Exploration Geophysics - Volume 44, Issue 4, 2013

A discrete conductive sphere model in which current paths are constrained to a single planar orientation (the ‘dipping sphere’) is used to calculate the secondary response from Geotech Ltd’s VTEM airborne time domain electromagnetic (EM) system. In addition to calculating the time constants of the B-field and dB/dt responses, we focus on the time-constant ratio at a late time interval and compare numerical results with several field examples. For very strong conductors with conductivity above a critical value, both the B-field and dB/dt responses show decreasing values as the conductivity increases. Therefore response does not uniquely define conductivity. However, calculation of time constants for the decay removes the ambiguity and allows discrimination of high and low conductivity targets. A further benefit is gained by comparing the time constants of the B-field and dB/dt decays, which co-vary systematically over a wide range of target conductance. An advantage of calculating time constant ratios is that the ratios are insensitive to the dip and the depth of the targets and are stable across the conductor. Therefore we propose to use their ratio rτ = τB/τdB/dt as a tool to estimate the size and conductivity of mineral deposits. Using the VTEM base frequency, the magnitude of rτ reaches a limiting value of 1.32 for the most highly conductive targets. Interpretations become more complicated in the presence of conductive overburden, which appears to cause the limiting value of rτ to increase to 2 or more.

The incident angle is a very important piece of information in many processing steps for seismic data, but it cannot be easily and directly estimated in many typical and familiar migration processes, such as shot-profile wave equation migration and reverse time migration. In this paper, we first revisit and analyse some popular schemes of estimating the incident-angle field. Then we present a robust method to estimate the incident-angle field in a 2D/3D heterogeneous isotropic media based on a one-way wave propagator. Unlike the band-limited wavefield, the incident-angle field is estimated by the division of two impulse responses of the monochromatic wavefield in order to reduce computation. The impulse responses are the derivative of the angle-weighted image extracted by multiplying an extra imaging weight in the conventional migration process and conventional image. The tilted coordinate system is adopted in our method to avoid the steep-angle limitation of one-way wave propagators. By comparison with other methods, our method can estimate the incident-angle field more accurately with higher efficiency and less memory cost. Computed incident-angle fields of a 2D layered model and 3D field data example demonstrate the generality and flexibility of the method.

The Ifal (Midyan) Basin is one of the well defined basins along the Red Sea coast, north-western Saudi Arabia. Location, geometry, thick sedimentary cover and structural framework qualify this basin for groundwater, oil and mineral occurrences. In spite of being studied by two airborne magnetic surveys during 1962 and 1983, structural interpretation of the area from a magnetic perspective, and its uses for hydrogeological and environmental investigations, has not been attempted. This work thus presents interpretation of the aeromagnetic data for basement depth estimation and tectonic framework delineation, which both have a role in controlling groundwater flow and accumulation in the Ifal Basin. A maximum depth of 3.5 km is estimated for the basement surface by this study. In addition, several faulted and tilted blocks, perpendicularly dissected by NE-trending faults, are delineated within the structural framework of the study area. It is also observed that the studied basin is bounded by NW- and NE-trending faults. All these multi-directional faults/fracture systems in the Ifal Basin could be considered as conduits for groundwater accumulation, but with a possibility of environmental contamination from the surrounding soils and rock bodies.

Three alternative local wavenumber methods are proposed to estimate the depth and the nature (structural index) of the 2D magnetic source simultaneously using various combinations of different forms of the local wavenumbers to compute the source parameters without any prior information about the source. A clustering method is also provided to get more accurate results. The proposed local wavenumber methods are demonstrated on synthetic noise-free and noise-corrupted magnetic data, and they successfully estimate the location parameters and structural index of the causative sources. The actual application of the proposed methods is demonstrated on a magnetic anomaly from southern Illinois.

Undercover mineral exploration is a challenging task as it requires understanding of subsurface geology by relying heavily on remotely sensed (i.e. geophysical) data. Cost-effective exploration is essential in order to increase the chance of success using finite budgets. This requires effective decision-making in both the process of selecting the optimum data collection methods and in the process of achieving accuracy during subsequent interpretation. Traditionally, developing the skills, behaviour and practices of exploration decision-making requires many years of experience through working on exploration projects under various geological settings, commodities and levels of available resources. This implies long periods of sub-optimal exploration decision-making, before the necessary experience has been successfully obtained.

To address this critical industry issue, our ongoing research focuses on the development of the unique and novel e-learning environment, exSim, which simulates exploration scenarios where users can test their strategies and learn the consequences of their choices. This simulator provides an engaging platform for self-learning and experimentation in exploration decision strategies, providing a means to build experience more effectively. The exSim environment also provides a unique platform on which numerous scenarios and situations (e.g. deposit styles) can be simulated, potentially allowing the user to become virtually familiarised with a broader scope of exploration practices.

Harnessing the power of computer simulation, visualisation and an intuitive graphical user interface, the simulator provides a way to assess the user’s exploration decisions and subsequent interpretations. In this paper, we present the prototype functionalities in exSim including: simulation of geophysical surveys, follow-up drill testing and interpretation assistive tools.

Vertical electrical sounding (VES), a resistivity sounding technique, has been applied at two important archaeological sites in the eastern part of the Nile Delta to trace the paleoenvironment, particularly the defunct canals. Like many other archaeological sites in the Nile Delta of Egypt, these two sites have been subjected to urbanisation and agricultural invasion from the local farmers. Therefore, studying the paleoenvironment is an important task for guiding the excavation process and highlighting the importance of these two archaeological sites. The VES stations were arranged to cover the two sites, in the form of traverse profiles for tracing the subsurface sand and gravel facies that intercalated with clay deposits. The acquired VES data were processed based on the available borehole lithological information for the purpose of establishing the resistivity-depth models. Both 1D and 2D processing schemes were applied to the VES data sets to increase the confidence of the obtained results. The clay and silt deposits are characterised by low resistivity values, whereas the sand facies has a relatively high resistivity character. From the constructed cross-sections at the two sites, it was possible to define a consistent character for the clay deposits, which can be inferred as the defunct canals that supplied water to the two sites.

Knowledge of orientations and magnitudes of present-day stresses is important for different applications including fault reactivation, borehole stability and CO2 injection studies. As part of the West Hub Carbon Capture and Storage project, the GSWA Harvey-1 well was drilled in early 2012. It is located ~115 km south of Perth and is used to assess the suitability for CO2 underground storage. The aim of this study is to estimate the mechanical properties and state of stress fields in the Southern Perth Basin. The analysis is based on the newly acquired log and VSP data and results of a rock mechanical model including vertical profiles of elastic and strength properties as well as identified breakout zones. The results indicate that the stress regime in the region is dominantly strike-slip. It changes to a reverse faulting system at shallow depths of below ~900 m. Stress field orientation is obtained from borehole breakout analysis. The average azimuth of the maximum horizontal stress is 106° and the standard deviation is 10°. This direction of the maximum horizontal stress is broadly consistent with the east–west direction earlier reported for the Perth Basin.