The FDEMS method was introduced in the former USSR at the turn of the 50s and 60s of the last century as an integral part of the triad of induction EM methods (MT, FDEMS, TDEM), which were actively developed in the 50s after the grand discoveries by A.N. Tikhonov and L. Cagniard. The method was not widely used, primarily due to lack of suitable hardware and software for data processing and interpretation. Nevertheless, FDEMS was actively developed in certain regions of Russia and Ukraine until the present days. Interest in the method is supported by the potentially high accuracy of mapping high-resistivity boundaries, since in the FDEMS method there is a direct relationship between the ratio (R / H) of the sounding spacing (R) to the depth (H) to the high-resistivity reference horizon pronounced by significant points of amplitude and phase frequency characteristics (curves). A number of successful FDEMS surveys were completed on the Ukrainian Shield and its slopes, Dnipro-Donetsk basin (Ukraine) and different parts of Russia and Uzbekistan which achieved positive results (1977-2000). To date, the capabilities of modern multifunction and multichannel equipment, software for processing and interpreting field data allows to realize to a large extent the prospective capabilities of the FDEMS method for high-precision mapping of boundaries in the geoelectric section and mapping of low-contrast objects.

The individual abstracts for this session are available to read in the PDF.

Appearance at the turn of the century of the 5th generation multifunction electroprospecting instruments has made significant contribution to the development of electroprospecting techniques. New properties of the equipment include light weight, compact, low power consumption, simple operation, high accuracy of the recorded parameters. One of the defining features is the application of 24-bit ADC. Technological advancements in the microprocessor development and computer technology allowed significantly improving hardware capabilities of the 5th generation instruments, i.e. going to 5+. Super multifunction 4 and 8 channel receivers can be used in both autonomous and manual modes (operator control). Receivers have intuitive user interface implemented on touch-screen display and several communication modes with a PC, touchpads, and smartphone. Instruments have flexible configuration of electric and magnetic channels (channels are interchangeable), which allows to use same receiver for such seemingly incompatible methods Electrical Profiling, IP, Electrotomography, TDEM, MT and MVP. Application of wideband high frequency ADCs and corresponding processors can significantly improve the frequency range of field investigations, as well as noise filtering capabilities. Data acquisition systems are equipped with a wideband electric and magnetic components EM field sensors, as well as a wideband portable transmitter.

Magnetovariational profiling method (MVP) was introduced into the field practice in the 195060s of the last century subsequent to the works of Parkinson (1959), Wiese (1965) and Schmucker (1970). At that time, low frequency variant (10-1000sec frequency range) was widely used for the detection and parameter estimation of large electrical conductivity anomalies in the Earth's crust and upper mantle. The appearance of the 5 th generation of electroprospecting equipment in the 1970-80s allowed MVP method to become regular add-on to magnetotelluric soundings (MT) in the 5-component variant. At the turn of the century, active implementation into field survey practice of the 5 th generation digital multifunction EM instruments resulted in significant increase to the number of carried out AMT measurements, which were widely used for mining exploration. In turn, emphasized was the demand for the induction vector and tipper interpretation techniques. Besides the development of effective 2-D inversions, proposed were express interpretation methods which allowed to estimate the parameters (depth, conductivity, angle of inclination, etc.) of the conductive body during the course of the field survey. Consequently, as well as due to the application of precision field tripods for quick and accurate installation of the magnetic sensors, the 3-component MVP method became an effective independent ground electroprospecting technique for solving wide range of mining exploration tasks.

Seabed EM surveys in shallow water environments have a number of specific particularities. Specifically, there is difficulty with hermetical sealing of EM instruments and the necessity of overcoming electromagnetic noise caused by underwater currents and sea surface disturbances. Another problem is the inability to employ large ocean going vessels in many areas, while the use of small vessels increases the demands on the size and weight of the equipment. There are also certain positive aspects in this, pertaining to the ability to use reliable acoustic communication between the marine EM instrument positioned on the seabed and the sea surface, as well as the use of beacons for identifying the location of the bottom apparatus. In this paper, discussed is a shallow marine EM data acquisition equipment complex that allows efficient use of standard ground EM instruments for carrying out investigations at a sea depth interval of 0-200m. This complex consists of 2x - 4x - 5-channel marine EM systems.

The individual abstracts for this session are available to read in the PDF.

The individual abstracts for this session are available to read in the PDF.

