Geology & Geophysics Group
Controlled Source Electromagnetic Surveying
Contact: Prof. Martin Sinha
The NOC is fully equipped to carry out field research programmes in seafloor EM methods – and specifically CSEM surveys – for both academic and applied research topics. Our equipment base includes:
- The DASI-2 deep-towed electromagnetic transmitter system
- LEMUR and LC-2000EM autonomous receiver instruments
- Access to winches, full-ocean-depth tow cables, and acoustic and satellite based positioning systems which are managed at NOC by National Marine Facilities Division.
DASI is an acronym for Deep-towed Active Source Instrument. Our DASI-2 transmitter system was originally developed in the late 1990s, and has been substantially upgraded. The main elements are the deep-tow vehicle itself, which can output a current of 300 A p/p at frequencies of 0.125 to 256 Hz; the shipboard control and data logging systems; the shipboard power supply unit which provides 10 kiloWatts of conditioned, frequency stabilised, high voltage power to the deep tow vehicle; and the antenna streamer. The latter is a neutrally buoyant array, 130 m in length, which acts as a 100 m horizontal transmitting dipole. DASI typically operates 30 to 50 m above the sea bed, with a source dipole moment of about 104 Am.
Upper right: The DASI-2 deep-tow vehicle being lifted outboard during a deployment in the North Atlantic. Lower right: Going down...DASI-2 disappears beneath the surface.
For further information see DASI specifications document (MS Word, 0.5MB) .
LEMUR is an acronym for Low-frequency Electromagnetic Underwater Recorder. These instruments are deployed from surface ships and sink to the sea bed, where they record electric field data from two orthogonal horizontal dipole sensor arrays. The dipoles and amplifiers are extremely sensitive - they can measure electric fields as small as 10 picoVolts per metre (10-11 Vm-1), enabling them to detect and record signals from the DASI-2 transmitter at ranges of up to 15 km. As a result we can use CSEM survey methods to determine resistivity to a depth of up to 4 to 5 km beneath the sea bed. The LEMURs are recovered by acoustic release systems which can be instructed to jettison a ballast weight, allowing the instrument to float back to the sea surface at the end of a deployment.
Above: Launching a LEMUR receiver, North Atlantic.
For further information see Lemur specifications document (MS Word, 0.3MB)
Our LC-2000EM receivers are a newer generation of seafloor instruments. They are based around generic instrument packages and recording systems, and can be configured either for seismic use, for electromagnetic use, or with combined seismic and EM sensors. Both the LEMUR and LC-2000EM instrument systems are managed through the UK Ocean Bottom Instrument Consortium.
Right: Recovering an LC-2000EM instrument at the end of a survey.
Ancillary equipment needed for conducting CSEM surveys includes a suitable winch and fibre optic-electro-mechanical deep tow cable; high resolution surface ship navigation systems; and appropriate long-base-line or ultra-short-base-line acoustic positioning systems for determining the locations of the DASI transmitter and each receiver during survey operations. This equipment is available through the National Marine Facilities Division at the NOC.
