Leif H. Cox

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.

Large Scale 3D Inversion of HEM Data Using a Moving Footprint

Helicopter electromagnetic surveying is a standard reconnaissance method in the mining industry. Typical surveys may cover tens to hundreds of square kilometers with hundreds of thousands of multifrequency soundings. Interpreting this volume of data is problematic. Yet each sounding location is sensitive to an area of less than 1 km, not the entire survey area. We suggest an inversion scheme, based on the integral equation method, in which the entire survey is inverted simultaneously with each transmitter-receiver pair being sensitive only to a relatively small area around each sounding location. We show that this method is as accurate as standard integral equation inversion methods, yet it inverts the data much faster. The technique is able to invert entire HEM surveys with over 500,000 inversion cells and tens of thousands of transmitter positions in less than one day on a single PC.

Airborne electromagnetic modelling options and their consequences in target definition

Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that the 2D and 3D methods used to extract models that are geologically meaningful would be favoured over 1D inversion and transforms. We do after all deal with an Earth that constantly undergoes, faulting, intrusions, and erosive processes that yield a subsurface morphology, which is, for most parts, dissimilar to a horizontal layered earth. We analyse data from a survey collected in the Musgrave province, South Australia. It is of particular interest since it has been used for mineral prospecting and for a regional hydro-geological assessment. The survey comprises abrupt lateral variations, more-subtle lateral continuous sedimentary sequences and filled palaeovalleys. As consequence, we deal with several geophysical targets of contrasting conductivities, varying geometries and at different depths. We invert the observations by using several algorithms characterised by the different dimensionality of the forward operator. Inversion of airborne EM data is known to be an ill-posed problem. We can generate a variety of models that numerically adequately fit the measured data, which makes the solution non-unique. The application of different deterministic inversion codes or transforms to the same dataset can give dissimilar results, as shown in this paper. This ambiguity suggests the choice of processes and algorithms used to interpret AEM data cannot be resolved as a matter of personal choice and preference. The degree to which models generated by a 1D algorithm replicate/or not measured data, can be an indicator of the data’s dimensionality, which perse does not imply that data that can be fitted with a 1D model cannot be multidimensional. On the other hand, it is crucial that codes that can generate 2D and 3D models do reproduce the measured data in order for them to be considered as a plausible solution. In the absence of ancillary information, it could be argued that the simplest model with the simplest physics might be preferred. Given the range of geological conditions under which airborne EM surveys are conducted, there is an expectation that 2D and 3D methods used to extract models of geological significance would be favoured over 1D inversion and transforms. We analyse data from the Musgrave province, South Australia, used for mineral and for hydro-geological prospecting.

Three-dimensional inversion of towed streamer electromagnetic data

ABSTRACT A towed streamer electromagnetic system capable of simultaneous seismic and electromagnetic data acquisition has recently been developed and tested in the North Sea. We introduce a 3D inversion methodology for towed streamer electromagnetic data that includes a moving sensitivity domain. Our implementation is based on the 3D integral equation method for computing responses and Frechet derivatives and uses the re‐weighted regularized conjugate gradient method for minimizing the objective functional with focusing regularization. We present two model studies relevant to hydrocarbon exploration in the North Sea. First, we demonstrate the ability of a towed electromagnetic system to detect and characterize the Harding field, a medium‐sized North Sea hydrocarbon target. We compare our 3D inversion of towed streamer electromagnetic data with 3D inversion of conventional marine controlled‐source electromagnetic data and observe few differences between the recovered models. Second, we demonstrate the ability of a towed streamer electromagnetic system to detect and characterize the Peon discovery, which is representative of an infrastructure‐led shallow gas play in the North Sea. We also present an actual case study for the 3D inversion of towed streamer electromagnetic data from the Troll field in the North Sea and demonstrate our ability to image all the Troll West Oil and Gas Provinces and the Troll East Gas Province. We conclude that 3D inversion of data from the current generation of towed streamer electromagnetic systems can adequately recover hydrocarbon‐bearing formations to depths of approximately 2 km. We note that by obviating the need for ocean‐bottom receivers, the towed streamer electromagnetic system enables electromagnetic data to be acquired over very large areas in frontier and mature basins for higher acquisition rates and relatively lower cost than conventional marine controlled‐source electromagnetic methods.

Multinary Inversion for Tunnel Detection

We introduce multinary inversion to explicitly exploit the physical property contrasts between different objects and their host medium, e.g., between air-filled tunnels and their surrounding earth. Conceptually, multinary inversion is a generalization of binary inversion to multiple physical properties. However, unlike existing realizations of binary inversion which are solved using stochastic optimization methods, our realization of multinary inversion can be solved using deterministic optimization methods. This is significant as the method can be applied to both linear and nonlinear operators and easily extends to joint inversion of multimodal geophysical data. Using synthetic models of full-tensor gravity gradiometry data, multinary inversion is demonstrated to be robust for tunnel detection relative to the presence of significant geological noise.

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.

Rapid and rigorous 3D inversion of airborne electromagnetic data

We address the challenging problem of interpreting frequency domain helicopter-borne electromagnetic data in areas with rough topography. Our method is based on localized quasi-linear (LQL) inversion followed by rigorous inversion, if necessary. Terrain corrections are also included. The LQL inversion serves to provide a fast image of the target. These results are checked by rigorous successive iterations of the domain equation, allowing more accurate calculation of the predicted data. If the accuracy is poorer than desired, rigorous inversion follows, using the LQL result as a starting model. We test this method on synthetic data and field data using the new code LQLRigInvTOPO. The results of the inversion are very encouraging with respect to both the speed and the accuracy of the algorithm. The numerical study shows that the new code with terrain correction provides a useful tool for airborne EM interpretation.

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 AEM Data Based on a Hybrid IE-FE Method and the Moving Sensitivity Domain Approach with a Direct Solver

The moving sensitivity domain method has been very successful in 3D inversion of the data from large airborne EM surveys. The modeling method was based on the integral equation (IE) method. However, in survey areas with rough topography and very high contrast, modelling based on the finite element (FE) method has advantages. We use the combined advantages of the IE and the FE methods in a hybrid scheme with the moving sensitivity domain to create a stable and efficient modeling method. To increase computational efficiency, we have reformulated the moving sensitivity domain so that multiple transmitters are contained in each sensitivity domain. This allows the economical use of a direct solver, because the sensitivity domain’s system matrix can be directly decomposed and then used for multiple transmitter positions. We show that there are an optimal number of transmitters to place in each subdomain: too many and the domain size become unwieldy, too few and the advantages of the direct solver are lost. We apply this technique to data from Germany’s Federal Institute of Geosciences and Natural Resources frequency domain AEM system. We validate the method by comparison with previously published results and our own 1D and 3D inversion results.