The MAPPEM System
The MAPPEM system
The MAPPEM System has been specifically developed for coastal marine conditions (patented).
It is towed close but a few meters above the seabed to measure the electrical resistivity of the substratum.
The MAPPEM System is an electromagnetic imaging system composed of a fish towed at about 3 knots near the seabed (a few meters depending on local conditions) by a surface vessel, followed by a sensor cable. Every 0.5 seconds, the system injects a strong current (typically 40A in sea water) and simultaneously measures a series of induced electric potential at the 20 receivers. These potentials are a function of the resistivity of the subsurface and the distance between the injection and the measuring electrodes, which determines the penetration depth. The inversion of the electric data provides a section of the actual electrical resistivity as a function of depth and position.
The MAPPEM System can be used in both marine and freshwater environments.
The instrument is in a Dipole-Dipole configuration, which allows good penetration and resolution.
Penetration and resolution are adapted to the expected target, from a penetration of some meters down to about 40m in the substratum.
The whole system consists of the main fish that contains the electronic acquisition and control, a winch and cable, sensors cable and a tail equipped with navigation sensors. The total length of the towed system is from 15 to 100m depending on the sensor and injection configuration chosen. It is towed near the bottom (a few meters) depending on the bathymetry of the area. The fish (1.5m, 20 cm diameter) is easily handled on deck by 2 people and the sensor cables are deployed by hand.
Additional information on the navigation (DGPS position, compass, depth, altimeter, attitude sensors…) are also recorded simultaneously. All these data are displayed in real time on the ship and stored on computer (Figure 2). The tail is also equipped with depth, altimeter, compass & attitude sensors post-processed to refine the x,y,z position of each potential and injection electrodes. The length of the towed cable is recorded with a wireless counting sheave to recalculate the precise position of the device on the bottom.
The raw data is recalculated in terms of apparent resistivity versus the offset between the potential receivers and the current dipole (increasing distance corresponds to an increasing depth). Hence each injection provides 19 potential differences at increasing distance (hence depth) from the injection dipole. One advantage of our system is that the 19 potential dipoles are measured simultaneously using a single reference. This approach allows for the recalculation all possible configurations in terms of offsets and dipole length which contributes to improve the final resolution in the resistivity model. The injection is repeated every 0.5 s. These sections are the basic data for the inversion.
The apparent resistivity profiles are then inverted to obtain a model of the true resistivity according to the depth and position.
Every 0.5 seconds, the system injects a current (typically 40A in sea water)
There is no harm to the environment as the voltages involved are low (about 12V)
A series of induced electric potential on 20 receivers are simultaneously measured
These potentials are a function of the resistivity of the subsurface and the distance between the injection and the measuring electrodes, which determines the penetration depth
Electrode spacing is adjusted depending on the target requirements