2D Resistivity Sounding

OUTLINE

2D Resistivity Imaging uses an array of electrodes (typically 64) connected by multicore cable to provide a linear depth profile, or pseudosection, of the variation in resistivity both along the survey line and with depth. Switching of the current and potential electrode pairs is done automatically using a laptop computer and relay box. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased and the process repeated in order to provide an increased depth of investigation.

DETAIL

Measurement of ground resistivity involves passing an electrical current into the ground using a pair of steel or copper electrodes and measuring the resulting potential difference within the subsurface using a second pair of electrodes. These are normally placed between the current electrodes.

Unlike conventional resistivity sounding and lateral profiling surveys, 2D resistivity imaging is a fully automated technique that uses a linear array of up to 64 electrodes connected by multicore cable. The current and potential electrode pairs are switched automatically using a laptop computer and control module connected to a ground resistivity meter (that provides the output current). In this way a profile of resistivity against depth (‘pseudosection’) is built up along the survey line. Data is collected by automatically profiling along the line at different electrode separations. The computer initially keeps the spacing between the electrodes fixed and moves the pairs along the line until the last electrode is reached. The spacing is then increased by the minimum electrode separation (the physical distance between electrodes which remains fixed throughout the survey) and the process repeated in order to provide an increased depth of investigation.

The maximum depth of investigation is determined by the spacing between the electrodes and the number of electrodes in the array. For a 64 electrode array with an electrode spacing of 2m this depth is approximately 20m. However, as the spacing between the active electrodes is increased, fewer and fewer points are collected at each ‘depth level’, until on the final level only 1 reading is acquired (see figure). In order to overcome this the array is ‘rolled-along’ the line of investigation in order to build up a longer pseudosection.

The raw data is initially converted to apparent resistivity values using a geometric factor that is determined by the type of electrode configuration used. Many 2D resistivity imaging surveys are carried out using the Wenner Array. In this configuration the spacing between each electrode is identical. Once converted the data is modeled using finite element and least squares inversion methods in order to calculate a true resistivity versus depth pseudosection.

RESULTS
The modeled results are displayed as scaled resistivity-depth pseudosections as illustrated. Blues represent areas of low resistivity whilst reds are relatively higher. image060The wedge shape of the plot illustrates the gradual reduction in the amount of data acquired as the current and potential electrode spacing are increased. As discussed earlier this is overcome by gradually rolling the electrode array along the survey line. The interpretation of the resistivity-depth pseudosection is normally provided as a separate diagram beneath the data or overlain directly on top. The results are calibrated using any available borehole or trial pit information together with modeled results from 1D resistivity soundings taken on the 2D resistivity imaging line.