13th SAGA Biennial Conference & Exhibition

The success of geophysical exploration depends on the characteristics and ore genetic processes of the deposit being sought after. These properties include mineralogy, structural setting, hydrothermal alteration, geochemistry and weathering processes. The Ysterkop North Prospect is located in the Western limb of the Bushveld Igneous Complex, and its mineralization is endogranitic and hosted within anorogenic and altered Nebo Granite of the Lebowa Granite Suite. The ores at Ysterkop North consist of quartz, hematite, and goethite which are locally accompanied by actinolite and fluorite with hematite commonly replacing the precursor actinolite rock or granite (Crocker et al., 2001). Results showed that the Ysterkop North Prospect is very responsive to geophysical techniques owing to the contrasting physical properties existing between the Nebo granite and hydrothermally altered mineralized part.

The Vaalbos project area is situated in the Northern Cape Province, South Africa, and contains diamondiferous gravel deposits. Eluvial Kalahari sand covers most of the area and limits the surficial exposure of these gravel deposits. Shallow gravel channels are challenging targets to directly resolve with airborne geophysics, however bedrock geology and structure, which are controlling factors in gravel concentration, can be mapped successfully. A GENESIS electromagnetic, magnetic and radiometric survey was interpreted, resulting in a number of likely alluvial gravel deposit targets in the project area to be followed up.

This paper demonstrates how to automatically obtain the location and depth of magnetic field sources of known structural index, such as contacts and dykes, for both profile and map datasets. The method does not require that the magnetic data be pole-reduced or have a known magnetisation vector. The method is applied both to synthetic data and aeromagnetic data from South Africa.

A TEMPEST airborne fixed wing Time Domain Electromagnetic (TDEM) and magnetic survey was flown over the Nuqrah – Mardah project area of central Saudi Arabia for Ma’aden Gold and Base Metals. The Nuqrah massive-sulphide sedex deposit is located within graphite schist and carbonates near the top of the pyroclastic Halaban formation. The survey has identified known mineral occurrences, and located new mineral targets. The north and south zones of the Nuqrah Deposit have been imaged, and further drilling may be indicated at Nuqrah North based on a strike length interpreted from CDI profiles of 1.6 km.

The Goedgevonden and Syferfontein kimberlites are located ~1 km apart and are ~150 km from Johannesburg, and ~25 km NNE of Klerksdorp in South Africa. Both pipes are small (~0.2 ha) and Goedgevonden is diamondiferous, although uneconomic due to low grade and quality. While the location of the Goedgevonden kimberlite has been known since the 1930s, the Syferfontein kimberlite was discovered by a speculative airborne EM survey in 1994. Exploration in the area is complicated due to the presence of thick layers of ferricrete and calcrete which partially resorb and trap heavy minerals making traditional soil sampling methods ineffective. The area around the Goedgevonden kimberlite hosts the highly magnetic shales of the West Rand Group, magnetically variable Ventersdorp lavas and several meters of ferricrete, rendering traditional airborne magnetic data virtually useless due to large numbers of anomalies not associated with kimberlite. However the frequency domain EM method was instrumental in discovering the Syferfontein pipe and also shows a pronounced response over the Goedgevonden pipe. Data for various ground geophysical methods (gravity, magnetic, TDEM, HLEM) were also collected over both pipes and show prominent anomalies. Further follow up with fixed wing and helicopter TDEM shows a significant response over both kimberlites. The EM responses are most likely due to the weathered kimberlite, which is well exposed in the Goedegevonden pipe. In spite of the presence of alluvial diamonds and diamondiferous kimberlites, this region remains under explored; we suggest that EM surveys could prove effective for exploring in this region.

