Glenn A. Wilson

Spatial and temporal variation in fish-assemblage structure in isolated waterholes during the 2001 dry season of an arid-zone floodplain river, Cooper Creek, Australia

Spatial and temporal variation in fish-assemblage structure within isolated waterholes on the floodplains of Cooper Creek, Australia, was studied during the 2001 dry season, a period of natural drought in this arid-zone river. Spatial variation in fish-assemblage structure and the abundance of five species in disconnected waterholes early in the dry season (April 2001) were related to the extent of floodplain inundation 14 months previously, and to the interconnectedness of waterholes and waterhole habitat structure. As the dry season progressed, waterhole volumes decreased owing to evaporative water loss and structural habitat elements (anabranches, bars, boulders) became exposed. Marked changes in fish assemblage structure between the early (April) and late (September) dry season were related to habitat loss but not to water chemistry. Interactions between flow and habitat across a nested hierarchy of spatial scales (the floodplain, the waterhole and habitat patches within waterholes) were crucial to the persistence of fish assemblages through the 2001 dry season. We conclude that the magnitude, timing and frequency of floodplain inundation and natural variations in waterhole volume must be maintained if we wish to sustain the distinctive habitats and fish assemblages of this arid-zone floodplain river.

3D inversion of airborne electromagnetic data using a moving footprint

It is often argued that 3D inversion of entire airborne electromagnetic (AEM) surveys is impractical, and that 1D methods provide the only viable option for quantitative interpretation. However, real geological formations are 3D by nature and 3D inversion is required to produce accurate images of the subsurface. To that end, we show that it is practical to invert entire AEM surveys to 3D conductivity models with hundreds of thousands if not millions of elements. The key to solving a 3D AEM inversion problem is the application of a moving footprint approach. We have exploited the fact that the area of the footprint of an AEM system is significantly smaller than the area of an AEM survey, and developed a robust 3D inversion method that uses a moving footprint. Our implementation is based on the 3D integral equation method for computing data and sensitivities, and uses the re-weighted regularised conjugate gradient method for minimising the objective functional. We demonstrate our methodology with the 3D inversion of AEM data acquired for salinity mapping over the Bookpurnong Irrigation District in South Australia. We have inverted 146 line km of RESOLVE data for a 3D conductivity model with ~310 000 elements in 45 min using just five processors of a multi-processor workstation.

3D Inversion of Time-lapse CSEM Data For Reservoir Surveillance

Recent studies have inferred the feasibility of time-lapse controlled-source electromagnetic (CSEM) methods for the surveillance of offshore oil and gas fields. However, quantitative interpretations have not been shown to ascertain what information about the reservoirs that may be recovered. We present a 3D inversion study of synthetic time-lapse CSEM data for the lateral water flooding of a reservoir unit where the hydrocarbon accumulation is trapped by a thin dome structure. We demonstrate that even with few constraints on the model, the flooding front can be recovered from 3D inversion. In this paper, synthetic time-lapse CSEM responses are simulated with the threshold about the noise floor and subject to multiple 3D inversion scenarios. The time-lapse CSEM inverse problem is highly constrained though inherently 3D since the geometry of the reservoir is established prior to production from high resolution seismic surveys; rock and fluid properties are measured from well logs; and multiple history matched production scenarios are contained in dynamic reservoir models.

Potential field migration for rapid 3D imaging of entire gravity gradiometry surveys

The geological interpretation of gravity gradiometry data is challenging. With the exception of the vertical gradient, maps of the different gravity gradients are often complicated and cannot be directly correlated with geological structure. 3D inversion has been the only practical tool for the quantitative interpretation of gravity gradiometry data. However, it is a complicated and time-consuming procedure that is very dependent on the initial model and constraints used. To overcome these difficul- ties for the initial stages of an interpretation workflow, we introduce the concept of potential field migration and demon- strate its application for rapid 3D imaging of entire gravity gradiometry surveys. This method is based on the direct integral transformation of the observed gravity gradients into a subsurface density distribution that can be used for interpretation, or as an initial model for subsequent 3D regularized inversion. We present a case study for the interpretation of gravity gradi- ometry data acquired in the Nordkapp Basin. We find agreement between the results obtained from potential field migration and those obtained from 3D regularized inversion, and show that the migration result are comparable to smooth inversion. For regional-size datasets, runtimes for migration are in the order of minutes compared to hours for inversion.

