Moving-Source Method

The moving-source method is a geophysical survey technique in which the energy transmitter and the measurement receivers travel together as a unit, maintaining a fixed spacing relative to each other throughout the survey. As the equipment moves, the source continually illuminates the rock beneath it while the receivers, riding at a fixed offset, record the response. The approach is most common in electromagnetic surveys, including marine controlled-source electromagnetics (CSEM), where the relationship between the source and the receivers must stay consistent to make measurements comparable from one point to the next across the survey area.

Key Takeaways

In the moving-source method, the source and receivers are physically linked (or electronically synchronized at a fixed offset) and move together across the survey area. The source-to-receiver offset remains constant throughout acquisition.
This contrasts with fixed-receiver methods where ocean-bottom nodes or land electrodes stay in place while a source moves across them. In marine CSEM, both approaches are used: moving-source towed systems and fixed-receiver node systems with a towed source.
Moving-source acquisition provides uniform lateral coverage and simplifies data processing because the source-receiver geometry is identical at every measurement location. It avoids the offset-distance variability that complicates interpretation in some fixed-receiver setups.
The technique is sensitive to variations in subsurface resistivity. Hydrocarbons (particularly oil and gas) are electrically resistive compared to brine-saturated rock, so a resistive anomaly detected by the moving-source EM system can indicate a potential hydrocarbon accumulation.
Moving-source methods are also used in land seismic acquisition (vibroseis or explosive source systems driven across a survey grid) and in airborne geophysical surveys where the source is mounted on the aircraft alongside receiver coils.

What Is the Moving-Source Method?

Imagine you are scanning a document with a hand scanner. You slide the scanner across the page and it captures an image as it goes. The sensor in the scanner is always at the same position relative to the light source inside the scanner. That fixed source-to-sensor relationship is exactly what the moving-source method maintains in a geophysical survey.

In marine CSEM, the most technically demanding application, a horizontal electric dipole transmitter is towed 20 to 50 metres above the seafloor. Towed behind it at fixed offsets are electric field receivers. As the ship moves at about 1 to 2 knots, the transmitter emits a low-frequency alternating current signal (typically 0.25 to 4 Hz) that propagates through the seawater and into the seafloor sediments. The receivers measure the amplitude and phase of the signal after it has traveled through the subsurface.

If the subsurface is uniformly resistive or uniformly conductive, the measured signal changes smoothly and predictably with distance. If a resistive body (such as a hydrocarbon-filled reservoir) is present, the signal amplitude at certain offsets is higher than expected because the resistive layer traps the electromagnetic energy and guides it laterally before it re-emerges at the seafloor. This resistive anomaly is the CSEM signal that interpreters map to assess potential hydrocarbon saturation.

Fast Facts

Marine CSEM was first used for commercial hydrocarbon exploration in 2002 when Statoil (now Equinor) and ExxonMobil contracted Scripps Institution of Oceanography to survey a prospect on the Girassol field offshore Angola. The survey correctly identified the resistive hydrocarbon reservoir and distinguished it from a brine-saturated lookalike structure nearby. Since then, CSEM has become a standard risk-reduction tool in deepwater exploration, particularly on the Norwegian Continental Shelf, the Gulf of Guinea, and offshore Brazil. Norwegian service company EMGS has operated extensive towed-source CSEM surveys on the NCS.

Moving Source vs. Fixed Receiver Acquisition

The alternative to the moving-source method is to deploy a grid of fixed receivers on the seafloor (ocean-bottom nodes or OBN), then tow only the source across them. Each receiver records signals from the source at many different offsets and azimuths as the source crosses overhead. This provides richer azimuthal sampling of the subsurface but requires significantly more time and cost to deploy, retrieve, and process the receiver nodes.

The moving-source towed system is faster and cheaper to acquire. A towed CSEM vessel can cover 50 to 100 kilometres of inline survey track per day. Node-based systems cover much less ground but provide superior 3D spatial sampling. The choice depends on survey scale, water depth, and the geological complexity of the target.

In land electromagnetic surveys, the moving-source method includes transient EM (TEM) systems where a transmitter loop is laid on the ground, a pulse is sent, and the receivers (in the same frame) measure the decay of the induced current in the ground. As the frame moves from station to station, the geometry remains constant. This technique is used in mineral exploration, groundwater mapping, and to a lesser extent in petroleum exploration in areas like the Alberta Basin and the Cooper Basin in Australia.

Applications in Hydrocarbon Exploration

Marine CSEM with a towed moving source has been used on every major deepwater margin. The Norwegian Continental Shelf, where EMGS and CGG Electromagnetic have conducted multi-client surveys, uses CSEM data to rank prospects before drilling. A positive CSEM anomaly (a resistive body at the right depth and geometry) reduces the probability of a dry hole. A negative result on a seismically defined structure shifts capital to the next prospect on the list.

Offshore Australia, the North West Shelf and Carnarvon Basin operators have used CSEM to evaluate carbonate and clastic reservoir targets. In deepwater Gulf of Mexico, the method has helped distinguish between salt-related resistivity effects and genuine reservoir resistivity, which look similar on seismic alone.

The moving-source method is also called a towed-source method in marine contexts and a continuous-profile EM method in land surveys. Related terms include controlled-source electromagnetics (CSEM, a marine geophysical technique that uses a towed electric dipole transmitter to measure the electromagnetic response of subsurface rock; sensitive to resistivity contrasts and used to detect hydrocarbon-filled reservoirs), resistivity (the property of rock that describes how strongly it resists the flow of electric current; brine-saturated rock has low resistivity; hydrocarbon-filled rock has high resistivity), ocean-bottom node (a self-contained seismic and electromagnetic recorder deployed on the seafloor; used in fixed-receiver acquisition where a moving source traverses above them), transient electromagnetics (TEM, a ground-based EM method that measures the decay of an induced current in the ground after the transmitter is switched off; used for mineral exploration, groundwater, and some petroleum applications), and airborne electromagnetic (AEM, an EM survey method flown on a helicopter or fixed-wing aircraft; the source and receiver coils are mounted in the aircraft in a fixed moving-source geometry, providing rapid coverage of large land areas).

How a Moving-Source CSEM Survey Prevented a 300-Million-Dollar Dry Hole

An exploration team working a deepwater block on the Norwegian Continental Shelf had a seismically bright four-way structural closure at 2,400 metres below the seafloor. The structure was large enough to hold a meaningful volume if gas-saturated. Before committing to a NOK 2.5-billion exploration well, the operator contracted a CSEM survey with a towed transmitter system.

The CSEM data showed a strong resistive anomaly at the expected depth over the crest of the structure, consistent with a hydrocarbon-filled reservoir. The anomaly terminated abruptly on the eastern flank at a depth that coincided with the mapped gas-water contact from the seismic amplitude response. The resistive anomaly's lateral extent matched the structure's closure area almost exactly.

The well was drilled and encountered a gas column consistent with the CSEM anomaly's dimensions. The field was developed. A separate structure on the same block that showed no CSEM anomaly was not drilled, saving an estimated exploration well cost of NOK 2.3 billion. The moving-source method, by keeping the source and receiver geometry constant across the entire survey, allowed the interpreters to compare resistivity measurements at every point on a consistent basis. That comparison is what made the anomaly visible against the background.