The SIO seafloor magnetotelluric recorder consists of the L-CHEAPO logger
and acoustic system equiped with custom electric field preamplifiers, low-noise
electrodes designed for seafloor use, and commercial broad-band magnetic
sensors in custom underwater housings. Together, these provide a full 4-component
MT sounding system capable of recording between 50 Hz and 10,000 seconds
(somewhat dependent on water depth).
The logger pressure case is supported in a polyethylene framework which
protects the instrument from damage during handling and supports five glass
flotation spheres, acoustic release package, two magnetometer coils, four
5 m electrode arms, and a concrete anchor. A magnetic compass, equipped
with a timed release to lock the needle mechanically, records the orientation
of the system after deployment on the seafloor. The acoustic release unit
serves both to locate the instrument underwater and to release the package
from the seafloor at the end of the recording period.
Electric field (telluric) sensors - The electric field sensors are grounded
dipole antennas. Two orthogonal 10 metre dipoles are used in order to measure
the complete vector electric field. The dipoles consist of insulated copper
wire installed within semi-rigid plastic pipe affixed to the deployment
package, installed in an `X' configuration. The grounds at the dipole ends
are provided by silver-silver chloride electrolyte-filled electrodes specifically
designed for the offshore environment by SIO.
Magnetic field sensors - The magnetic field is sensed using multi-turn mu-metal
core coils identical to those used for land MT surveys (specifically BF-4
sensors manufactured by EMI). For the marine survey the magnetic field coils
were encased in aluminum pressure cases roughly 6 cm in diameter and 1.3
meters long. The coils include integral broad-band low-noise preamplifiers,
powered by an external battery installed in the logger package and connected
to the coils by short high pressure waterproof cables.
Postamplifiers - Output from the magnetometers is input to a 2-channel AC-coupled,
40 dB postamplifier of relatively simple design. Gain setting is by means
of jumpers on the board, with x1 to x100 available. The long-period -3 dB
point is at 2500 s. Alias filters are switched by replacing an integrated
16-pin DIP module.
Each pair of electric field sensors is connected to a more sophisticated
amplifier developed specially for the marine environment by SIO. This amplifier,
also AC coupled, has low noise, low input impedance, and very high gain
(120 dB). The -3 dB point of 0.1 Hz is designed to match the response of
the BF-4 coils, and because of the very red nature of the electric field
spectrum allows signals at periods of 2000~s and longer to be monitored.
These instruments have been deployed in the Gulf of Mexico and the Mediterranean
on both academic and commercial surveys designed to assist in the search
for oil and gas. See Seafloor Electromagnetic
Methods Consortium