CSEM
Controlled-Source Electromagnetics
A powerful transmitter dipole injects a low-frequency current while receivers along a line record the diffused electric and magnetic fields. Resistive bodies such as hydrocarbon or salt distort the field in a way that is measurable tens of kilometres away.
Crew
Party chief, 2 EM engineers, navigator, vessel or heavy-vehicle crew
Mobilisation
5-7 days including deck layout, dipole assembly and receiver drops
Daily coverage
10-18 line-km of towed acquisition per 24 h
Equipment and specifications
| Transmitter | Horizontal electric dipole, 100-300 m, 1,000-1,500 A peak, 0.05-10 Hz square wave |
|---|---|
| Power plant | 150-400 kW generator with transformer / rectifier and current monitor |
| Receivers | Seafloor or ground nodes, orthogonal E-field arms plus 2-3 axis induction coils |
| Navigation | Acoustic ranging or RTK GNSS, 1-5 m node positioning |
| Timing | Rubidium or GNSS-locked clocks, sub-millisecond drift over the survey |
Setup sequence
- 01
Design the line
Lay the tow line across strike with maximum offsets 3-4x the target depth.
- 02
Deploy receivers
Drop or plant nodes at planned offsets, log positions and clock offsets for every unit.
- 03
Rig the dipole
Check electrode continuity and insulation resistance before energising; verify current waveform on a dummy load.
- 04
Calibrate
Record a background segment away from the target for normalisation.
- 05
Acquire
Tow at 1-2 knots (or move the land dipole in fixed steps), holding transmit current within 2%.
- 06
Recover and merge
Recover nodes, merge time series against the transmit log, and stack per frequency.
Safety
High-current electrodes
Lock-out / tag-out on the transmitter, no wet handling while energised, insulated gloves.
Deck and winch operations
Exclusion zones, hard hats, no hands inside bight of line.
Man overboard / immersion
PFDs, harnesses, buddy system on deck at night.
Generator exhaust and fuel
Ventilated housing, CO monitors, bunded fuel storage.
Field vs lab
| Aspect | In the simulator | In the field |
|---|---|---|
| Signal-to-noise | Clean synthetic response; noise added only if you ask for it. | Cultural noise, wind and instrument drift set the usable dynamic range. |
| Airwave | Optional term you can switch on in the model. | Dominates shallow water at long offsets and must be modelled or suppressed. |
| Transmit current | Exactly the value on the slider. | Drifts with electrode contact and sea state; logged and used to normalise. |
| Geometry | Perfect inline dipole geometry. | Feather, tilt and node attitude must be measured and corrected. |
| Repeatability | Identical parameters return an identical curve every time. | Coupling, moisture and temperature move the response between repeats. |
Field QC checklist
- · Transmit current log within 2% of nominal for every stack
- · Clock drift below 1 ms per node over the deployment
- · Repeat line within 5% amplitude on the reference offsets