Exploration Geophysics - Volume 44, Issue 3, 2013

The 21-point finite difference scheme for the frequency-space domain elastic wave forward modelling is designed through optimising the impedance matrix, especially calculating the spatial derivative terms and the mass acceleration terms of the elastic wave displacement equation as accurately as possible. Comparative tests show that the 21-point finite difference scheme is much better in grid dispersion, memory requirement, and computation time than the 9-point scheme and slightly better than the 25-point scheme. The 21-point finite difference scheme is ~15% lower in memory consumption and computing time than the 25-point scheme. The numerical examples show that the 21-point finite difference scheme is valid in the sense of the numerical simulation of ideal models.

This paper presents a case study of fracture-system interpretation using reverse vertical seismic profiling (RVSP) with seismic reflection, refraction, and borehole televiewer data on Seokmo Island, South Korea. We first extracted fracture locations from a pre-existing image obtained by prestack phase-screen migration of RVSP data, and the strike and dip of each fracture from the borehole televiewer data. We established an initial velocity model using this fracture information and then generated synthetic common-receiver gather data through forward simulation. However, the synthetic data could not sufficiently reflect the characteristics of the field data. To resolve this problem, we added an upper alluvial layer, observed in the surface reflection and refraction data, to the velocity model. To improve the quality of the migrated image of the RVSP field data and the velocity model, we not only reprocessed the RVSP data, but also applied prestack generalized-screen migration, which is more accurate than phase-screen migration in computing steep fractures. The new synthetic data generated from the improved velocity model agreed well with the RVSP field data. As a result, we could describe a markedly improved subsurface structure including fracture locations. The workflow suggested in this study will be helpful for imaging fracture systems in oil and gas reservoirs as well as in geothermal reservoirs.

Self-potential anomalies observed over sulfide ore bodies can be closely associated with electrochemical reactions and the ohmic potential drop within the rocks. Self-potential surveys based on laboratory measurements of electrochemical potentials allow us either to measure the amplitude of the anomalies generated by this mechanism or to determine the model parameters. In order to achieve these goals, two sheets of zinc and copper were joined together to simulate sheet-like ore bodies. Self-potential surveys were conducted over 684 electrodes with the purpose of revealing the influence of various angles of the sheet. In a laboratory experiment, four different inclinations were chosen to perform the forward modelling. The last part of this paper involves the inversion of measured data to recover the distribution of generated self-potential signals. The inversion results show a satisfactory agreement with the laboratory measured data. Finding the geometry of the buried source from the shape of the SP response is not intended as it is fixed in advance. The first aim of this paper is to show how the SP response is affected under the presence of a 2D conductive structure (sheet-like) in tank experiments. The second aim is to obtain one of the model parameters (coefficient M) using data regression.

In this paper, we have developed a new least- squares minimisation approach to determine the depth of a buried faulted structure approximated by a 2D semi-infinite horizontal slab from second moving average residual gravity anomalies. The problem of depth determination has been transformed into a problem of finding the solution to a nonlinear equation of the form z = f(z) .The method can be applied not only to residuals but also to observed data. The method overcomes the problems associated with determining the depth from successive horizontal derivative anomalies obtained from 2D gravity data using filters of successive graticule spacings. The method is applied to theoretical data with and without random errors and is tested on a field example from Egypt. In all cases, the depth solution obtained is in good agreement with the actual depth.

In wellbore climatology the method of temperature inversion to determine the trends in ground surface temperature history (GSTH) assumes that the process of a well’s thermal recovery is practically completed. However, for deep wells (>100–300 m) the drilling process, due to the lengthy period of drilling fluid circulation, greatly alters the temperature of formation immediately surrounding the well. As a result, the determination of the formation temperature (with a specified absolute accuracy) at any depth requires a lengthy period of shut-in time. The objective of this study is to determine how long it takes before the error caused by mud circulation is small compared to the change arising from the change in surface temperature. In this paper we suggest two techniques, Slider’s method and utilisation of the γ-function, which enable us to estimate the rate of temperature decline and the difference between the formation and shut-in temperatures.

Calibration facilities for total-count radiation probes used to measure high-grade uranium (> 2 %) are scarce and the conventional method using correction factors to convert to equivalent uranium has severe shortcomings. In this paper gamma-ray transport modelling is utilised to directly convert logging data into in situ U grade estimates, taking into account casing, water, borehole diameter, probe size and detector shielding corrections which at high U grades are dependent on the U grade itself. The method uses interpolation between modelled results and avoids the usual calibration factors. It is demonstrated using data collected at the Angela U deposit, Northern Territory, and at the Kintyre deposit, Western Australia. At the Kintyre deposit, the uranium grades exceed 20% and both unshielded and shielded NaI(Tl) detectors were tested, with the latter showing the best performance at high grades.

New heat generation measurements for radiogenic granites were made for thirteen localities in the Darling Range, Western Australia. These are integrated with published data to estimate temperatures at depth within radiogenic-granite bodies for this region of the south-western Yilgarn Craton. The heat generation in the radiogenic granites is calculated from the concentrations of uranium, thorium and potassium measured in the field. A reliable relationship between total counts from a commercial portable spectrometer and Geiger Müller counter was found for the various granites measured. The relationship Ao = 0.34 + 2.16 CU, with a correlation coefficient of 0.98, was found between uranium (CU in ppm) content and heat generation (Ao in units of µW/m³) for those radiogenic granites measured in the Darling Range, and also for two granites in the Pilbara Craton. Measured heat generation in the Darling Range was found to vary between 4 and 10 μW/m³, the maximum of which is higher than previously known heat generation in granites for the Yilgarn Craton. Based on these new data, temperatures between depths of 3000 and 4000 m are modelled to fall between 60 and 110°C, depending on the thickness of the granitic bodies. These results are encouraging for potential low-temperature geothermal developments in this region, which is adjacent to the Perth metropolitan area.

Shallow warm water resources associated with low enthalpy geothermal systems are often difficult to explore using geophysical techniques, mainly because the warm water creates an insufficient physical change from the host rocks to be easily detectable. In addition, often the system also has a limited or narrow size. However, appropriate use of geophysical techniques can still help the exploration and further investigation of low enthalpy geothermal resources. We present case studies on the use of geophysical techniques for shallow warm water explorations over a variety of settings in New Zealand (mostly in the North Island) with variable degrees of success.

A simple and direct method for the exploration of warm water systems is shallow temperature measurements. In some New Zealand examples, measurements of near surface temperatures helped to trace the extent of deeper thermal water.

The gravity method was utilised as a structural technique for the exploration of some warm water systems in New Zealand. Our case studies show the technique can be useful in identifying basement depths and tracing fault systems associated with the occurrence of hot springs.

Direct current (DC) ground resistivity measurements using a variety of electrode arrays have been the most common method for the exploration of low enthalpy geothermal resources in New Zealand. The technique can be used to detect the extent of shallow warm waters that are more electrically conductive than the surrounding cold groundwater. Ground resistivity investigations using the electromagnetic (EM) techniques of audio magnetotellurics (AMT or shallow MT), controlled source audio magnetotellurics (CSAMT) and transient electromagnetic (TEM) methods have also been used. Highly conductive clays of thermal or sedimentary origin often limit the penetration depth of the resistivity techniques and can create some interpretation difficulties. Interpretation of resistivity anomalies needs to be treated in a site specific manner.