Fixed-Source Method: Electromagnetic Surveying, Receiver Profiles, and Subsurface Resistivity Mapping
The fixed-source method is a controlled-source electromagnetic (CSEM) acquisition technique in which the transmitting energy source is held in a single position while the receivers, detectors, or sensors are progressively moved across the survey area to construct a profile or map of the subsurface electromagnetic response. The technique is one of two major end-members in surface CSEM acquisition design, the other being the moving-source (or moving-transmitter) configuration, and each has distinct advantages for specific exploration problems. In a fixed-source survey the transmitter is typically a grounded electric dipole or a large magnetic-loop coil energized at a defined frequency or with a precisely timed pulse waveform, and the receivers measure the secondary electromagnetic field at a series of stations that progressively map the response of the subsurface as a function of distance and azimuth from the source. The method exploits the principle that subsurface resistivity contrasts (most importantly between conductive saline pore water and resistive hydrocarbons or gas hydrates) generate measurable electromagnetic anomalies that can be mapped quantitatively when source position, source moment, and receiver geometry are all known. In Western Canadian Sedimentary Basin and offshore Canadian East Coast exploration contexts, fixed-source CSEM is most commonly applied as a derisking complement to seismic data in frontier basins or in mature plays where seismic alone cannot distinguish between brine-filled and hydrocarbon-filled porosity. CSEM acquisition costs in offshore Newfoundland Grand Banks or Flemish Pass campaigns under CNLOPB jurisdiction typically run $1.2 to $2.5 million CAD per survey square kilometre depending on water depth, receiver spacing, and transmitter dwell time. Onshore the technique is used less commonly for hydrocarbon mapping but is widely deployed in groundwater mapping for SAGD operations targeting the Cretaceous Mannville aquifer or the deeper Devonian saline aquifers used for produced-water disposal. The dual unit convention applies to source-receiver offsets reported in metres alongside feet, and source frequencies are reported in hertz with typical CSEM bandwidths spanning 0.1 to 10 Hz for marine deep-target imaging. Survey design must consider source-to-receiver geometry, near-field versus far-field response distinctions, and corrections for source-transmitter coupling at close offsets that can saturate the receivers. The fixed-source approach simplifies the data processing chain because the source position contributes a single known quantity to every receiver measurement, eliminating the moving-source uncertainties that complicate position fixes and timing synchronization in moving-platform CSEM surveys.
Key Takeaways
- Acquisition geometry: A grounded electric dipole or magnetic loop transmitter is anchored at one location; receivers are deployed at a grid or profile of stations and either record simultaneously (multi-channel acquisition) or are moved sequentially through the survey footprint. Source-receiver offsets typically range from 100 metres (330 ft) for shallow targets up to 12,000 metres (39,370 ft) for deep marine prospects. Each receiver records electric and magnetic field components.
- Frequency selection: Survey frequency determines depth of investigation through the electromagnetic skin-depth relationship: skin depth equals approximately 503 times the square root of resistivity divided by frequency. At 1 Hz in a 1 ohm-metre marine sediment, skin depth is roughly 503 metres (1,650 ft); reducing frequency to 0.1 Hz extends investigation to about 1,590 metres (5,215 ft). CSEM surveys use multi-frequency transmissions to image multiple depth ranges in one acquisition.
- Resistivity contrast detection: The technique excels where target zones have high resistivity contrast against surrounding rock. Hydrocarbon-saturated reservoirs at 30 to 300 ohm-metres set against brine-saturated host rocks at 0.5 to 2 ohm-metres produce strong anomalies; gas hydrates against marine sediments produce equally robust signatures. Resistivity contrasts below roughly a 2:1 ratio are generally below detection threshold for offshore CSEM at realistic source-receiver offsets.
- Cost and crew structure: Marine CSEM with fixed-source acquisition under CNLOPB or CNSOPB jurisdiction typically uses a single transmitter vessel and 20 to 40 ocean-bottom receivers per deployment. Day rates for the full acquisition crew including vessel, transmitter, and receivers run $80,000 to $150,000 CAD per day, with full surveys lasting 14 to 30 days. Land CSEM with fixed source uses simpler crews and runs at $25,000 to $50,000 CAD per day with smaller receiver arrays.
