Andrew Pethick

Semiautomatic and Automatic Cooperative Inversion of Seismic and Magnetotelluric Data

Natural source electromagnetic methods have the potential to recover rock property distributions from the surface to great depths. Unfortunately, results in complex 3D geo-electrical settings can be disappointing, especially where significant near-surface conductivity variations exist. In such settings, unconstrained inversion of magnetotelluric data is inexorably non-unique. We believe that: (1) correctly introduced information from seismic reflection can substantially improve MT inversion, (2) a cooperative inversion approach can be automated, and (3) massively parallel computing can make such a process viable. Nine inversion strategies including baseline unconstrained inversion and new automated/semiautomated cooperative inversion approaches are applied to industry-scale co-located 3D seismic and magnetotelluric data sets. These data sets were acquired in one of the Carlin gold deposit districts in north-central Nevada, USA. In our approach, seismic information feeds directly into the creation of sets of prior conductivity model and covariance coefficient distributions. We demonstrate how statistical analysis of the distribution of selected seismic attributes can be used to automatically extract subvolumes that form the framework for prior model 3D conductivity distribution. Our cooperative inversion strategies result in detailed subsurface conductivity distributions that are consistent with seismic, electrical logs and geochemical analysis of cores. Such 3D conductivity distributions would be expected to provide clues to 3D velocity structures that could feed back into full seismic inversion for an iterative practical and truly cooperative inversion process. We anticipate that, with the aid of parallel computing, cooperative inversion of seismic and magnetotelluric data can be fully automated, and we hold confidence that significant and practical advances in this direction have been accomplished.

Open Source Interactive Electromagnetic Modelling

Summary The marine controlled source electromagnetic method has developed during the last decade for direct hydrocarbon indication. Marine controlled source electromagnetic software is still in its infancy with only a small number of open source algorithms and even fewer integrated software environments. We have developed an open source software package to encourage the development and use of the marine controlled source electromagnetic method in both industry and educational institutions. The software was written in Java and was made to perform interactive real-time synthetic modeling for varying earth models or survey parameters.

1D Magnetotelluric Forward Modelling Web App

Geophysical computing within the cloud appears to be the way of the future. The instantaneous, on-demand character of modern life is now firmly established. We present an integrated 1D magnetotelluric forward modelling web app. This basic web app combines a scientific python back end and a front end built upon PHP and HTML5 web technologies. It has also been packaged as an easy to install plugin for the popular Wordpress framework. The application simulates the 1D magnetotelluric response over any isotropic geo-electrical earth model. MT forward modelling can be performed on any internet enabled device containing a HTML5 compliant browser with our WebMT application. This includes mobile phones, tablets and desktop PC’s. This research demonstrates one approach to geophysical web application development and promotes future development and innovation within the geophysics community.

Unconstrained 2D and 3D Magnetotelluric Inversion - Detection of a Ni-Cu-PGE Ore Zone and Carbonaceous Phyllites - Kevitsa

The large Kevitsa Ni-Cu-PGE (platinum group elements) deposit in Finland is part of a complex geo-electrical setting. The ore zone is electrically conductive compared to the high resistivity olivine pyroxenite host rock. The ore body and host are surrounded by thin sheet like high electrical conductive carbonaceous phyllites. Unconstrained 2D and 3D inversion of magnetotelluric (MT) data was performed using seven densely sampled MT transverses ranging in length from 2500m to 7000m. Two dimensional inversions included a total of 312 MT stations along the seven transverses, while 3D inversion was completed on a subset of 270 MT stations focused around the ore zone. The inversions were conducted on the Pawsey Centre, Magnus Cray XC30 supercomputer. The challenge for this site was to resolve the ore body in the presence of the carbonaceous phyllites using sparse transverses. We compare the outcome of 2D and 3D MT inversion and find that 3D inversion appears to provide superior definition of the ore zone. This research and the resulting electrical conductivity distributions will inform our future work on full 3D cooperative inversion of seismic and MT data over the Kevitsa site.

Application of seismic attributes for constraining Magnetotelluric Inversion

Unconstrained inversion of surface magnetotelluric data generates non-uniqueness solutions. Boundaries derived from seismic reflectively images have the potential to substantially improve MT inversion. Seismic should be highly benefic ial where significant and strong reflectors can reasonably be associated with contrast in electrical conductivity across well-defined relatively continuous boundaries. We show how seismic reflections can assist in defining such inversion controls as the smoothness penalty across known boundaries. We apply and compare a range of cooperative inversion strategies using large scale co-located magnetotelluric and seismic reflection field data sets from the Carlin style gold district in Nevada USA.

