Exploration Geophysics - Volume 36, Issue 3, 2005

A set of temperature data from 61 deep petroleum wells drilled in the Cooper Basin of South Australia has been used to test the accuracy of four commonly used models of borehole thermal reequilibration. Models tested include the Horner plot as derived from Bullard (1947), the theoretical dual-media zero circulation cylindrical model of Cooper and Jones (1959), the empirical semi-log plot of Pitt (1986) and the exponential model of Nakaya (1953).

The selected models have been fitted to bottom-of-hole temperature data recorded during the recovery period of each well, and are used to predict a value for True Formation Temperature (TFT). These values are then compared with actual temperature measurements derived from Cement Bond Logs (CBL). Recorded on average 482 days after the end of drilling, these temperature measurements are likely to represent TFT.

Analysis of the accuracy of model prediction relative to CBL temperature suggests that on average most models tend to underestimate TFT to some degree. The magnitude of this bias is found to be dependent upon assumptions implicit in each model. In most cases, model prediction is improved where at least one perturbed bottom hole temperature (BHT) has been recorded ≥ 20 hours after the end of drilling. To date the best results are derived from the semi-log plot extrapolated to 50 hours.

Recent developments in theory and instrumentation have led to increasing interest in the use of geo-electrical techniques to map seafloor structure and to explore mineral deposits. Electrical experiments conducted at sea are difficult and costly to perform, reinforcing the need for theoretical design studies before any seagoing programs get underway.

I present in this paper the first theoretical investigation of geoelectrical responses associated with hydrothermal fluid circulation in a mid-ocean ridge flank environment. A 2D conceptual electrical model is constructed based on hydrothermal modelling results, and its responses to two major ‘galvanic’ techniques (magnetometric resistivity (MMR) and electrical resistivity methods) are calculated using a finite difference computer package. Forward modelling results reveal that the marine MMR method is capable of detecting off-axis hydrothermal convection cells with equivalent or even greater resolution than traditional seafloor heat flow surveys. However, the electrical resistivity method is not applicable because this system suffers a very severe ‘shorting effect’ of the overlying seawater layer, which almost totally masks the contribution from the underlying oceanic crust.

New algorithms for apparent resistivity, static corrections, and Bostick transforms of CSAMT data allow real-time production of parasections for resistivity versus depth from profiles of CSAMT data at all frequencies. This allows rapid visualisation of 2D and 3D CSAMT model data and comparison with field data. The resulting interactive fitting of parasections of model and field data provides an alternative to formal inversion.

The process is used to model the effect of glacial erosion features in CSAMT data from Western Tasmania in order to assess the sensitivity of the CSAMT method for the detection of targets such as steeply-dipping graphitic and mineralised alteration zones. The approach has advantages of real-time capability (with potential to flag data affected by cultural and power transmission line noise), of disriminating between effects of surficial conductive variations such as filled glacial erosion features, and of differentiating between genuine dipping conductors, vertical conductors, and artefacts from near-surface effects, which may generate apparent “vertical” conductive anomalies.

A similar approach using CSAMT data from basalt-covered terrain in the Victorian goldfields, combined with 3D EM modelling, was successful in identifying a prospective sulphidic conductor in the Ordovician basement, beneath variably-conductive basalt cover.

Conductivity surveys (Induced Polarization and Transient Electromagnetics) have been conducted on the southern bank of the Goulburn River, near Murchison, Victoria, to discriminate between clay zones and saline discharge streams. The combined surveys provide a consistent dataset and multiple conductive zones are delineated within the regolith. The ambiguity between salinity and clay content variations is resolved by means of the induced polarization parameters. In particular, normalised chargeability values are shown to highlight the location of clay lenses containing saline pore water.

Airborne electromagnetic (AEM) data were collected in the Geographe Bay region, Western Australia, using the QUESTEMTM 450 system operating at 25 Hz. The survey covered a region with shallow water up to 35 m depth (Geographe Bay), including a steel wreck, and two overlapping tie lines skirting Cape Naturaliste extending into deeper water. We interpreted the survey data using layered-earth inversion and Conductivity-Depth Imaging (CDI) processing to estimate sea depths and compared these findings with accurate bathymetric data. The root mean square (RMS) residual error of the estimated sea depths was found to be 3.2 m using layered-earth inversion of inline-component data measured in the depth range of 25 to 93.5 m off Cape Naturaliste. The CDI processing of vertical-component data in this region underestimated the sea depth but displayed conductivity variations representing seawater to depths of 60 to 70 m. In Geographe Bay, the RMS residual error of the estimated sea depths was found to be 0.9 m using layered-earth inversion of selected line data in the depth range of 25 to 35 m. In shallower regions, adjacent to the shoreline, the residual error of the estimated sea depth increases because of the influence of the edge effects at the boundary of the conductive (seawater) and resistive (land) layers.

