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Electrical tomography on the field: a guided tour logo anglais

by GIBERT Dominique - 4 March 2006

The aim of electrical tomography is to produce images of the interior of conductive bodies by studying how electrical current flows across them. In a volcano, the electrical resistiviy of rocks varies in a very wide range, from less than 1 \Omega.m for the most conductive rocks like the very altered rocks found in hydrothermal area of the volcano to more than 100000 \Omega.m for the most resistive ones like the massive unfracture andesite forming the Piton Dolomieu. Because of the large variations of the resistivity, electrical tomography is particularly well-suited to characterize the different geological materials forming a volcano. Since the presence of water strongly decreases the resistivity, electrical tomography is also a convenient mean to detect the hydrothermal reservoirs.

In practice, measurements are with four electrodes: two of them, traditionally named A and B, are used to inject the electrical current and the other two, named M and N, are used to measure the electrical potential. The Figure on the right shows how this method can be applied on the field. In order to probe a depth range, measurements are made with different electrode spacings, relying on the intuitive idea that the wider the space the greater the depth of investigation. The injected electrical current is of the order of 100 mA and the measured potentials varies from several Volts to several mV depending on both where the M and N electrodes are placed with respect to A and B and on the electrical conductivity of the rocks.

On the field, the measurements are made by deploying long cables on the ground and by pluging electrodes onto them. In our experiments, the cables measure about 1 kilometer and count 64 electrode plugs. The image below shows a typical equipment setup as deployed on La Soufrière. The cables must be long enough to place the A and B electrodes on opposite sides of the volcano and make the current flowing across it. A remote electrode may be used to augment the length of the cables. Such electrode configurations provide large-scale data about the deep global structure of the volcano. These data are completed with small-scale data obtained with nearby electrodes and providing information about the shallow geo-electrical structure of the volcano.

The picture on the left below shows the cables stored on their wheels. Each cable counts 11 electrodes plugs placed every 15 meters along the cable. A total of 6 cables is necessary to obtain a full-length setup with 64 electrodes. A cable weights about 22 kg. The storage wheels are too heavy (about 7 kg) to be carried on the field, and, as shown in the picture on the right, we prefer to put the cables into strong back-pads as used in caving and canyoning.

The helicopter of the civil security is of much help, especially since the road going from the Saint-Claude village to the base of the dome has been destroyed by a landslide caused by Les Saintes earthquake in November 21th, 2004 (see for more details). Flight conditions are often very difficult both because of clouds and strong winds.

On rough terrain conditions, rope access techniques are necessary to operate in safe conditions. Care must be taken against sudden floods caused by heavy rain on the volcano.

On the left, Ludovic Séverin is carrying a back-pad with the cable automatically unrolling and correctly placed on the ground by Florence Nicollin. On the right, Didier Mallarino progresses on a steep slope while unrolling the cable. If the slope is too steep, the cables must be secured in order to prevent them from sliding and braking under their own weight.

Once installed, the cables are connected to the station and the measurements may begin. The resistivity meter (with a lcd screen) injects the electrical current between electrodes A and B and measures the electrical potential between electrodes M and N. The resistivity meter also controls the relay matrix of the switcher unit onto which the cables are connected. An arbitrary sequence of A, B, M, N electrodes chosen among the 64 electrodes connected to the cables is permitted.

The electrodes are simple stainless steel rods with a length of about 30 cm and a diameter of 1 cm. High voltages (several hundreds of Volts) and intensities (up to 500 mA) may be found at the A and B electrodes. These lethal values imply to be very cautious when the apparatus is on, and warning pannels must be placed on the pathes where tourists may be present.

Heavy rain constitute a real difficulty and necessitate to protect all equipment. In particular, the plugs connecting the cables must be carefully taped in order to prevent water to produce short-circuits.

Corrosive gases is the second ennemy of both people and equipment ! Fighting against gasses is difficult and the apparatus needs a complete checking after each mission.

After the completion of the field measurements, the data are downloaded at the observatory and are processed to produce pseudo-sections of apparent electrical resistivity as shown in the picture on the right. By this way, it is possible to check both the quality and the integrity of the data set before moving the cables on the field and performing another electrical profile. The picture on the left was taken in May 2002 and shows Florence Nicollin (with the yellow shirt) together with students Julie Chaulet and Charles Poitou. The pictures on the walls show La Soufrière in 1976 when an important phreatic eruptive activity occurred.