Bathymetry, electromagnetic streamlines and the marine controlled source electromagnetic method*

Seafloor topography must influence the strength and direction of electromagnetic fields generated during deep ocean controlled source electromagnetic surveying. Neither mathematical equation nor rules of thumb provide a clear perspective of how changes in water column thickness alters electromagnetic fields that engulf hundreds of cubic kilometres of air, ocean, host and reservoir. We use streamline visualisation to provide a generalised representation of how electromagnetic fields propagate into a 2D geo-electrical setting that includes strong bathymetry. Of particular interest are: (i)’ dead zones’ where electric fields at the ocean floor are demonstrated to be weak and (ii) the ’airwave’ that appears in the electric field streamlines as circulating vortices with a shape that is clearly influenced by changes in ocean depth. Our analysis of the distribution of electric fields for deep and shallow water examples alludes to potential benefits from placement of receivers and/or transmitters higher in the water column as is the case for towed receiver geometries. Real-time streamline representation probably holds the most value at the survey planning stage, especially for shallow water marine EM surveys where ocean bottom topography is likely to be consequential. Ocean bottom topography significantly influences the application of marine controlled source electromagnetics. Distortions in the electromagnetic field shape and behaviour associated with the airwave, bathymetry and potential hydrocarbon targets can be observed using streamlines and can point towards survey designs that may not have been considered.

Computing, Brains and Geophysics?

Brain computer interface (BCI) systems emerging as a breakthrough technology of the 21st century. As is the case with other developing technologies, proof of concept must be demonstrated before advanced methods are pursued. This article presents the first published case study of a brain controlled geophysical software package. We show how brain computer interface systems can facilitate accelerated learning in the geoscience community. Our results show that processed brainwaves from the NeuroSky MindWave electroencephalography (EEG) device can be used to control various geophysical survey parameters with an acceptable degree of accuracy and to model the corresponding data in real-time.

Understanding the effect of Bathymetry on the Marine Controlled Source Electromagnetic Method using Electromagnetic Streamlines

Summary Understanding the impact of bathymetry can be a critical in application of the marine controlled source electromagnetic methods. Electromagnetic field strength and direction can be affected by small changes in water column depth. The bathymetry, the air-water interface and sub-surface resistivity variations will all contribute to any single electromagnetic measurement. In such complex geo-electric settings a deeper understanding of how and where each feature is expressed in the electromagnetic response is required. We compute the electromagnetic fields for a model with complex bathymetry and a hydrocarbon target. We compute the response with and without hydrocarbon to investigate the effect of bathymetry. A 2.5D finite element algorithm was used to forward model the MCSEM response. The interaction of the electromagnetic fields with the target and the bathymetry can be appreciated by viewing electric and magnetic streamlines. A key benefit of using streamlines is that they quickly show where the electric or magnetic fields would be most strongly altered by the target or alternatively by bathymetry. This aids both interpretation and survey design.

3D modelling for time-lapse cross-well CSEM monitoring of CO2 injection into brine filled reservoirs

Summary Carbon Dioxide (CO2) sequestration is one proposed solution to the possible detrimental effects of increased CO2 emissions into the Earth’s atmosphere. A proposed method for CO2 sequestration is capture and storage below the earth’s surface in deep saline reservoirs. Australia’s CO2CRC research group is currently trialling this method of CO2 sequestration by injection into the Paaratte formation in the Otway Basin Australia. As CO2 is injected into brackish or saline water saturated sediments it is expected to create a zone of increased electrical resistivity around the injector well. The cross-well controlled-source electromagnetic method may be capable of mapping the movement of injected CO2 as it expands out from the injection interval. We simulate time-lapse in-hole controlled source electromagnetic surveys using expected change in electrical resistivity that might be associated with CO2 injection. We demonstrate that controlled source electromagnetic methods will successfully monitor CO2 injection given; (i) suitable transmitter type and frequency range; (ii) a monitoring well design that can facilitate the electrical methods and (iii) correct monitoring well location relative to the injection well. In particular we find that, because of the large volume of CO2 that would likely be injected during a large sequestration project even relatively small changes of less than 10% in electrical resistivity associated should be readily detectable. We provide images of the time lapse cross well electromagnetic response for an expanding disk representing 0.1 to 10 kilo tonne of CO2.

Comparison of a vertical electric and a vertical magnetic source for cross well CSEM monitoring of CO2 injection

Controlled source electromagnetic transmitters create highly geometric coupled electric and magnetic vector fields that propagate in a way that is dependent on both the orientation of the transmitter and electrical conductivity distribution. There may be a good case for using cross well controlled source electromagnetic methods for monitoring injection of CO2 into deep saline or brackish sandstone reservoirs. The expected range of geo-electrical frameworks that can be used to represent CO2 injection into a saline or brackish sandstone water saturated reservoir is reasonable constrained. That is injection of CO2 would likely create an expanding zone of elevated electrical resistivity that would move out from the injector well into the reservoir. The reservoir would typically be confined above and possibly below by conductive clay or shale dominated sediments. Given this type geo-electrical framework we consider the relative merits of a time harmonic vertical electric and vertical magnetic source for monitoring CO2 injection. We compare numerically generated electric and magnetic fields created in a heterogeneous horizontally layered injection zone with and without injection of CO2. Examples are first provided for a layered earth and then for an expanding 3D volume within permeable layers. We provide images indicating that the vertical electric dipole source is sensitive to CO2 injection into thin resistive sandstone layers in a conductive background.