During 2010, PGS conducted one of the first field trials of their towed streamer electromagnetic (EM) system at the Troll field in the Norwegian North Sea. Obviating the need for ocean bottom receivers, the towed-streamer EM system enables CSEM data to be acquired simultaneously with seismic over very large areas in frontier and mature basins for higher production rates and relatively lower cost than conventional CSEM. The towed streamer EM data are currently processed and delivered as a spectrum of frequency-domain responses. In this paper, we review the large-scale 3D inversion of towed streamer EM data using a moving sensitivity domain. We present a case study for the 3D inversion of towed streamer EM data from the Troll field, and demonstrate our ability to image the Troll West Gas Province and Troll East Gas Province.
We have developed a deterministic method for directly inverting geophysical data to 3D lithological models. This method is based upon a lithology-based model transform of the model parameters and their sensitivities from their physical property basis to one of a lithology basis. This method is general, as it can be applied to both linear and nonlinear geophysical methods, and that the physical properties defining a lithology may have a statistical distribution. We demonstrate the method with a case study for the 3D inversion of airborne electromagnetic (AEM) data for bathymetry mapping in the Backstairs Passage in South Australia, where the 3D earth model is characterized as discrete lithologies of seawater, sediment, and basement. Our results are shown to be in very good agreement with LiDAR bathymetry from the same area.
Following recent advances in superconducting quantum interference devices (SQUIDs), airborne full tensor magnetic gradiometry (FTMG) is emerging as a practical geophysical exploration method that is intended to recover information about remanent magnetization. In this paper, we introduce 3D regularized inversion of FTMG data that recovers the total magnetization vector in each cell of the 3D earth model. If a priori information about the susceptibility or remanent magnetization is available, the 3D inversion can be constrained to recover the remanent magnetization vector. If a priori information is not available, it is possible to recover attributes such as the amplitude, components, and angle of the magnetization vector relative to the inducing field. We present a case study for data acquired over a dike swarm in South Africa that compares our 3D FTMG inversion for magnetization with a 3D total magnetic intensity (TMI) inversion for a positively-constrained susceptibility distribution. Given the significant remanent magnetization present, the 3D FTMG inversion for magnetization recovers results that are most consistent with the known geology.
We present a new method of analyzing borehole-to-surface electromagnetic (BSEM) survey data based on redatuming of the observed data from receivers distributed over the surface of the earth onto virtual receivers located within the subsurface. The virtual receivers can be placed close to the target of interest, such as just above a hydrocarbon reservoir, which increases the sensitivity of the EM data to the target. The method is based on the principles of downward analytical continuation of EM fields. We use Stratton-Chu type integral transforms to calculate the EM fields at the virtual receivers. Model studies demonstrate the effectiveness of the method.
We introduce 3D Cauchy-type integrals that extend the classic theory of Cauchy integrals to 3D potential fields. In particular, we show how we are able to evaluate the gravity and gravity gradiometry responses of 3D bodies as surface integrals over arbitrary volumes that may have spatially variable densities. This entirely new method of 3D spatial-domain modeling is particularly suited to the terrain correction of airborne gravity gradiometry (AGG) data. The surface integrals are evaluated numerically on a topographically conforming grid with a resolution equal to the digital elevation model (DEM). Thus, our method directly avoids issues related to prismatic discretization of the digital elevation model, and their associated volume integration. We demonstrate this with a model study for AGG data simulated for a 1 Eö/vHz system over the Kauring test site in Western Australia.
The Airborne Magnetic Tensor (AirMt) system measures the rotational invariants of the transfer functions for audio-frequency natural sources from helicopter platforms. AirMt, data are typically measured from 30 Hz to 720 Hz, giving detection depths down to 1 km or more, depending on the terrain conductivity. Given the airborne deployment, it is possible to acquire AirMt data over large areas for a relatively low cost compared to equivalent ground surveys. This makes it a practical method for mapping large-scale geological structures. We describe the theory of the AirMt system, and present a case study comparing 3D inversion results from ZTEM and AirMt surveys flown over the Nebo-Babel Ni-Cu-PGE deposit in Western Australia. Our 3D inversion results are shown to be in good agreement with the known geology of the deposits and the surrounding area.
The generalized effective medium theory for induced polarization (GEMTIP) enables one to model and invert the complex resistivity (CR) spectra for rock and fluid parameters such as matrix resistivity, grain size, grain resistivity, fraction volumes, porosity, fluid saturations, and polarizability. Moreover, GEMTIP can explain anisotropic resistivity in terms of grain orientation. In this work, the GEMTIP model is used to invert laboratory-based complex resistivity measurements for the aforementioned rock and fluid properties. We have applied the GEMTIP model to analyze the IP phenomena in both mineralized rocks, and hydrocarbon-bearing reservoir rocks. From laboratory measurements of mineralized rock samples, we show how the mineral properties recovered from GEMTIP analyses of CR spectra can be correlated with optical microscopy, QEMScan and x-ray tomographic mineralogical analyses.