In recent years a number of marine electromagnetic approaches have been developed to address the problem of mapping offshore electrical resistivity structure. Here we consider marine controlled source electromagnetic imaging (CSEMI) in the frequency domain, a technique which has been in use for over 20 years. Recently the method has been used to provide a direct measurement of resistive layers that can be indicative of the presence hydrocarbon bearing strata in the seafloor.
Many exploration strategies today are heavily weighted towards fields that have a strong seismic expression. However, in some instances there is little or no resolvable seismic anomaly over fields which have yielded commercial discoveries (for example the Alba field in the North Sea was poorly imaged by conventional seismic methods). In contrast perhaps 10-30% of prospects supported by a strong seismic amplitude anomaly result in dry holes . One cause of this is the presence of low gas saturations in the reservoir: although seismic amplitudes are good indicators of the presence of gas in a reservoir, their ability to resolve the gas saturation is limited. It can therefore be hard to distinguish low gas saturations from economic reserves.
CSEM sounding addresses these issues by exploiting the often large contrast in resisitivty between hydrocarbon bearing strata and the surrounding more conductive sedimentary layers. In contrast to P-wave velocity, electrical resistivity varies smoothly with gas saturation. This is illustrated in figure 1, which shows the expected CSEM anomaly for a simple rock property model based on Archie's law compared to the variation in P-wave velocity. For gas saturations greater than 15%, the variation in P-wave velocity with increasing gas saturation is relatively small. The expected CSEM anomaly varies continuously across the range of gas saturations, and can therefore be used to establish whether an observed seismic amplitude anomaly is supported by a high resistivity zone, that in turn is indicative of a high gas saturation.
Figure 1: Comparison of the seismic P-wave velocity (basedon Domenico, 1974 ), and CSEM anomaly relative to awater saturated reservoir as a function of gas saturation. The CSEM anomaly is calculated from a rock property model based on Archie's law. (Available in full paper)
The CSEM method uses a horizontal electric dipole (HED) source to transmit low frequency (typically 0.01 - 10Hz) signals to an array of receivers that measure the electromagnetic field at the seafloor. By studying the variation in amplitude and phase of the received signal as the source is towed through the receiver array, the resistivity structure of the sub-surface can be determined at scales of a few tens of metres to depths of several kilometers.
Since the fields of an electric dipole are 3-dimensional in nature, the response obtained depends on the source-receiver geometry, even in the simplest 1-D earth structures. Fields of an HED source can be decomposed into two components: the radial component (parallel to the line joining source and receiver) is sensitive to the presence of thin resistive layers in the seafloor.