The vertical resolution of inverted CSEM data remains an important issue for its optimal exploitation. In this paper we use a modified simulated annealing (SA) algorithm to remap the inverted CSEM data, in the form of resistivity profiles in depth, into the seismic resolution. Our modification of the SA algorithm solves not only for a best-fitting model, but also for parameters describing the probability density functions for the resistivity in each layer. This allows us to monitor the uncertainty of our resistivity estimates within the layers. In fact we find that the expected value of the PDF shows better agreement with the well log than the best-fit model, although the latter has a lower misfit when compared with the surface EM data. In application to the Nuggets-1 field, we find that the method correctly locates the anomaly associated with the gas sand. The seismic layer is thicker than the gas sand as determined from the well logs, and therefore our estimate of resistivity remains lower than the well log values.
It is widely recognised that the vertical resolution of CSEM data inverted to resistivity with depth is poorer than that of seismic data, due to the diffusive nature of EM energy propagation. In order to investigate rock and fluid properties at seismic resolution, we would like to sharpen the resistivity image before combining it with the seismic information. Separating the resolution enhancement from the process of combination allows us to calibrate the rock physics models used in the latter, as described by Harris and MacGregor (2006). Our starting point is a resistivity profile in depth, inverted from the recorded CSEM data using the methods of MacGregor et al (2006), and a set of seismic events. These may be picked automatically, by interpretation, or by a combination of both. In this study we used an automatic technique for event identification from seismic attributes developed by M.T.Taner (pers. comm.). The resistivities are then mapped into the seismically-derived layers. This process is justified by the observation that the product of thickness and resistivity is well-resolved by the CSEM inversion (MacGregor et al, 2006) even though the individual factors of thickness and resistivity are not. To deal with the high degree of non-uniqueness in this process, we use a modified simulated annealing algorithm to perform the remapping. The result of the remapping is a probability density function (PDF) for the resistivity within each layer, allowing us to investigate the uncertainties in the result. In theory this method does not require any well data. However, in practice the seismic events must be mapped from time to depth and this has to be verified by a well-tie. In addition, the resistivity inversion may require depth constraints in order to fix the absolute depths at which anomalies occur. Thus in practice well information is a requirement, although our remapping does not use it explicitly.
The simulated annealing (SA) algorithm is described in many publications, for example Kirkpatrick et al (1983).
