Audio frequency magneto-telluric (AMT) was first implemented for mineral exploration applications back in 1980/1990. Starting from 2002-2003, through the numerous projects accomplished by Nord-West, it became a widely used technique. Nowadays, AMT is definitely an inalienable part of subsurface imaging technology and one can hardly imagine geophysical exploration of ore minerals without using this method. So far, Nord-West has collected over 15 thousand AMT stations.

Many of the ore deposits are related to the intrusions. Intrusive massifs often occupy large volume of rocks and contain ore-bearing formations. Electrical properties of the intrusions can vary significantly, depending on a mineral composition. Usually, intrusive bodies display higher resistivity compared to a host rock. Intrusive rocks could be characterized by lower resistivity, if the formation or it's part contains impurities with electronic conduction. Thus, in a majority of situations, electromagnetic imaging is in favorable conditions and demonstrates the advantages in studying the intrusion's configuration and properties.

In some ore provinces, the bedrock and ore deposits are covered by loose younger sediments. In those settings AMT is used to study the composition of the folded basement and for mapping its top, imaging the structures, revealing ore-bearing formations, mapping the top of the basement. Refining the boundaries between different geological units becomes really important if the deposits in the survey area are known to be related with some particular geological formation. If those formations are characterized by different resistivity values, this problem is efficiently solved with AMT which also allow both to trace the boundaries buried under the quaternary sediments and images the structure between them as a whole.

Ore-bearing metasomatites are usually significantly reworked by hydrothermal alteration, foliation and other mechanisms, which lead to the change in physical properties. The decrease in magnetic susceptibility as well as in density and electrical resistivity is usually observed due to the alteration processes like chloritization and kaolinization. In contrast, silification leads to higher resistivity values and lower naturally occurring radioactivity of the host rock.

In most cases, the resistivity of ore minerals is several orders of magnitude lower than the host rock resistivity. Massive ores and vein ores display intensive conductivity anomalies and EM techniques, including magneto-telluric are being the main tools for geophysical exploration of such targets.

The efficiency of EM exploration gets lower (especially in case of galvanic field excitation, used in DC resistivity method) if the ore component is present in a host rock in form of impurities. However, our experience shows that in such conditions AMT remains efficient enough to image the ore-bearing formation even if the ore concentration is a few percent.

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Spectral IP with phase measurement was developed by the Soviet geophysical research institutions back in 1960-1980. In contrast to the conventional time domain induced polarization, this technique provides higher accuracy of the measured data and better productivity in the field.

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It is known, that Induced Polarization (IP) can be characterized in terms of chargeability (expressed in %) and the rate of medium charging-discharging. Studying the IP responses either in Time or Frequency domain allows to distinguish the IP anomalies of different origin.

One of the main challenges of IP application in mineral exploration is that sulfides, being the main targets in IP prospecting, and widely spread coal or graphite rocks, display high chargeability values, which makes the distinguishing between them a complicated problem. For example, ore deposits often occur within black shale formations. In order to solve this problem and efficiently recognize the nature of IP anomaly we analyze the shape of the IP decay response. Graphite is characterized by slower decay and could be clearly distinguished from sulphides, based on this criterion. Studying the responses implies measuring to be carried out with an unprecedentedly high accuracy, which is achieved by the use of powerful current sources as well as phase measurements.

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Since 1998 we’ve been developing and manufacturing our own geophysical portable instruments and signal processing software for DC resistivity imaging (including tomography), IP, CSEM in frequency domain and other techniques. Among our products there are MARY multifunctional receiver, IMVP multichannel receiver, ASTRA portable transmitter, Octopus software package for signal processing and more. Our advantage, compared to other geophysical service companies, is that we have the possibility to constantly improve both hardware and software and implement innovative designs, keeping our technology at the highest level of industrial and research standards.

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mplementation of multi-electrode acquisition arrays for DC resistivity imaging of essentially inhomogeneous media (tomography) has become a general trend in recent years. Resistivity tomography is an entire complex, including measurement methodology, as well as an approach to process and interpret the data, which is usually done with high-performance 2D and 3D inversion codes.

By the use of standard resistivity tomography instruments (for example, the French-made Syscal Pro system) one can study the resistivity structure up to the depth of 100-120 m, which is normally not enough for ore prospecting. Nord-West has developed improved tomography techniques based on our hardware designs, allowing to infer the resistivity structure up to 300-400 m in depth.

