Joint Transient CSEM Inversion
Publication Date: 2012-Jan-25
The IP.com Prior Art Database
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Combined acquisition and interpretation of marine transient electromagnetic (TEM) and marine magnetotelluric (MMT) measurements Introduction
This method relates to the combined acquisition and interpretation of marine transient electromagnetic (TEM) and marine magnetotelluric (MMT) measurements for the enhanced detection of thin resistive formations, including oil bearing reservoirs, gas injection zones, and high quality fresh-water aquifers. The marine transient electromagnetic method includes a vertical (or nearly vertical) electromagnetic field source (positioned near the sea bottom) that generates electric currents with a sharply defined termination. The electromagnetic fields generated by these pulses are measured by at least one vertical (or nearly vertical) electric dipole receiver antenna, during the interval when the current in the transmitter antenna is switched off. The distance between the electromagnetic field source and the receiver is small compared to the depth of the target object (Figure 1). For the marine magnetotelluric method, a separate receiver is employed at or near the location of the TEM receiver to measure naturally occurring time-varying electromagnetic fields.
One of the major issues related to current marine CSEM surveying methods for detecting thin resistive targets is the depth of investigation, which currently limits the application of such techniques to subsea formations shallower than approximately 3km. Newly developed marine TEM methods measure electromagnetic signals in the absence of direct source fields and have much greater depth sensitivity compared to CSEM (~4km), but have limited vertical resolution due to small source-receiver offsets (See ). While the MMT method have the ability to sense very deep below the seabed (10's of km), it is well known that the MMT method, which measures the response of a vertically 'propagating' EM field is not sensitive to thin resistors at depth. So no single marine EM method has the necessary depth penetration and vertical resolution to solve the problem of detecting thin resistors deeper than 3km. This patent memo outlines a method for the joint acquisition and inversion of a plurality of both TEM and MMT measurements in order to better characterize subsurface resistivity and infer formation fluid content in thin resistors below 3km depth.
The marine CSEM method is well-known in the prior art for the interpretation of thin resistors to depths approaching 3km (e.g.,  and ). The MMT method (e.g., ) and TEM methods (e.g.,
 and ) are also known in the art for estimating subsurface properties. It was also shown by
 that the TEM method is sensitive to thin resistors and has a depth penetration that is greater than CSEM and as much as 4km. However, as far as we are aware, the combination of TEM and MMT is not described in the prior art (we have not performed a complete patent search on this). Here we propose the use of two separate...