Principles of Aquares resistivity sounding
Land based applications
An electrical current is injected into the subsurface by means of two currrent electrodes. The voltage gradient associated with the electrical field of this current is measured between two voltage electrodes placed in between the current electrodes. Based on the measured values of current and voltage the average resistivity of the subsurface is calculated for a subsurface volume down to a certain penetration depth. The penetration depth depends on the distance between the current electrodes. Larger electrode distances are associated with increasing penetration depths. If the measurements are repeated with progressively increasing current electrode distances information is obtained from progressively deeper geological structures. As such, a fieldcurve is obtained showing the resistivity as a function of the (horizontal) distance between the current electrodes.
After computermodelling this fieldcurve is transformed into a real geophysical subsurface section showing the resistivity as a function of depth. The resistivity of a geological structure depends on it's porosity, water saturation and the pore water resistivity value. Gravel usually has a lower porosity than sand and it's resistivity value thus is higher. Clay with generally very high porosities shows very low resistivities. Solid rock, on the other hand, has a low porosity and shows very high resistivity values. Each geological structure tends to have it's own specific resistivity value. In fresh water the resistivity value of a geological structure generally depends on it's clay content.
Fluvial and marine applications
For water based applications the electrodes are placed on a multichannel cable trailing behind a survey vessel. The survey vessel may be a large seagoing ship, a tugboat, normal survey launches, inflatable rubber ducks, a dugout canoe or even a pick-up truck running on the beach.
While the survey vessel is sailing the measurements are carried out and stored automatically without any intervention from the operator. As such, an entire electrical sounding may be obtained every 2.5 seconds. At a boat speed of 2 m/s this corresponds to a horizontal resolution of 1 sounding every 5 meters. In applications concerning the exploration of alluvial diamonds the resolution can be increased to 1 sounding per second which would be needed to detect even the smallest diamond bearing "potholes" and buried channels. During the fieldsurvey qualitative results are already shown on computer screen. The quality of the fielddata are monitored on line so the operator can intervene at any moment to adjust and optimise the survey parameters. As each volume element of space can be assigned a resistivity value resistivity survey results can be interpolated into a full 4D model of the subsurface (X,Y,Z, resistivity) which can be visualised with a number of horizontal and vertical resistivity sections at any level and orientation.