Technical Test Of CORIM in France

Continuous Resistivity Imaging System

 

The CORIM equipment measures the resistivity of the ground at a few meters depth through an AC capacitive electrode system using one transmitting (Tx) carpet and up to six receiving (Rx) carpets pulled on the surface behind a towing vehicle (Fig. 1 and Fig.2).

A field test of CORIM system has been carried out along the Loire river, near Orleans city, on sedimentary formations consisting in clayey sands overlaying thick limestone layers.

In a first step, a profile has been measured two times to check the repetitivity of the readings.  Fig.3 shows that the two sets of readings for both channel 1 (shallowest investigation, highest signals) and channel 6 (deepest investigation, lowest signals) are quite repetitive.

In a second step, the same profile has been successively measured with the various values of current intensities available from the transmitter :2, 4, 6 and 8 mA.  The linearity of the system is checked on Fig. 4 by the stability of the resistivity values when the current changes; it is as valid for channel 6 as for channel 1.  on this site, the noise level appears to be equivalent to less than 1 ohm*m for channel 6.

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Fig. 1 : Principle of CORIM -Continuous resistivity imaging system
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Fig. 2 : CORIM set-up to be pulled by vehicle.

 

Finally, a comparison has been carried out between the readings of the CORIM system with its AC capacitive (plate) electrodes and those of the SYSCAL R2 standard resistivity meter using DC resistive (stake) electrodes.  The SYSCAL R2 measurements have been taken with the same equatorial dipole-dipole array as the CORIM system.  A good correlation between the two sets of data can be observed in Fig. 5.
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Fig. 3 : CORIM repetitivity test : Receiving channel 1 and 6 measured two times
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Fig. 4 : CORIM linearity test : Receiving channel 1 and 6 measured witht he various intensities of current : 2,4,6,8 mA
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Fig. 5 : Comparison of CORIM AC capacitive electrodes system and SYSCAL R2 DC resistve electrodes system on the test profile.

The analysis of the pseudo-sections shows an increase of resistivity with depth corresponding to the transition of the clayey sands to the limestones.  Also the lateral increase on the right part of the section suggests that limestones are almost outcropping between stations 30 and 42.

As a conclusion, CORIM system proves to give an electrical image of the ground both qualitatively and quantitatively comparable with standard DC measurements.  The main advantage of CORIM system for shallow resistivity surveys consists in the high speed of acquisition it provides (0.5 to a few km/h) thanks to its continuous measuring capability.