ASEG Extended Abstracts - 24th International Geophysical Conference and Exhibition – Geophysics and Geology Together for Discovery, 2015

The seismic reflection method provides the possibility for delineation of very complex geological and this method might be good for detecting the presence of Iron Oxide Copper-Gold (IOCG) deposits. Despite many technically superior attributes, no arguments for regional seismic exploration have been proposed; probably because a cost-benefit analysis has never been conducted at such a scale. In this study we analyse such a case by modelling a Hillside IOCG deposit scenario where 2D seismic with relatively sparse source-receiver geometry is used to detect the presence of a possible intrusive package near a deep fault.

The modelling results show that seismic reflection using 20m geophones and 40m shot spacing as an exploration tool is feasible, and that with the spacing halved we can definitely recover reasonable images of the upper parts of the mineralisation. The presences of such intrusives are clearly detectable and with the seismic method are detectable from 100m to 1000m deep. Thus, we propose that using 2D seismic is viable for IOCG exploration as it can detect mineralised intrusive structures along known favourable corridors or structures.

Unconstrained inversion of surface magnetotelluric data generates non-uniqueness solutions. Boundaries derived from seismic reflectively images have the potential to substantially improve MT inversion. Seismic should be highly benefic ial where significant and strong reflectors can reasonably be associated with contrast in electrical conductivity across well-defined relatively continuous boundaries. We show how seismic reflections can assist in defining such inversion controls as the smoothness penalty across known boundaries. We apply and compare a range of cooperative inversion strategies using large scale co-located magnetotelluric and seismic reflection field data sets from the Carlin style gold district in Nevada USA.

In March 2012, a helicopter-borne VTEM electromagnetic (EM), magnetic and radiometric survey was flown over the Cerro Quema high sulphidation epithermal gold deposits in Panama. Geophysical signatures, including Airborne Inductive Induced Polarization (AIIP) effect, characteristic of high sulphidation epithermal gold deposits were observed in the EM, magnetic and radiometric data over the known deposits. This success points to the applicability of regional helicopter EM-Mag-Spec surveys for the exploration of similar high sulphidation epithermal gold deposits to depths <500m in weathered terrains.

Conductivity-depth sections (CDI) produced from a recent airborne TEM exploration survey showed a poor fit to the expected geology of the area (a known conductive layer was appearing deeper than expected). The source of the problem was found to be the use of an incomplete description of the system geometry which had the effect of dramatically scaling the secondary field. In many modelling programs, including in this case EMFlow, the system geometry may be used to determine transmitter-receiver coupling which is used to compute the apparent primary field. This paper explains why system geometry can be critical for precise modelling of TEM data. By specifying the correct transmitter orientation and de-rotating receiver pitch for both primary and secondary fields, the match between known geology and CDI depth was greatly improved.

A geoprocessing methodology has been developed to capture potential field anomalies from residual gravity and reduced to pole total magnetic intensity (RTP TMI) datasets. Anomalies captured using this process are converted to GIS polygons and attributed with descriptive statistics of the underlying grids. The polygons are subsequently included as criteria in a GIS analysis to target IOCG-style deposits beneath extensive cover in the Eastern Gawler Craton. The application of gravity anomaly polygons within this study was as potential trap sites, based on the assumption that a localised increase in density manifested as an anomalous gravity response may be associated with mineral accumulation. The characterisation of residual potential field anomalies for use in prospectivity modelling has resulted in the accurate identification of existing deposits of IOCG-style mineralisation and has suggested additional targets warranting further investigation.

Most of the migration techniques require an input velocity model. Velocity analysis is one of the most critical stages in seismic data processing. Standard ways to find the velocity model are constant velocity stack and Semblance velocity analysis, which can be time consuming and labour intensive. In this work, we introduce a new approach to obtain the migration velocity and the relevant pre-stack time migration algorithm that is time effective and does not require any input velocity model prior to imaging. The velocity components, in each point in a common source gather, are achieved by calculating the radius of the curvature of seismic reflected wave-front. The corresponding velocity formula is a function of local derivatives of two way travel times with respect to the position of receivers. Computational experiments with synthetic seismic data examples confirm the theoretical expectations and demonstrate the feasibility of the proposed technique.