Exploration Geophysics - Volume 43, Issue 1, 2012

Regional scale gravity modelling is an effective and fast way to gain geological understanding of large scale structures like the Bowen Basin. Detailed deep 3D geological knowledge has become an important component of many types of exploration and resource modelling. Current interest in the Bowen Basin for geothermal exploration highlights the need for a complete basin scale model which is compatible with thermal modelling software. The structure of the Bowen Basin is characteristic of a typical asymmetrical extensional rift basin, with up to 5 km of sediment overlying the basement. By combining gravity modelling, calibrated by boreholes and seismic reflection profiles, we produce geologically reasonable 3D surfaces and structures to create a model of the Bowen Basin. This model is the final part in the completion of the 3D Sydney–Gunnedah–Bowen Basin system geological model and provides both an important framework from which detailed thermal models can be derived and a platform from which to expand with new information.

Studies of the Sydney–Gunnedah–Bowen Basin (SGBB), one of the largest extensional rift sedimentary basins on the east coast of Australia, lack an understanding of the 3D upper crustal structure. Understanding of the subsurface structure is essential for many areas of resource exploration, development and management, as well as scientific research. Geological models provide a way to visualise and investigate the subsurface structure. The integrated regional scale gravity modelling approach, which uses boreholes and seismic data constraints, provides an understanding of the upper crustal structure and allows the development of a 3D geological model which can be used as the architectural framework for many different applications. This work presents a 3D geological model of the SGBB developed for application in high resolution thermal models. It is the culmination of geological surfaces derived from the interpolation of previous regional scale 2D gravity models and numerous borehole records. The model outlines the basement structure of the SGBB and provides information on depth to basement, depth to basal volcanics and thickness of overlying sediments. Through understanding the uncertainties, limitations, confidence and reliability of this model, the 3D geological model can provide the ideal framework for future research.

Electromagnetic anomalies covering a wide range of frequencies from ultra low frequency (ULF), very low frequency (VLF) up to very high frequency (VHF) have been observed before earthquakes. However, the ULF range emissions provide a greater source of information regarding the earthquake precursor. One of the main techniques of investigating such a precursor is by using a magnetic sensor. In this paper, we have carried out a study of spectral density (magnetic field intensity) and polarization ratio methods to extract earthquake precursory signatures of the ULF data for moderate earthquakes (magnitude Mb = 3.7–4.8), using a three-component induction coil magnetometer installed at Shivaji University, Kolhapur (16.40°N, 74.15°E), India. We have applied a Fast Fourier Transform (FFT) procedure to calculate the spectral density of the ULF time series. We have found enhancement in ULF magnetic field intensity 3 to 5 days before the main shock and this specific enhancement appeared ±3 h around the main shock time in the 1–5 Hz frequency range. We have examined ULF variations with polarization values and Kp index data. Magnetic field intensity of ULF data can give important information about earthquake preparation processes and it can be involved in the development of earthquake prediction methodology.

Geophysical methods, especially seismic inversion, have improved considerably in recent years. The prediction of elastic behaviour is important to decrease risk in mining operations. The investigation of rock physics is a way to predict rock behaviours, especially reservoir geomechanical parameters. The first step in rock physics studies is to diagnose and introduce a suitable rock physics model. In this paper, we review rock physics models, such as the Rymer–Greenberg–Castagna model, and we compare them with real data trends in two oil wells of a carbonate reservoir (the Fahliyan Formation) in the Zagros Basin of southwestern Iran using sonic, density and porosity logs. After omitting the effect of water saturation and clay content, the best model for clean carbonate of the Fahliyan Formation was developed in two oil wells (A1 and A2).