ELECTROMAGNETIC RECEIVER FOR MARINE GEOPHYSICALEXPLORATION

living resources [14].

M1Sessions

J-F. D’Eu (1), P. Tarits (1)INSTRUMENTATION VIEWPOINT

8

Laboratoire Domaines Océaniques UMR 6538
IUEM-OSU
Technopôle Brest-Iroiseplace nicolas Copernic
29290 Plouzané.
+33 298498730
deu@univ-brest.fr
1. Introduction
Electromagnetic techniques for marine geophysical exploration haveconsiderably improved in the last decade. Both Research and Industry benefices of these new data to better understand the structureof the upper crust or to refine the geological setting of natural resources. Both short period magnetotellurics (SPMT) and controlled
sources techniques (CSEM) are now increasingly used in offshorestudies. While it may be sufficient for CSEM to measuring the vector
electric field only, the combination with the vector magnetic field isof great interest. Simultaneous electric and magnetic fields are necessary for SPMT.
A great number of geological targets in marine exploration are below
several hundred of metres up to a few kilometres of seawater. Additionally, the frequent very high conductivity of the marine substratum implies that EM techniques must be used at very low frequencies(10 Hz or much less). The magnetic sensors generally used in marine
environment to record data at these frequencies are search coils.
While these sensors have a remarkable sensitivity at high frequencies(above several 10 Hz), their performance drops rapidly toward low
frequencies. Furthermore, their bulk and weight make them difficult
to adapt for marine experiments and make them very sensitive tomotion on the sea bottom.

This receiver was originally designed for SPMT but is also fully suitable for CSEM measurements, adding the magnetic information to
the recorded signal. The instrument has been designed and recently
greatly improved to be small and light, easy to deploy and unexpensive to built, with no compromise on performances. Using glass
spheres both as electronics and sensor housing, we managed to geta instrument of only 70kg in air, with all maintenance and configuration done from outside with no need to open the instrument. Energy is provided by rechargeable batteries. Using glass as instrument
housing and buoyancy also has no effect on electric nor magnetic
measurements. The instrument is deployed on the seafloor, with aconcrete anchor. An acoustic release and its natural buoyancy is used
to recover the instrument after a few days of measurements (with rechargeable batteries).
3. Performances
Sensors capabilities, integration and management in the system are
key issues for the overall performances of the instrument. We paid
particular attention to reduce and avoid noise electronics can induceon the nearby sensors.
The figure 2 shows a sample of seafloor magnetic and electric signals
recorded with our system. As the signals recorded can be very small,noise level caracterisation is of great importance.

Fluxgate magnetometers are different sensors that may be used formarine exploration. While their sensitivity at high frequencies cannot
compete with search coils and their power consumption is higher,
the new generation coming from space research is increasingly improving and becomes very reliable up to a few 10 Hz. These sensors
are very small (a few 10 to 100 g) and hence easily packaged for marine environment. The full vector magnetic field is readily measured.
2. Instrument description
We developed a new patented marine electromagnetic receivermeasuring 5 (and optionally 6) components of the electromagnetic
field (2 Electric and 3 Magnetic components) (Figure 1). The magnetic
field is measured with a 3-component fluxgate magnetometer. The
electric field is obtained from the potential differences between either Pb-PbCl2 or Ag-AgCl impolarisable electrodes, placed at the endof 5m-long tubes.
The current version of our system samples the EM field at about 30Hz. The fluxgate magnetometer gives the instrument orientation,with the help of precise tiltmeters.

The fluxgate magnetometer has a factory noise level of 20pT/√Hz at1Hz. The main field can be compensated to increase the sensor dynamics. Electric field is sample via a 24-bit sigma delta converter, withDC-coupling of the electrodes. The preamplifier is rated at 6nV//√Hz
at 0.1Hz, with no 1/f noise. This allows to get non distorded data at
the lowest frequencies.
We shall present in more details the noise levels and performances
measured on the seafloor and in our test facilities, to show the greatinterest to use such sensors and techniques for electromagneticmeasurements.
4. Conclusions
We have developed a reliable new electromagnetic receiver, suitable
for magnetotellurics and controlled source electromagnetic soundings, using 3-components fluxgate magnetometer and 2 (optionally3) electric dipoles.

Figure 1 Marine EM receiver