In May 2010 a time domain airborne electromagnetic (TD-AEM) survey was carried out for the Council for Geoscience over two selected areas in the traditional West Rand and Free State gold mining areas of South Africa. The goal of the TD-AEM survey was to acquire a 3D electric conductivity model of the survey areas reflecting the geological layering and potential pathways for rising acid mine waters or leaking contaminant fluids from tailings and slime dams. The TD-AEM Genesis system which was a fairly recent development mounted on a single engine Cessna Grand Caravan 208 was provided by Fugro Airborne Services (FAS). The survey was flown at 200 m flight line spacing, with flight lines oriented N-S and tie lines oriented W-E at 2000 m spacing. The EM transmitter and stinger mounted Caesium vapour magnetometer were flown at about 90 m terrain clearance. The EM receiver bird was closer to surface (45 m) and dragged by the aircraft. The transmitter-receiver (Tx-Rx) configuration is non-symmetric. Base frequency of the transmitter (Tx) was 75 Hz, peak moment 60300 Am2 and peak current 450 A. In high conductivity areas the depth of investigation of the Genesis system has been limited to little more than 100 m, only in low conductivity environment a maximum depth of investigation of about 300 m was achieved. Despite these constraints, the 3D conductivity voxels display the geological layering, highlight tectonic features (folding, faults) and identify individual conductive spots which are possibly associated with leaking tailings. As a whole, the usefulness of fast and cost-effective AEM surveying when tackling the issues of acid mine waters and contaminant seepage has successfully been proven. However, the use of a helicopter-borne AEM system may be preferred with regard to spatial power of resolution and system geometry.

The Orange Basin underlies the Atlantic Ocean off the South African west coast and covers an area of approximately 160 000 km2 to the 2 000 m isobath and the drift sequence can be as thick as 7 km in the north, thinning to 1-3 km in the south. The Orange Basin is largely underexplored with only one well per 4000 km2 and has potential for the successful accumulation of hydrocarbons, possibly in significant quantities. The seismic reflection data obtained in the Orange Basin portray the subsurface in recorded two-way time. All the interpretation for this basin has been done in the time domain, which is quick and adequate for many situations. Stratigraphic interpretation in the time domain can be fine for seismic facies and sequence stratigraphic analyses, where the interpretation remains largely the same with changing structure. However, structural interpretation in the time domain is risky as it means accepting the risk of assuming a constant velocity model, or that all possible velocity deviations can be accounted for by the interpreter. The aim of this paper is to discuss the benefit of acquiring prestack depth migration seismics in future, compared to prestack time migration gathered in the Orange Basin to date. Further 2D and 3D seismic acquisition, which is depth migrated, as well as drilling programs will be of great benefit in mitigating the geological risk. Improved integrity of seismic data through depth imaging, will potentially reduce the exploration cycle time, allow for improved well planning and increase accuracy especially when fault delineation is critical.

New Broadband marine solutions have significantly changed marine Imaging Landscape reaching up to 6 octaves bandwidth. These solutions are the results of a combination of innovative solutions in the fields of Equipment, Acquisition and Subsurface Imaging. Following the same approach we illustrate that similar improvements are achievable in Land.

Mining-induced earthquakes pose a risk to workers in deep mines, while natural earthquakes pose a risk to people living close to plate boundaries and even in stable continental regions. A 5-year Japan-SA collaborative project "Observational studies in South African mines to mitigate seismic risks" was launched in 2010. Here we report on progress since the report at Geosynthesis 2011 in Cape Town. Acoustic emission sensors, accelerometers, strainmeters, and controlled seismic sources have been installed at sites in Cooke, Moab-Khotsong and Sibanye gold mines to monitor the deformation of the rock mass, the accumulation of damage during the earthquake preparation phase, and dynamic stress as the rupture front propagates. These data are being integrated with measurements of stress, stope closure, stope strong motion, and seismic data recorded by the mine-wide network.

The dependency of the quality factor derived from S wave coda (Qc) on frequency is analysed in order to understand the effect of fracturing ahead of a mining stope. Micro seismic events recorded using acoustic emission sensors in a mining environment were used in the analysis. Acoustic emissions due to rock extraction were compared to those due to fault slip. The comparison is done by considering the ray paths of the two groups of events. The ray paths of the face events pass through fractured rock around the mining stope while those due to fault slip pass thought competent rock, away from active mining. Qc was computed from averaging events from the two groups and normalising by the number of events. It was found that the Qc dependency on frequency follows a power law. For face events, the power law is given by Q(f)=0,06f0,99 and Q(f)=0,003f1,58 for fault slip events. Qc derived from face events was found to be lower than that derived from fault slip events. The degree of dependency which is given by the power law exponent was found to be lower for face events. The low dependency value is interpreted as the effect of fracturing ahead of the mining stope. The degree of dependency was found to be comparable to those found for natural earthquakes. This suggests scale invariance of the power law since the results obtained from acoustic emission events are comparable to those derived from natural earthquakes.