TOWED STREAMER ELECTROMAGNETIC
Towed-streamer CSEM allows for the acquisition of high-quality marine CSEM data in a fast and efficient manner with minimal environmental impact.
The system was developed by PGS Geophysical AS and first deployed commercially in 2009 for hydrocarbon exploration. Since then, over 20,000 line km of EM data have been collected. OFG completed the acquisition of the system in July 2021.
Controlled Source Electromagnetic (CSEM) methods, which measure the electrical resistivity of the seafloor, are used to complement other geophysical approaches across a range of applications. In the past they have been applied successfully to resource evaluation in oil and gas, methane hydrate, seafloor massive sulphides and groundwater mapping. Multiphysics workflows which combine electromagnetic data with other sources of information such as seismic, well logs or gravity/magnetics, can lead to improved imaging and more robust characterisation of rock and fluid properties of the sub-surface.
The towed streamer EM system uses a high powered horizontal electric dipole source towed behind a survey vessel to transmit EM fields through the seawater column and into the seafloor. The resulting signals are recorded by an array of electric fields receivers, also towed behind the survey vessel. This configuration provides a fast and efficient way of acquiring high resolution CSEM data at significantly lower cost than other EM methods, paving the way for large area surveys in parallel with seismic programs.
Dense Source and Receiver Coverage
Both source and receiver coverage are continuous along survey lines
High resolution of sub-surface resistivity structure within the system’s depth of investigation
Efficient Data for a Fixed Budget
Data acquired three times, or more faster than possible using other offshore EM methods, allowing more data to be acquired within a fixed budget
Can be operated from a vessel of opportunity, in combination with other survey technologies
Real-time Data Monitoring
Data recorded in real time on the survey vessel to give an early indication of data & data quality
Allows problems to be rapidly resolved
Low Environmental Impact
No contact with the seafloor
Transmitted fields have little to no effect on marine wildlife in the survey area
Oil & Gas
Applications in the past have included prospect ranking to improve probability of success and assist in drill or drop decision making. Quantitative interpretation approaches that combine CSEM and seismic data in a rock physics framework have been used successfully to distinguish commercial hydrocarbon accumulations from sub-commercial fizz gas in prospect evaluation, allowing exploration efforts to be more effectively targeted. Similar systems have been used to map marine gas hydrates, both as a resource and as a potential hazard.
Offshore Groundwater Mapping
The importance of water security is recognised globally, and there is increased pressure on traditional water supplies. Offshore groundwater resources have the potential to supply very large additional reserves of fresh water, however these are at present poorly understood. Recent proof of concept surveys have demonstrated the use of EM surveys for the detection and characterisation of offshore groundwater resources. The towed streamer EM system is ideal for mapping sub-surface resistivity data across extended areas on continental shelves, and once acquired, multiphysics approaches developed for reservoir characterisation in the oil and gas industry can be used to make a quantitative estimate of new freshwater resources that may be available to address critical water shortages.
Understanding the Effects of Climate Change
Numerous studies have indicated that Arctic permafrost may be thawing in response to global changes in climate and rising sea levels. Such thawing affects the hydrogeology and ecology of the region, and is a likely source of large to huge emissions of methane, a potent greenhouse gas. Understanding the extent of the permafrost (and any associated methane hydrate), its stability and how this changes over time is therefore a key goal in understanding the effects of climate change on the Arctic region. Electromagnetic methods have proven effective at mapping permafrost and methane hydrates on continental shelves. The towed streamer EM system can rapidly map resistivity in such areas, providing accurate measurements of resistivity from which permafrost extent can be estimated.
Release of hydrocarbon or other contaminants into the environment is to be avoided at all costs. However, should the worst happen, it is important to understand the extent of the contamination and how it varies over time in order to design effective mitigation strategies and verify their effectiveness. In many settings, electrical resistivity, mapped using a towed streamer EM system, provides a non-invasive method for evaluating the spread and concentration of any contamination. As an increasing number of offshore hydrocarbon production facilities head towards a decommissioning phase, this application becomes increasingly important.
Gas Hydrate Mapping as a Resource or a Hazard
Marine gas hydrates and shallow gas can be seen as either a resource or a hazard depending on context. In both cases, surveys have demonstrated that such accumulations can be effectively mapped using EM methods. The towed streamer EM method is applicable here, allowing efficient mapping of such features over a wide area.