Exploration Geophysics - Volume 40, Issue 3, 2009

The HeliGEOTEM system was introduced in 2005 to provide higher resolution data than fixed-wing electromagnetic (EM) systems. The characteristics of HeliGEOTEM are illustrated by comparing the system with other airborne EM systems. A comparison with previous versions of HeliGEOTEM shows that, since 2005, the early-time information has improved allowing rapidly decaying responses to be identified. An improvement in the signal-to-noise ratio means the system is able to detect bodies at greater depth. A height attenuation test over the Nighthawk conductive body indicates that the latest system could see that target if it were buried 380 m below surface. Another target that is difficult to detect (Caber) is clearly seen on the HeliGEOTEM data.

A comparison of field data at the Maimon deposit indicates that the helicopter DIGHEM frequency-domain system and the HeliGEOTEM time-domain system both acquire data with similar spatial wavelengths. Data collected away from the main ore body and along strike indicate that the HeliGEOTEM sees a less attenuated response from a deeper part of the body. Also, the HeliGEOTEM is able to estimate the conductivity, whereas the DIGHEM system cannot discriminate the conductance, it can only indicate that the body is highly conductive. The DIGHEM data, however, is better able to resolve the near-surface conductivity, and the spatial form of the DIGHEM data is simpler. The data acquired with multiple transmitter-receiver coil pairs (DIGHEM and HeliGEOTEM) provides information superior to single-component data.

Tools used to display fixed-wing airborne EM data have been modified to work with HeliGEOTEM data. These tools can image the structure in cases where the ground is assumed to be comprised of a) horizontal layers or b) discrete conductors.

A comparison of HeliGEOTEM with the helicopter RESOLVE and fixed-wing GEOTEM systems shows that the HeliGEOTEM is able to map most of the shallow features seen on the RESOLVE and to image the resistivity to depths comparable to the GEOTEM (in a moderately conductive environment).

A traverse line from Australia over what is considered a difficult target (the Nepean Mine) demonstrates that the HeliGEOTEM system provides good signal-to-noise ratios. Using the z- and x-component data does a better job of defining the geometry of the target than using the z-component data alone.

Availability of a fracture map of a producing reservoir aids in increasing productivity. Generally, accurate information related to fracture orientation is only available at a few sparse well log locations. However, fractures introduce velocity anomalies in seismic data by making the medium azimuthally anisotropic. When multi-azimuth data is available then it is possible to map the fracture attributes in the entire reservoir zone by analysing the anisotropy induced velocity anomalies in the seismic data. In the absence of 3D data, seismic anisotropy induced velocity anomaly from 2D data (as fracture strikes are not constant and data contains multi-azimuthal effect even when it is 2D) can still be used as a secondary source of information for the purpose of fracture strike simulation. To validate the above hypothesis, fracture strike information in a reservoir from the Mexican part of the Gulf of Mexico is derived using Markov-Bayes stochastic simulation. In this simulation process, accurate well log derived fracture information is used as hard or primary data and seismic velocity anomaly/uncertainty based fracture information is used as soft or secondary data. The Markov-Bayes Stochastic simulation provides multiple realisations of the fracture patterns and thus helps to estimate the uncertainty associated with the fracture strikes of the reservoir. Accuracy of the simulation process is also estimated and the simulation result is compared with simple and ordinary kriging methods of fracture strike simulation.

The Torrens Hinge Zone is a long but narrow (up to 40 km wide) geological transition zone between the relatively stable Eastern Gawler Craton ‘Olympic Domain’ to the west, and the sedimentary basin known as the Adelaide Geosyncline to the east. The author hypothesised from first principles that the Torrens Hinge Zone should be prospective for high geothermal gradients due to the likely presence of high heat flow and insulating cover rocks. A method to test this hypothesis was devised, which involved the determination of surface heat flow on a pattern grid using purpose-drilled wells, precision temperature logging and detailed thermal conductivity measurements. The results of this structured test have validated the hypothesis, with heat flow values over 90 mW/m² recorded in five of six wells drilled. With several kilometres thickness of moderate conductivity sediments overlying the crystalline basement in this region, predicted temperature at 5000 m ranges between 200 and 300°C.