pulls a tail of electrodes along with the processing electronics. The caterpillar, with a height of 80 cm and a width of 70 cm can pass under fences and in between trees. A small 3 m platform is carried along and serves as a bridge for crossing small creeks and ditches. Eight electrode arrays are measured simultaneously allowing for sounding data to be acquired continuously along the profile (Fig. 4.5.5b). A crew of two workers can record profile lengths of 10–20 km/day. Two electrodes with a current of 2–30 milliamperes are maintained as sources. The current limit of 30 milliamperes is for the safety of personnel. The current is maintained at a constant level with a current generator in order to facilitate data processing. Processing electronics with a high input resistance of 5-10 Megaohms (MΩ) are mounted inside the potential electrodes to diminish earth contact problems, cross talk and capacitive coupling between receiver– and transmitter cabling in the tail. Light weighted mild-steel electrodes with a maximum of 10 kg are necessary as current and potential electrodes. Electrochemical interactions between the rapidly changing soil environment and metal potential electrodes are by far the largest noise sources. The decay time of these noise potentials is of the order of seconds. Clearly this is not an issue when using traditional metal rods, but in the case
4.5 Geoelectrical methods Fig. 4.5.5: The PACES method. a) Sketch of the PACES system, where a tail of electrodes is pulled by a small caterpillar. b) A diagram of the eight electrode configurations, which all use the same two current electrodes. c) Photo of the PACES system in the field. of moving electrodes the influence may be severe. The electrochemical noise is suppressed by applying synchronous detection techniques with a frequency of 20 Hz followed by adapted filtering (Munkholm et al. 1995). Digital data acquisition is performed on-line by equipment on the caterpillar. The data are sampled with a frequency of 80 Hz and reduced to a dataset for every second, and the speed of the tail is approximately 1 m/s (3 km/h). The spatial averaging width in the post-processing stage is related to the electrode spacing and is usually 0.1 – 0.25 times the spacing, giving a detailed spatial resolution and smooth datasets. To monitor data quality, the system has the ability to detect insufficient galvanic contact at the current and potential electrodes while measuring. In addition, the magnitude of the emitted current is adapted automatically to ground contact to optimize the signal-to-noise ratio. The amplification in each channel adjusts automatically to the level of the incoming signals (Sørensen 1996). All parameters related to the measuring process are stored together with the data sets to insure high data quality. 4.5.3 Data processing and inversion Data processing Geoelectrical data are normally processed by simply removing data influenced by noise. Figure 4.5.6a shows an example of a data profile collected using gradient arrays (Dahlin & Zhou 2004). A very simple data processing is done to the data and only data with a “jiggered” appearance are removed. The removal is done manual. The pseudosection in Figure 4.5.6b shows colour contour of the same data as in Figure 4.5.6a. Inversion In the following we will discuss inversion methodologies for CVES and PACES data focusing on the 1-D and the 2D laterally constrained inversion (LCI) which uses sharp layer boundaries. The 1D-LCI was originally developed for inverting PACES data (Sørensen 1996). The data quantities from the PACES system are extremely large, and 2D smooth inversion is therefore not practically possible on a routine basis. Because the PACES 81
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4.5 Geoelectrical methods - LIAG

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