Efficient acquisition of controlled source electromagnetic data using towed streamer technology
CHALLENGE
There are numerous situations where ambiguities in the interpretation of sub-surface structure or properties can be resolved by combining different geophysical data types. In recent years the marine controlled source electromagnetic (CSEM) method has proven a valuable complement to seismic and other methods. Multiphysics workflows lead to improved imaging of structure in geologically complex areas (for example around basalt or salt) and more robust characterisation of rock and fluid properties of the sub-surface.
Traditional CSEM methods use a deep towed electromagnetic source to transmit signals through the seafloor to an array of receivers (typically 50-100), which are deployed on the seafloor. The requirement to deploy and recover arrays of receivers, along with the slow tow speeds necessitated by the deep-towed source, mean that such surveys can be slow, limiting the amount of data that can be acquired. Deployment of receivers on the seafloor may also disturb sensitive benthic environments, and leave traces of bottom weight material.
SOLUTION
The towed streamer CSEM method provides an alternative approach to the acquisition of CSEM data in water depths up to about 500m. 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.
The method 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 acquired this system in June 2021, and offer a complete suite of pre-survey feasibility analysis, acquisition, processing and interpretation services, including proprietary multiphysics approaches to integrate EM and seismic data.
The source is housed in a 20’ container, and takes 360 kW 3-phase as input from the vessel. The output is a GPS synchronised switched DC waveform, which is tailored to offer a frequency content that optimises sensitivity to the targets of interests. The output waveform is logged continuously at a sample rate of 128Hz. The signal is transmitted into the water via two metal electrodes, towed 400-800m apart behind the vessel, and positioned using RGPS in surface buoys. Emergency stop and lockout systems, and over voltage and current protection ensure that the system operates safely.
The receiver array comprises 150m long streamer segments. Each segment houses up to four Ag-AgCl electrodes, which can be configured to form receiver dipoles that vary from about 200m close to the source, up to 1km at the longest offsets to ensure that signal-to-noise ratio is maintained across the lengths of the array. Electric field data is sampled at 128Hz in the streamer receiver units, and telemetered in real time to the vessel, allowing online QC and analysis during acquisition. The position and orientation of the receiver array are monitored continuously through USBL acoustics on the front and compass birds along the length of the streamer.
RESULTS
The towed streamer CSEM system offers several advantages:
Dense source and receiver coverage: Both source and receiver coverage are continuous along survey lines, allowing high resolution of sub-surface resistivity structure within the depth of investigation of the system.
Real time data monitoring: Data are recorded in real time on the survey vessel to give an early indication of data quality and content. This allows problems to be rapidly identified and resolved.
Efficient data acquisition – more data for your budget: CSEM data are acquired three times for more faster using the towed streamer system compared to conventional CSEM approaches, allowing more coverage in a fixed budget and paving the way for large area surveys in parallel with seismic.
Minimal environmental impact: There is no seafloor contact during a towed streamer CSEM survey, safeguarding the sensitive benthic environment. Studies have shown that the transmitted EM fields are harmless to wildlife.
Flexible operations: The system can be operated from a vessel of opportunity, in combination with other survey technologies.
PUBLICATIONS
Buchanan, R.A., R. Fechhelm, P. Abgrall, and A.L. Lang. 2011. Environmental Impact Assessment of Electromagnetic Techniques Used for Oil & Gas Exploration & Production. LGL Rep. SA1084. Rep. by LGL Limited, St. John’s, NL, for International Association of Geophysical Contractors, Houston, Texas. 132 p. + app.
Engelmark, F., Mattsson, J., McKay, A. & Du, Z., 2014. Towed streamer EM comes of age, First Break, 32, 75-78
