EAGE/DGG Workshop on Deep Mineral Exploration

We are proposing a new electromagnetic system that is intended for deep exploration of the subsurface. The system comprises one or more three-component transmitters, which can be magnetic dipoles, or for marine exploration electric dipoles. The three transmitter components can be combined so as to direct the primary field in any direction at any location in the subsurface. This can result in maximal coupling of the fields with the structures of interest. A large number of transmitter locations can be used to increase the signal at the subsurface and hence enhance the response. In a similar way, it is possible to combine the signals from multiple transmitters to reduce the response from other features that are not of interest. When multiple receivers are used, the signal-to-noise ratio can be increased so as to provide maximum depth penetration.

The combination of multiple transmitters and multiple receivers can be undertaken after the survey is completed and this allows the data to be reprocessed to look for different targets in different locations, or for the volume to be investigated in great detail.

Different applications of seismic interferometry have arisen in the last decade, however the potential of this technique to improve reflection seismic processing in hardrock environments has not been regarded explicitly. Therefore, in this paper we investigate the potential of retrieving the first arrivals originally hindered by high noise level in the exploitation of controlled-source data acquired over the apatite-iron deposit at Grängesberg (Sweden) and its mining-induced structures. The supervirtual first arrivals generated using interferometry methodologies allowed first-breaks to be picked more extensively than in the original data. Revised static corrections significantly improved the linearity of the first arrivals and continuity of reflections in the source gathers. Especially, reflections considerably enhanced in the source gathers stacked constructively in the final seismic section. Comparison with geologic data, supported by reflection-traveltime forward modelling, indicates that these reflections represent the deep (> 700 m) and unmined part of the deposit. Other reflections at shallower depth are interpreted as anthropogenic faults possibly located at lithological contacts (pegmatites). Even though the potential of first-arrivals retrieval is likely case-dependent, this study illustrates that interferometry may substantially improve the accuracy of field static corrections and subsequent stack for hardrock imaging and deep mineral exploration.

The Kylylahti copper mine is located in the Outokumpu Cu-Co-Zn-Ni ore district in eastern Finland. We used high resolution reflection seismic profiles imaging the subsurface structures down to depth of 5 km and ZTEM (Z-axis Tipper Electromagnetic) inversion results revealing deep conductivity anomalies to study the deep exploration potential of the Kylylahti area. Regional interpretation of these deep penetrating data suggest that peridotite body of Kylylahti has substantial down plunge extend towards south and underneath the current mine. Additionally to regional interpretation, we have interpolated the densities measured in laboratory from drill-core samples to create a 3D-subsurface density distribution grid and compared it with the seismic reflection data. Overall correlation of reflectivity and density patterns is excellent: high density values correlate particularly well with high amplitude reflections while no prominent reflectivity is observed in the areas where density is uniformly low. The combination of reflectivity and interpolated density distribution suggest that the high density rock units possibly hosting the sulfide ore occur in a steeply dipping feature continuing northwest from the Kylylahti mine along the seismic profile. Results of this study highlight the deep exploration potential of the rock volumes below currently drilled depths in the Kylylahti area.

The two types of Deep Sea Mineral resources which we are exploring at the moment (Polymetallic Nodules and Seafloor Massive Sulfides, SMS) are being found in very different settings and hence require different approaches for exploration with geophysical methods. In both cases, exploration begins with overview measurements of multibeam bathymetry, magnetics, and gravity over large areas carried out at the sea surface with research vessels.

In the case of Polymetallic Nodules, especially the acoustic backscatter data have proven to be highly useful and reliable for classification of the seafloor according to the abundance and size of the Nodules. Further investigations mainly involve geologic sampling.

The occurrence of SMS is connected to active or inactive seafloor hydrothermalism for which indications can be found in morphological structures and magnetic anomalies observed from the sea surface. Since the SMS are confined to very small spots at the seafloor and in some cases buried by sediments, extensive near bottom observations with high resolution bathymetry, magnetic, and electromagnetic methods are required for ground-truthing in prospective areas.

Potential and geological methods are traditionally used to discover Cu-Au deposits. Whilst these methods seem effective, they lack resolution needed to confidently detect deeper mineral deposits. As the search for ore-bodies under regolith cover expands due to depletion of the near surface deposits, there is need to develop new exploration technology to explore these ore-bodies within 1–3km depth range. Seismic reflection method represents this new technology as it offers distinct advantage over all other geophysical techniques because of its depth of penetration and superior spatial resolution. Further, in many different geological environments, it offers a greater “3D mapability” of units within the stratigraphic column. We present two prospect case studies within the Gawler craton (Olympic Dam and Vulcan) deposits. Results show that seismic method could be a useful tool for exploration of deep seated IOCG deposits. The migrated section was not only able to identify and trace various and the complex structures but also shows reflections around the edges of intrusion. In the case of Gawler Craton, IOCG deposits tend to be a later addition to the regional metamorphic events hence seismic reflection surveys might be good for such exploration especially along known faults or edges of major intrusive complexes

New approach for multivariate analysis of EM data MsDEMPCA (Missing data EM Principal Components Analysis) is applied to several large arrays: the Baltic Electromagnetic Array Research (BEAR), Electromagnetic Mini Array (EMMA), Magnetotellurics in Scandes (MASCA), IMAGE obesrvatory data and various EarthScope MT arrays, with the goal of demonstrating how the MV approach can clarify signal and noise characteristics, improve estimates of TFs, and, perhaps most importantly, quantify biases in these estimates.

We present feasibility of applying Discrete Curvelet Transform (DCT) to pre-stack data acquired in the hardrock environment using the Lalor dataset. Two algorithms were developed to help reduce random and coherent noise at various stages of the processing sequence and to obtain a high-quality seismic volume representative of the subsurface. First we demonstrate the ability of DCT to separate coherent events by employing it for ground-roll attenuation. Secondly we tackle the problem of velocity model building for pre-stack time migration (PSTM). We design an automatic workflow for gathers conditioning and moveouts picking which require only minimal human interaction. Robustness of our method is demonstrated by examples convincing that DCT can be effectively used to solve variety of problems encountered during processing and imaging of the seismic data acquired in the ore exploration context.

A combination of audiomagnetotelluric (AMT) surface and borehole measurements can be expected to reduce the ambiguity in models of electrical resistivity derived from surface measurements. For a synthetic 2-D model with a dipping mineralisation zone of ~ 1 Ohmm resistivity and 1.5 km depth extent embedded in a resistive host of 10000 Ohmm underneath weathered overburden of 100 Ohmm, we present a comparison of 2-D inversion models derived from combinations of surface impedance measurements and borehole measurements in the form of vertical magnetic transfer functions and vertical electric transfer functions. We have observed that based on combinations of ground and borehole AMT measurements the resulting inversion models have better pronounced resistivity contrasts at depth than models computed exclusively from surface measurements. To use the borehole transfer functions suggested by us, 3-component borehole sensors sufficiently sensitive to the AMT magnetic field need to be developed. A miniaturised 3-component SQUID magnetometer seems to be a promising solution.