Marine Time Domain CSEM: an Emerging Technology
- Kurt Strack (KMS Technologies) | Norman Allegar (KMS Technologies) | Svein Ellingsrud (Electromagnetic Geoservices)
- Document ID
- Society of Exploration Geophysicists
- 2008 SEG Annual Meeting, 9-14 November, Las Vegas, Nevada
- Publication Date
- Document Type
- Conference Paper
- 2008. Society of Exploration Geophysicists
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- 38 since 2007
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Recent advances in electronics now enable time-domain, or transient CSEM data, to be reliably acquired in an offshore environment and make the leap from its dominant position in land EM to the marine world. Multiple surveys using autonomous receiver nodes have successfully acquired marine time domain CSEM data. The approach has shown particular benefit in shallow water since the method records only the secondary fields and the signal is not dominated by the primary field.
For the past 40 years, the seismic method has been the geophysical workhorse of the oil industry. While it offers the best description of the reservoir shape and stratigraphy, it falls short on describing the fluid properties of the pore space as elastic waves predominantly travel through the rock matrix. In particular, many of the changes that take place during the production life of a reservoir do not exhibit a detectable acoustic property change. Recently, marine controlled-source electromagnetic (CSEM) methods have found hydrocarbons after their response to thin resistors was understood (Eidesmo et al. 2002). The use of marine CSEM has gained momentum, and has become one of the most significant technology developments in oil exploration since the advent of 3D seismic.
Among the CSEM methods, we use a time domain (tCSEMT) version which employs time variant magnetic fields of either natural or artificial origin that cause eddy urrents within the conductive sediment layers (Strack 1992, 1999). These eddy currents are time variant as well and they cause a secondary EM-field that can be sensed with magnetic or electric sensors placed on the sea floor or in the wellbore. High resistivity lithologies and pore fluids are the resistors that alter this artificial electric field. Today, most service providers of CSEM technology transmit a frequency-targeted source into the Earth, often this is referred to as frequency domain CSEM or fCSEM. This source is almost always a continuous square-wave and both the active source and the subsurface response are recorded at nodes distributed along the seafloor. As a consequence, the much larger primary field often swamps the weaker Earth response (secondary field), particularly at short source receiver offsets and in water depths of less than 200 m. This shallow water affect is commonly called the airwave phenomena.
With recent advances in electronics, time-domain, or transient CSEM data can now be reliably acquired in an offshore environment and make the leap from its dominant position in land EM to the marine world. With timedomain CSEM one transmits a current into the Earth and charges the subsurface. The current is then switched off and the charge drains from the Earth. Transient responses to this artificial electric field are then measured by sensors that record both the electric and magnetic components. Because the time domain method is only measuring the secondary field it offers a solution to the shallow water limitations that confront frequency-targeted techniques(Weiss 2007, Avdeeva et. al. 2007). The duration of these “on” and “off” times of the source are optimized to eachparticular problem. Additionally, every current switch represents the initiation time, or time zero, for a given transient.
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