Near Surface Geoscience 2016 - First Conference on Geophysics for Mineral Exploration and Mining

Several physical properties including full-waveform triple sonic, natural gamma, susceptibility, formation and fluid conductivity from downhole geophysics as well laboratory measurements (density, RQD and magnetite content) were carried out and analysed in order to characterize iron-oxide mineralization and their host rocks in the Blötberget mining area in south-central Sweden. It was observed that a strong seismic signal from the mineralization would be generated and that full-waveform sonic data could be used as proxy for fracture mapping and rock quality definition. Fracture systems were also believed to generate modest reflectivity from the calculation of reflection coefficients. While properties like magnetite content or density were clearly indicative of the magnetite mineralization, magnetic susceptibility plots indicated little paramagnetic effect; even host rocks showed ferromagnetic behaviour. We attribute this to a general enriched magnetite mineralization in most types of rocks. This observation is interesting since it may allow joint inversion of gravity and magnetic data possible and thus an estimation of the mineralization tonnage and magnetite content.

The Pindan Falcon survey was flown as part of a mineral exploration program over an area where multiple mafic and ultrabasic bodies intrude metasediments and granitoids. Many of the intrusions have expressions in both the AGG and magnetic data. I present modelling and inversion of one particular co-sourced anomaly pair for which the magnetic anomaly is due to a reverse remanent magnetization. Matched modelling and inversion of the two anomalies provides both density and magnetization information, and substantially increases confidence in interpretation. Both model results support the intrusion having considerable vertical extent, rather than just being a relict of a wider horizontal sheet.The two inversions also identically map their respective property distributions between flight lines, whereas the total gradient anomalies (which should be expected to overly those property distributions), are confined to the flight lines where the measured gradients are steeper than gradients between lines interpolated by the gridding operator.

An aeromagnetic survey of the Coompana area of South Australia has detailed multiple anomalies due to reverse remanent magnetization due to sources beneath a cover of more recent sediments. Inversion of these anomalies has been used to evaluate potential drill sites to investigate the source of this magnetization.Model sensitivity tests have been performed to gain confidence in location of the proposed drill holes. We found that horizontal uncertainty in the location of magnetization due to having to simultaneously solve for magnetization direction is less than the standard vertical uncertainty in determining of depth to the magnetization (which is not strongly influenced by magnetization direction). The shallowest reverse remanent magnetization we have located appears to be at a depth of 200 to 250 metres below surface, and to have an intensity above 10 A/m. We suggest that this high intensity may be due to lamellar magnetization.

The Baomahun deposit in Sierra Leone is likely to become one of the country’s first modern gold mines, and there has been significant interest in exploring adjacent areas for similar deposits.

After commissioning a magnetic/radiometric survey to understand regional geology, Cluff Gold accepted a recommendation from Reid Geophysics to use VTEM for direct targeting of sulphide-hosted gold mineralization within its Baomahun and Victoria project areas.

The geological model of mineralization was pyrrhotite-hosted gold closely associated with BIF and preferentially concentrated in steeply-plunging fold noses.

The following indicators were applied for defining and prioritizing target zones: High magnetic susceptibility to indicate BIFs, and specifically zones associated with folding or discontinuities; High conductivity values to indicate presence of massive pyrrhotite, although medium to lower conductivities (due to more disseminated sulphides) should not be completely disregarded; Elevated arsenic and/or gold soil geochemistry values; Artisanal mining activities in close proximity.

Using the above indicators, eight target zones were selected for follow up drilling and/or ground survey programmes. In 2010, Cluff Gold announced a 6,000m diamond drilling programme to test seven of these target zones.

In January-February 2016 an integrated multi-parameter helicopter-borne geophysical system was flown over the Vredefort Dome Structure in South Africa. The system consisted of an on-board GT-2A gravimeter and a towed-bird ZTEM(Z-Axis Tipper Electromagnetics) with caesium magnetometer. The survey consisted of nine (9) approximately 70km long NW-SE oriented flight lines, totaling 640km, acquired at a nominal spacing of 1000m over an area of approximately 650 square kilometers. The survey data are presented and analyzed with the support of 2D and 3D inversion. The results appear to correlate well with previous geophysical survey data and the known geology. These results demonstrate that high quality gravity measurements can be made concurrently with helicopter ZTEM and magnetics, and therefore represents a potentially useful multi-parameter and deep penetrating geophysical mapping tool.