- Processing advantages: Because the source is stationary, source-position uncertainty drops out of the inversion. This simplifies the forward modelling problem and reduces ambiguity in the recovered resistivity model. Joint inversion with seismic data is straightforward because both data types share known geometry. Workflows commonly use commercial codes like EMGS Mainframe, Geosystem CSEM, or open-source frameworks such as SimPEG and emg3d.
Marine Versus Land Application Differences
Marine CSEM with a fixed source typically uses a horizontal electric dipole transmitter towed 30 to 50 metres above the seafloor while ocean-bottom receivers sit on the seabed measuring the resulting electromagnetic field. The seawater layer provides a conductive return path that makes the technique particularly sensitive to resistive hydrocarbon targets. On land in WCSB or Saskatchewan groundwater surveys, the source is a grounded electric dipole 500 to 2,000 metres long, and receivers are placed on the surface with electric and magnetic field sensors. The land setting has higher ambient electromagnetic noise from power lines, pipelines, and railway grounding currents, requiring more sophisticated noise rejection and longer stacking times to achieve usable signal-to-noise.
Comparison to Moving-Source Configurations
The principal alternative is the moving-source method, in which the transmitter is towed continuously while receivers are stationary, producing a dense grid of source positions for each receiver. Moving-source surveys generate higher-resolution lateral resistivity maps at the cost of more complex source-position bookkeeping and more demanding GPS, INS, and timing synchronization. Fixed-source methods are preferred for deep-target mapping where source dwell time matters more than lateral density, and where the survey is operating at the limits of receiver sensitivity. The choice between fixed and moving source depends on target depth, lateral extent, budget, and the resistivity contrast expected from the geological model.
Fast Facts
The first commercial marine CSEM survey using a fixed-source configuration was acquired in 2000 by ExxonMobil over the Girassol field offshore Angola, where the technique successfully discriminated hydrocarbon-filled from brine-filled reservoirs in a previously drilled play. By 2010 CSEM had become a standard frontier-exploration derisking tool for major operators including Statoil (now Equinor), Shell, and ExxonMobil. The Canadian first commercial CSEM survey was acquired over the Mizzen prospect in the Flemish Pass under CNLOPB jurisdiction in 2011.
Related Terms
The fixed-source method is one acquisition variant within the broader discipline of electromagnetic survey techniques, which also include magnetotelluric (MT) methods that use natural-source signals from solar and atmospheric activity. The data interpretation step relies on resistivity contrasts in the subsurface, the same fundamental quantity measured during well-log acquisition by induction log tools. CSEM data is commonly integrated with surface seismic survey data through joint inversion to produce a quantitatively consistent earth model that reduces exploration risk in frontier and mature basins alike.
Real-World Scenario: Flemish Pass Frontier Survey
In 2019 a major operator conducted a fixed-source marine CSEM survey over a 220 square kilometre area in the Flemish Pass offshore Newfoundland under CNLOPB exploration licence. The survey was designed to derisk three Cretaceous-Jurassic sandstone prospects identified on prior 3D seismic but flagged as uncertain on direct hydrocarbon indicator analysis. The transmitter operated at 0.25 and 1.0 Hz with a 270-metre horizontal electric dipole, and 32 ocean-bottom receivers were deployed on a 3-kilometre triangular grid. Total acquisition time was 18 days at vessel day rate of $145,000 CAD plus receiver mobilization for a total survey cost of $3.1 million CAD.
Processing and joint inversion with the pre-existing 3D seismic identified one of three prospects as showing a strong resistive anomaly consistent with hydrocarbons; the other two showed flat resistivity consistent with wet sands. The operator subsequently drilled only the CSEM-confirmed prospect, avoiding an estimated $90 million CAD in unsuccessful drilling cost across the two derisked prospects that would otherwise have been tested.