The receivers are capable of measuring field strengths that vary greatly in magnitude, from weak naturally occurring magnetotelluric signals to strong direct signals from the source. Measuring the EM phase with respect to the source requires precise measurement of the timing of the signal.
For the receivers to be accurately positioned on the seabed, they have been designed to descend rapidly when dropped from the survey vessel, and, if large surveys are to be performed, to remain autonomously operational for several weeks.
See also:
Main features
Comprehensive field measurement
EMGS's receivers have four electric (E) and two magnetic (H) channels that record vertical and horizontal field components. Comprehensive measurement of the electric and magnetic fields enables successful EM surveys to be performed in shallow water.
Large dynamic range
EMGS' data loggers have 24-bit analogue-to-digital converters and automatic gain control, which enables high-resolution data to be recorded at all source-receiver offsets without signal saturation.
Low noise
Ridged 4-m arms hold each electrical sensor at a fixed distance from the receiver body. This gives a constant and large (8-m) separation between sensor pairs, which ensures a high signal-to-noise ratio. The arms are flexible in the vertical direction but stiff in the horizontal direction to minimise drag when the receivers are moving through the water column.
High sensitivity
EMGS' receivers can detect weak electric and magnetic fields, which makes them suitable for CSEM and magnetotelluric surveys.
Synchronisation
The timing of received signals can be synchronised accurately to the source waveform using GPS time-reference signals backed up by onboard clocks.
Dissolvable concrete anchors
EMGS' receivers use concrete anchors to provide rapid sink rates of about 1 m/s. This helps to achieve accurate receiver placement and reduces survey times. The anchors also provide a stable noise-limiting platform. The anchors remain on the seabed after the receivers are retrieved. EMGS uses patented soluble cement that reduces the concrete to disaggregated sand within six to nine months to ensure no seabed hazards remain after this time.
Long seafloor life
EMGS' receivers have proprietary electronics that combine low noise levels with low power consumption for high sensitivity and a long seabed life. The battery endurance and large memory capacity enable large EM imaging surveys to be deployed and conducted. The long seabed life also gives EMGS the ability to perform magnetotelluric surveys (passive measurements of the earth's magnetic field).