In the Baveno “Seula” quarry, in North-West of Italy, the mining in the area has three aspects: 1) quarrying the pink granite for the production of ornamental stone blocks; 2) recycling of the debris and of the production scraps from previous mining for the production of cliff’s boulder and smashed inert material; 3) mining the k-feldspar mineral from the mining landfill. In this context, the aim of the geophysical survey was to map the bedrock topography and evaluate the volume of the debris (overburden), which would be used for the processing of feldspar mineral. Secondly, we want roughly to characterize the debris (granulometry) and the quality of the underlying stone. We used the ERT methodology, since we expect a significant resistivity contrast between debris (more conductive due to the presence of water in voids and at the interface with the bedrock) and the compact stone (more resistive). The geophysical survey allowed the identification of different layers, and the top of the granite to be mapped. The excavation operations in some portions of the investigated area reached the bedrock: the electrical resistivity sections clearly show the coincidence between the estimated bedrock location from the ERT results and the real position.

This paper uses a novel approach to constructing an effective preconditioner for finite-difference (FD) electromagnetic modeling in applications to mineral exploration. This approach uses a FD contraction operator, similar to one developed for integral equation modeling. We demonstrate the effectiveness of this new modeling method and corresponding code in the sensitivity study of magnetotelluric data to an ore deposit, typical for Norilsk ore region.

This paper develops a new approach to 3D full-waveform inversion of the acoustic field data. This approach is based on a novel technique to compute the data gradient in the framework of seismic full-waveform inversion carried out in the frequency domain. The quasi-analytical (QA) approximation, which was initially developed for electromagnetic field modeling, is applied to the acoustic field. The advantage of this approach is that the QA approximation is almost as fast as the Born approximation, but much more accurate. We use this approach to reduce the computational load needed to calculate the Fréchet derivative operator required for the inversion. To demonstrate feasibility of the new approach, we inverted synthetic pressure responses of a model mimicking salt tectonics.

Extrapolation of seismic wavefields is useful for determining the location of the seismic source, as well as analysing how the wavefront propagates given different local conditions. One type of such extrapolation is known as reverse-time migration, which is used successfully for various modelling tasks. In a typical mine setting reverse-time migration struggles due to microseismic monitoring arrays being sparse. Sparsity of data makes it difficult to achieve accurate results from extrapolation, so a way of overcoming this is explored. Given the true 3-dimensional geometry within a microseismic array, a method of reconstructing a wavefront using sparse data is introduced. The radial component of each seismic trace is time-shifted to a reference radius, followed by an interpolation on the sphere with this reference radius. This densely sampled wavefront is then used as a starting point for reverse-time migration, and can thus be used to analyse wave characteristics of interest such as peak particle velocities and accelerations. This can lead to a better assessment of hazard, and ultimately a safer working environment.

The Cabinet Office, Government Japan started the Cross-ministerial Strategic Innovation Promotion Program (SIP) in which “Next-generation technology for ocean resources exploration” is scheduled from 2014 to 2018. J-MARES (Research and Development Partnership for Next Generation Technology of Marine Resources Survey) is a private partnership participating in this SIP program organized from four companies (JAPEX, JGI, NSENGI and MMTEC).

Since 2014, 6 research cruises have been carried out by J-MARES, performing mainly vertical cable seismic (VCS) and time-domain electromagnetic (TDEM) surveys of known seafloor hydrothermal fields, such as Okinawa Trough and Izu-Ogasawara Trench. Based on the successful results of these surveys, we are improving these systems including new deep-towed seismic system (ACS, Autonomous Cable Seismic), which is more efficient than VCS. Other than geophysical surveys, we carried out the sampling on seafloor by ROV. J-MARES finally aims to develop and standardize multi-stage and integrated scheme for SMSs exploration by combining geophysical, geological and mineralogical exploration methods.