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EMS-IP

Overview

Technology of Electromagnetic Sounding and Induced Polarization (EMS-IP) is a non-intrusive, geophysical method for obtaining subsurface resistivity-conductivity and induced polarization (IP) data.

By using the EMS-IP technology it is possible to identify values of electrical parameters of rock that have a close correlation with the matter composition. These electrical parameters are related with the petrophysical properties of rock and allow to reliably determine the position of sulphides in high- and low-resistance media to a depth of 1 km or more.

The EMS-IP technology is a very promising method for search of base metals, gold-mineralization, kimberlite pipes and hydrocarbon deposits. Due to the possibility to obtain continuous data with the highly detailed step of shooting, to track promising areas and to create three-dimensional models of the geological cross-sections the EMS-IP technology is applicable for the solution of a wide range of ore geophysics problems.

Data of EMS-IP technology are processed by the use of robust regression analysis combined with 1D and 3D inversion within the model of polarizable medium. This approach simultaneously allows to determine the apparent resistivity of the medium and the polarization characteristics of geoelectric cross section.

How it works

The EMS-IP technology uses the complete information about the transient processes recorded by receiving electrodes by passing current pulses through a grounded electric dipole. High-resistivity screens are transparent for induction currents that arise during the turning off the current pulse. In addition, the ground is polarized by the transmitted electric current and when it is switched off, there an effect of induced polarization (IP) related to the chemical composition and physical properties of rocks is appeared. Simultaneous use of information about induction currents and the IP process allows to select low contrast polarizing objects (rocks with disseminated sulfide mineralization) in high resistivity rocks (granite, basalt, etc.).

Applications

The EMS-IP technology is used to map geologic structure in search of geothermal sources, groundwater, kimberlite pipes, ore and hydrocarbons deposits on land and in the transit zone of the shelf. Environmental and engineering uses range from delineating salt-water intrusion and contaminant migration to determining permafrost and depth to bedrock.

EMS-IP advantages over DC resistivity

In modern electrical measurement systems based on the DC registration a sequence of bipolar electric current pulses is used for excitation of geological cross section. As a general rule, a pulse sequence with pause is used to measure IP whereas the pulse sequence without pause is needed for apparent resistivity registration. In the EMS-IP technology registration of transients allows to pass from the use of apparent electrical parameters of the medium to the one-dimensional or three-dimensional inversion of electromagnetic field formation of horizontal electric dipole taking into account the frequency dispersion of electrical conductivity.

This approach significantly increases the information content of the traditional methods of DC registration and gives the opportunity to study lithology of geological cross section in more details. In addition, high-resistance screens as a permafrost, basalts, traps, etc., are transparent for induced currents, while for DC methods they significantly limit the range of possible applications. Rather than to suppress the induction effect, which is a hindrance to the traditional methods of the IP, in the EMS-IP technology it is recorded with minimal distortion. Data of transients recorded from the grounded source of electric current by grounded receiving multielectrode line are processed by the use of robust regression analysis combined with 1D and 3D inversion within the model of polarizable medium. This approach simultaneously allows to determine the apparent resistivity of the medium and the polarization characteristics of geoelectric cross section to the depth up to 1 km.

Logistics

The EMS-IP is non-intrusive and low-impact technology. For large-scale surveys a typical field crew consists of three or four people, with one pick-up truck. Equipment for the receiver site is carried by backpack. Equipment consists of a generator, a grounded generator line, a receiver with grounded receiving line, field notebook.

Principles

The EMS-IP technology begins with transmitting a time-domain, square-wave bipolar pulses into a long grounded generator line.  Pulses are separated by pauses which allows to extract information about transients from recorded signal on receiving line.

By using the EMS-IP technology it is possible to identify values of electrical parameters of the medium that have a close correlation with the matter composition. For a description of the frequency dispersion of electrical conductivity the Cole-Cole model is used, where the transients are described by three variables: polarizability, relaxation time and index of degree. These variables are related with the petrophysical properties of the medium and allow reliably determine the position of sulphides in high- and low-resistance media to a depth of 1 km or more.

Duration time of pulses and pauses is 125 milliseconds. Each decay waveform is recorded at 50 discrete time intervals or “time gates” following the primary inducing pulse. This typical time pulses and pauses intervals is enough to confidently record IP effects.