Typical resistivity tomography array specs:

• Step along the profile - 6m
• Electrode separation - 3m
• 3-electrode array AMN+MNB
• АО separation (4.5-121.5m)
• Number of measurements - 1600
• Profile length - 360m

Array specifications used for the imaging up to 200-4000 m in depth:

• Source dipole length – 200m
• Receiver dipole length 25 to 4000m
• Number of source-receiver separations: 10. Maximal separation – 1100m
• Step along the profile 100m

or

• Source dipole length – 1km
• Receiver dipole length 20 to 200m
• Number of source-receiver separations: 15. Maximal separation – 1480m

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When the soviet geophysical industry collapsed, most of the mineral petrophysics labs were ruined. Measuring petrophysical properties of the ore specimens are very different from the measurements, done in oil petrophysics, because in the first case we mostly deal with igneous and metamorphic rock instead of the sedimentary rock. Due to the low porosity, high contact resistance and magnetization makes the laboratory measurements a challenging process which requires specialized instrumentation and high specimen quality.

In recent years, we’ve begun doing petrophysical measurements (mostly focusing on measuring IP responses) which are of great importance for recognition of the anomaly origin and help to significantly increase the accuracy and reliability of the interpretation.

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The depth of mineral exploration is constantly growing. For example, in Norilsk Ore Region, the targets are located at depth of approximately 1000-1500 m; Polymetallic ore deposits in Altai Region – about 1000 m and so forth. Large depth makes the conventional methods (first of all, DC resistivity/IP) inefficient. In such applications, the successful results can be expected to come from borehole-onland, and borehole-borehole (interwell) imaging. The simplified approaches, like charged-body method, in most situations don't provide any additional information compared to onland surveys. The only way to significantly increase the information outcome is to use multielectrode interwell data and 2D inversion codes for its interpretation. This technology is called interwell imaging or tomography.

During the last years we have been developing an interwell resistivity tomography technique for imaging the rich sulfide ore, embedded into the bottom part of the layered intrusions in the setting of Norilsk ore region. This work is being done by the specialists of Nord-West’s research and development department in cooperation with Moscow state university (Faculty of Geology) and Norilsk Nickel mining company.

The interwell survey carried out in that area, covering more than 1 km in depth, has shown that acquiring multi-electrode interwell data and their subsequent 2D-inversion allows to recover the resistivity and chargeability structure between the existing boreholes. All what is required to get the reliable result is a sufficient contrast between the target object and the background rocks in terms of electrical properties.

Interwell tomography emerged as a modification of a standard onland resistivity imaging, which has been developing in Russia and overseas during last decades. The physical concept used in this technique is similar to that used in DC resistivity method, namely, the depth of the study increases with the increase of source-receiver separation. The thing which is different from conventional DC resistivity is multiple overlapping of the source (current) ant receiver (potential) electrodes. For this reason, multichannel loggers and multielectrode multiwire streamers are used in this technology. This approach allows to recover the resistivity image of the essentially heterogeneous media, which is the main difference from the conventional geometrical sounding.

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Nord-West owns lots of software tools for geophysical data interpretation, analysis and integration of the results into generalized projects. We have developed and are currently using up-to-date software with capabilities for signal processing and data inversion, which can be used for interpretation of DC Resistivity, Induced Polarization and Controlled Source EM Frequency Domain data.

A number of software packages are available for Resistivity Tomography data inversion (RES2DINV, ZOND2DRES, ZONDCHT), as well as gravity and magnetic data analysis and inversion (Geosoft Oasis Montaj, Coscad 3D, TM-2D, TG-2D).

Also, we have some MT-related software tools (MT-Corrector, MTS-Prof, MT2DTools, REBOCC, WSInv3DMT and some 2D inversion codes).

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Nord-West is constantly in touch with the leading Russian scientific institutions, including universities and research centers. We organize conferences, workshops and educational courses for students. Some 20 students from different universities attend our field training courses each year.

• Lomonosov Moscow State Univercity
• Gubkin Russian State University of Oil and Gas
• Sergo Ordzhonikidze Russian State Geological Prospecting University

• Schmidt Institute of Physics of the Earth
• Shirshov Institute of Oceanology
• Geological Institute

• Siberian Research Institute of Geology, Geophysics and Mineral Resources – SNIIGGiMS
• Karpinsky Russian Geological Research Institute