Inverting airborne geophysical data for mega-cell and giga-cell 3D Earth models

Today’s mineral exploration is driven by the simple fact that discovery rates have not kept pace with the depletion of existing reserves. To improve discovery rates, there is an industry-wide consensus on the need to increase the “discovery space” by exploring under cover and to greater depths. This attracts increased risks which may be mitigated by improved targeting. To do this, mining geophysics needs to shift toward 3D geological models founded upon improved petrophysical understanding and geophysical inversion. Regardless of the inversion methodology used, all geological constraints manifest themselves in the user’s prejudice of an a priori model, upper and lower bounds, and choice of regularization. However, the practice of geologically constrained inversion is not the major problem needing to be addressed. It is known (and accepted) that geology is inherently 3D, and is a result of complex, overlapping processes related to genesis, metamorphism, deformation, alteration and/or weathering. Yet, the mining geophysics community to date has not fully accepted that geophysics should also be 3D, and most often relies on qualitative analysis, 1D inversion, and depositscale 2D or 3D inversion. There are many reasons for this unfortunate deficiency, not the least of which has been the lack of capacity of existing 3D inversion algorithms. To date, these have not been able to invert entire surveys with sufficient resolution in sufficient time to practically affect exploration decisions. This problem is most critical for airborne geophysical sur

3D inversion of SPECTREM and ZTEM airborne electromagnetic data from the Pebble Cu–Au–Mo porphyry deposit, Alaska

Geological, geochemical, and geophysical surveys have been conducted in the area of the Pebble Cu–Au–Mo porphyry deposit in south-west Alaska since 1985. This case study compares three-dimensional (3D) inversion results from Anglo American’s proprietary SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and Geotech’s ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble deposit. Within the commonality of their physics, 3D inversions of both SPECTREM and ZTEM recover conductivity models consistent with each other and the known geology. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West. The high grade CuEqn 0.6% ore shell is not consistently following the high conductive trend, suggesting that the SPECTREM and ZTEM responses correspond in part to the sulphide distribution, but not directly with the ore mineralization. As in any exploration project, interpretation of both surveys has yielded an improved understanding of the geology, alteration and mineralization of the Pebble system and this will serve well for on-going exploration activities. There are distinct practical advantages to the use of both SPECTREM and ZTEM, so we draw no recommendation for either system. We can conclude however, that 3D inversion of both AEM and ZTEM surveys is now a practical consideration and that it has added value to exploration at Pebble. This case study compares 3D inversion results from SPECTREM 2000 fixed-wing time-domain airborne electromagnetic (AEM) and ZTEM airborne audio-frequency magnetics (AFMAG) systems flown over the Pebble Cu–Au–Mo porphyry deposit in south-western Alaska. Both 3D inversions recover conductors coincident with alteration associated with both Pebble East and Pebble West.

3D Inversion of SQUID Magnetic Tensor Data

Developments in SQUID-based technology have enabled direct measurement of magnetic tensor data for geophysical exploration. For quantitative interpretation, we introduce 3D regularized inversion for magnetic tensor data. For mineral exploration-scale targets, our model studies show that magnetic tensor data have significantly improved resolution compared to magnetic vector data for the same model. We present a case study for the 3D regularized inversion of magnetic tensor data acquired over a magnetite skarn at Tallawang, Australia. The results obtained from our 3D regularized inversion agree very well with the known geology of the area.

3D inversion of towed streamer EM data — A model study of the Harding field and comparison to 3D CSEM inversion

A towed streamer electromagnetic (EM) system capable of simultaneous seismic and CSEM data acquisition has been developed and tested in the North Sea. The towed EM data are processed and delivered as a time-domain impulse response. In this paper, we use 3D modeling and inversion to investigate the ability of the towed EM system to detect and characterize the Harding field, a typical North Sea-type target. The 3D model of the Harding field itself was constructed from dynamic reservoir simulations. We have compared our 3D inversion of time-domain towed streamer EM data with 3D inversion of conventional frequencydomain CSEM data. We observe similarities in the recovered models. 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.

3D Inversion of Time-lapse CSEM Data from Dynamic Reservoir Simulations of the Harding field, North Sea

Recent studies have inferred the feasibility of time-lapse controlled-source electromagnetic (CSEM) methods for the monitoring of offshore oil and gas fields. The time-lapse CSEM inverse problem is highly constrained though inherently 3D since the geometry of the reservoir is established prior to production from high resolution seismic surveys; rock and fluid properties are measured from well logs; and multiple history matched production scenarios are contained in dynamic reservoir models. Using Archie’s Law, rock and fluid properties from dynamic reservoir simulations of the Harding field in the North Sea were converted to resistivity, from pre-production in 1996 to decommissioning in 2016. CSEM data were simulated for each state. We demonstrate how 3D inversion can be used for monitoring the oil-water contact from pre-production to end of oil production in 2011, and for monitoring of the gas-water contact 2011 to 2016 during gas production. In particular, we show that focusing regularization is able to recover sharp resistivity contrasts across the oil-water and gas-water boundaries, whereas smooth regularization fails to recover an adequate resistivity contrast.

3‐D inversion of electromagnetic data based on the quasi‐analytical approximation for inhomogeneous background conductivity

The traditional implementation of the integral equation (IE) methods for three-dimensional (3-D) electromagnetic (EM) modeling and inversion requires the background model to be horizontally layered. Any deviations from that model must be treated as inhomogeneous inclusions. Practically, this can be very limiting when attempting to solve those 3-D inverse problems where the background model is known to be inhomogeneous. To overcome this problem, we have extended the IE method for 3-D EM modeling to include an inhomogeneous background conductivity distribution and applied this to rapid 3-D inversion based on the quasi-analytical (QA) approximation. We demonstrate this technique on the 3-D inversion of synthetic EM data.