GTEC has formed a joint work alliance with DEMCO NV, Belgium (http://www.demco-surveys.com) to provide the Aquares resistivity surveys for sand search project, dredging exploration projects and port development projects. A brief detail of the ARS applications are given below.
Aquares is a high resolution geophysical method based on the old principles of geoelectrical sounding as well known for land based systems. In the Aquares method a number of elements have been added to this originally land-based resistivity technique turning it into a very powerful tool for fluvial and marine applications.
Applications - Marine / Fluvial
Application - Land
Principles of Vertical Electrical Sounding
Land based applications
An electrical current is injected into the subsurface by means of two current 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 field curve is obtained showing the resistivity as a function of the (horizontal) distance between the current electrodes. After computer-modelling this field curve 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. Gravel usually has a lower porosity than sand and it's resistivity thus is higher. Clay with generally very high porosities shows very low resistivity. Solid rock, on the other hand, has a low porosity and shows very high resistivity. Each geological structure tends to have it's own specific resistivity.
Fluvial and marine Applications
For water based applications the electrodes are placed on a multichannel cable trailing behind the survey vessel. According to the circumstances the cable may be floating or towed on the seafloor. A floating cable may be more efficient in shallow water or if obstacles on the seafloor hamper the use of a bottom towed cable. The electrode geometry is chosen such that good quality data may be obtained even for shallower targets.
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 3 or 4 seconds. At a boat speed of 1 m/s this corresponds to a horizontal resolution of 1 sounding every 3 to 4 meters. In applications concerning the exploration of alluvial diamonds this resolution is needed to detect even the smaller diamond-bearing "potholes" and buried channels.