Night-time measurements of the Earth’s undisturbed, quiet magnetic field at satellite altitudes provide models of the main field and its secular variation, and of the crustal magnetic anomaly field. Fields from the regular daily variations of solar and lunar origin are most pronounced in daytime data and can affect aeromagnetic and ground magnetic surveys. Satellite magnetic data in total intensity are now available across all local times, and for the first time it is possible to determine the regular daily variations in the ionosphere-magnetosphere region. We give the daily variation in total intensity at satellite altitudes for six equally spaced Universal Time epochs each covering daytime and night-time, showing clearly the equatorial electrojet and Sq current system above the ionosphere during daytime hours. Also given are crustal magnetic anomaly maps, based on high-degree residuals in main field data, for the world and the Australian region at satellite altitudes. The daily variations and crustal magnetic anomalies at satellite levels are both of the order of 20 nT.

Finsch Mine is a world-class diamond mine in the Northern Cape province of South Africa, producing roughly 2.4 million carats annually. The mine is currently conducting resource evaluation to examine the feasibility of block caving below the current extraction level, which is located 630 m below surface (Block 5). Resource evaluation requires the accurate determination of the kimberlite pipe morphology.

As part of the evaluation, borehole radars (BHR) were deployed down two boreholes that fanned outwards at ~30° to one another from a drilling station at 650 m level in the host dolomite. Both holes dipped at ~60°. Both struck the near-vertical flank of the di treme ~120 m below their collars. Single-hole profiling as well as cross-hole BHR scanning surveys were run to determine whether coherent reflection from the kimberlite–country rock interface could be obtained, and to establish the limits on the operational parameters such as range and resolution.

Coherent reflections from the kimberlite pipe contact were clearly observed from distances up to 60 m through the dolomite country rock. The kimberlite–dolomite interface appeared to be sharp, cemented, smooth, and curved. The BHR results showed that the kimberlite pipe surface was not that of a smooth cone. Inversion by interactive forward modelling revealed three horizontal rounded ridges, or corrugations, spaced by ~30 m. These penetrated 4 m to 8 m into the country rock, across the portion of the flank covered by the survey.

The range, resolution, and accuracy obtained from the BHR surveys at Finsch Mine are of high quality. The survey results improve knowledge of the detail of, and confidence in, pipe morphology significantly. This allows for more-accurate geological modelling and resource estimations, meaningful project evaluation, better tunnel design, and ultimately, sound investment decisions.

Removal of radiometric anomalies due to airborne radon is one of the most demanding steps in airborne gamma-ray spectrometry data processing. The contribution from airborne radon gas at the usual survey altitude is often several times that emanating from uranium ground sources. If left uncorrected, the radon background component can corrupt the whole chain of radiometric processing and preclude the production of seamless uranium and ternary data images.

The concept of the use of ²¹⁴Pb photopeaks for radon removal has been known for many years (Grasty, 1982), however until now it has not been introduced into regular data processing practice. The main obstacle in the implementation of this concept has been the fact that ²¹⁴Pb photopeaks are found in the low energy part of the spectrum where there is a high degree of noise caused by scattered radiation. However, advances in modern instrumentation and in multichannel processing techniques allow the utilisation of these ²¹⁴Pb photopeaks for radon removal in daily practice. The use of low-energy photopeaks is now possible following the release of a new generation of spectrometers, which produce improved spectrum linearity and high resolution in 512 channels for each crystal in the sensor array,

The spectral ratio method suggested by Minty (1992, 1998), based on the 609 keV and 1765 keV photopeaks of ²¹⁴Bi, has been to date the most accepted technique for radon removal. However, it has an inherent difficulty when processing data with a high ¹³⁷Cs contribution, as is widespread in the Northern Hemisphere. In addition, this method suffers from the inability to resolve in a satisfactory way the overlap of the ²⁰⁸Tl photopeaks at 511 keV and 583 keV with the ²¹⁴Bi 609 keV photopeak, in cases where there is strong thorium radiation.

During recent surveys totalling 290 000 line kilometres, flown for the Geological Survey of Namibia, a Pico Envirotec GRS10 Spectrometer was used to collect 512 channels of raw data per second from each of eight NaI(Tl) crystal sensors. Observation of the 214Pb photopeaks at 295 keV and 352 keV, instead of the more usual ²¹⁴Bi 609 keV photopeak, allowed successful radon removal.

The violent eruption of the Piparo mud volcano, Trinidad, in February 1997 demonstrated its destructive capability by completely burying 16 houses and a number of livestock under a 5 m thick mud pile. Unlike magmatic volcanoes, mud volcanoes involve very low energy, making geophysical methods such as seismology unsuitable for monitoring. Three-dimensional gravity modelling over the Tabaquite mud volcano suggests the presence of a large density contrast (−0.70 t.m^-3). The density contrast being large and dynamic (i.e., it is absent at recently active mud volcanoes like Piparo) makes the gravity method a potential tool for monitoring mud volcanoes.