The electromagnetic (EM) methods in the frequency domain (FDEM) have become increasingly popular in the past decade due to their high resolution and sensitivity, as well as high immunity to EM noise. They could be separated in two groups: a) Shallow FDEM profiling techniques which use high frequency bands and mainly utilize only the magnetic components of the EM field; b) Electromagnetic soundings in wide depths intervals, consisting of two sub methods - the impedance soundings (CSAMT) and the individual components EM soundings (FDEMS). The last sub method combines geometric and induction soundings techniques. Both electromagnetic soundings sub methods actively use magnetic and electrical EM field components and, whereas in the CSAMT only the far zone of the control source is being used, the FDEMS engages all three control source zones: nearest, middle and far. Such feature provides a number of significant advantages during field data interpretation. At the present time, hardware capabilities of recently developed wideband (50 000 - 0.0001 Hz) multifunction EM data acquisition systems allow to take full advantage from the application of the high resolution FDEMS method for solving a variety geological engineering and groundwater exploration tasks.

In recent years the 5-component audiomagnetotellurics method (AMT) became very popular for mining exploration and geological mapping. This method actually combines two electroprospecting methods based on the use of the Earth’s natural electromagnetic (EM) field: audiomagnetotellurics (AMT) and magnetovariational profiling (MVP). The first method is highly sensitive to the behavior of the sub-horizontal layers, and the second to subvertical boundaries or bodies. The MVP method, besides being a perfect complement to the AMT, can be used independently for mapping tectonic elements, as well as for exploration of conductive bodies with different shapes. The MVP method allows carrying out cost-effective, environmentally friendly, year-around surveys on any terrain since there is no grounding requirement, as well as the application of 3-component precision field tripods for the accurate and quick installation of induction magnetic sensors. Any 3or 5component fifth generation AMT/MT equipment be used for the application of the method in the field. However, the most cost effective would be 4 or 8 channel multifunction EM receivers Gepard with the MVP method incorporated in the design. The method of express estimation of conductive body parameters (shape, position, depth, inclination and conductivity) based on the MVP data provides unique opportunity to target drilling of the subsurface objects immediately during the MVP field survey.

The preference given to multifunction geophysical EM equipment in recent years is due to two things. On the one hand, the new technical features of modern instruments such as accuracy, wide frequency and dynamic band, similar software design for realization of different methods, intuitive operation, low weight, and portability. On the other hand, the economic considerations such as low cost, reduced net cargo shipping weight, smaller field crew size, required training time, etc. Another recent tendency is to add as many functions as it is economically feasible to multifunction equipment. The multifunction EM receiver GEPARD is largely in line with these tendencies. It has a wide frequency band (50,000-0.00005Hz), high sensitivity, wide signal band (0.1μV-10V), low power consumption, low weight, intuitive operation via touch screen user interface and several data transfer modes. GEPARD has a flexible configuration of channels (4 or 8), where any of the channels could be either electrical or magnetic and each channel has good EM noise protection and a combination of switchable, precise digital and analog filters. GEPARDs come equipped with induction coil magnetic sensors and low-noise non-polarizing electrodes, as well as a portable multifunction current source AT-100 with the corresponding frequency band.

Multifunction seabed EM receivers for electroprospecting field surveys in transition zones (SMMT) and coastal shelf (AUSS) are intended for the use of land-based multifunction EM receivers GEPARD and induction coil magnetic sensors AMS-37. However, it is also possible to use other 2-ch and/or 5-ch geophysical EM receivers that meet the technical specification of the 5th generation instruments. Due to their low weight and physical dimensions, marine EM receivers could be deployed from relatively small vessels. Custom designed retrieval subsystems guarantee the recovery of the instruments and its environmental compliance with the maritime regulations. SMMT and AUSS marine EM receivers could be successfully used for both saltwater and fresh water environments. Special precautions have been taken into account during the design to reduce the noise effects of underwater currents and surface waves.

Typical users of fifth-generation multifunctional equipment, which was put on the market in the late 1990s, showed a preference for a relatively high number of sites measuring two horizontal electric components, and only limited numbers of sites measuring five components of the natural EM field. Five-component measurements were used as local reference sites to calculate apparent resistivity using two horizontal magnetic components for normalization of electric components. Use of the vertical magnetic field was limited mainly to regional surveys. The principal reason for this approach was to lower costs and increase productivity by eliminating the expensive magnetic field sensor, which also requires a relatively longer time for installation in the field compared with the grounded electric-field sensors. As well there are a lot of areas in the world with surface conditions that create significant difficulties for reliable vertical magnetic sensor installation, such as severe winter conditions (frozen ground).