| A finite-element numerical modelling computer program is used to calculate the time-domain electric and magnetic field responses to several petroleum-relevant 2-D conductivity models. These models include single or multiple layers, faults, stratigraphic changes, and near-surface resistive layers. The EM system consists of a horizontal electric source-dipole and inline horizontal electric and magnetic field receivers oriented perpendicular to the source-dipole. The finite element program solves an EM boundary value problem based on the governing Maxwell;s equations in the Laplace s, and wavenumber q domains. The fields are transformed back to the time and space-domains via an inverse Laplace transform and cosine and sine transforms respectively. Response curves for the fields are analyzed to determine the sensitivity of the system.;Generally, the modelling result show that the CSEM system modelled is more sensitive to conductivity structures with large vertical extent than to thin, layered structures. The results also show a greater sensitivity to near-surface structures than to deep ones. In particular, the CSEM system is capable of resolving whether a discontinuity in conductivity is present across a subsurface fault. In contrast, relatively small changes in conductivity due to changes in stratigraphy appear to have virtually no affect on the EM response. The most promising target in terms of hydrocarbon exploration appears to be near-surface resistive zones, which have a large EM response, making the determination of the location and size of the zone possible. A resistive layer can be created by hydrocarbon induced alteration or gas hydrates. To detect such a resistive zone, a source dipole moment of 10... |
