Electrical Methods
Shallow EM is a useful non-destructive testing tool for mapping near-surface changes in bulk electrical conductivity. A transmitted EM field is generated by a transmitter coil to induce an electrical current in the subsurface. The induced subsurface secondary field which is created by the induced subsurface electrical current is detected by the EM receiver. The Shallow EM equipment measures the conductivity value at the location the instrument is situated and with GPS tracking of those locations a 2D contour plan of the bulk conductivity distribution of the investigation area can be created. Salt, clay, water, contaminates and also metallic buried objects in the near surface (e.g. pipes, cables, underground storage tanks) can affect the near surface bulk conductivity, which make Shallow EM a useful tool to map this type of subsurface feature.
EM measures the bulk conductivity changes across the site and in general the output conductivity contour plots do not provide depth information. Depths of interpreted geophysical target features/anomalies can be further investigated with the complimentary geophysical technologies Electrical Resistivity Imaging (ERI) and/or Ground Penetrating Radar (GPR). Depending on depth range and EM line spacing, GPR can resolve the position of objects and contacts to an accuracy of centimetres. The effective depth range of Shallow EM can be up to 5 m depth in some geological settings, but the effective depth is dependent on the distribution of subsurface materials. An example of a 2D contour plan of bulk conductivity distribution is illustrated in Figure 1.
Figure 1. Example of an Apparent Conductivity contour plan.
Moving Loop EM (MLEM) is a useful non-destructive testing tool for mapping sub-surface changes in electrical conductivity. At a specified station, or sounding location, a transmitted EM field is generated by a transmitter coil to induce an electrical current in the subsurface. The induced subsurface secondary field which is created by the induced subsurface electrical current is detected by the EM receiver. The MLEM equipment measures the conductivity value with time at the sounding location. The measured conductivity at the station location can be modelled and with GPS locations of the stations, a 2D cross-section of modelled conductivity can be created. Electrical conductivity is the reciprocal of electrical resistivity, therefore if the units of conductivity (mS/m) are known it is possible to present the same data set in units of resistivity (Ohm.m). An example of a 2D cross-section of modelled resistivity data collected using MLEM is shown in Figure 1.
Figure 1. Example of a modelled resistivity TEM cross-section with geological interpretation.
(Image source. http://zond-geo.ru/english/services/equipment/aie-2-instruments/).
Saline groundwater, clay, fresh water and contaminates in the sub-surface can affect the sub-surface apparent conductivity. This makes MLEM a useful tool to map this type of sub-surface feature. If the Client has requested the MLEM stations and/or profile lines to form a grid, it is possible to create contour plan maps of the modelled resistivity/conductivity distribution across the investigation area. The modelled resistivity/conductivity contour maps can be presented at Client specified depth intervals.
Draig Geoscience has extensive experience in the design, acquisition and interpretation of downhole electrical conductivity logging surveys for use as a tool in groundwater, environmental and geotechnical engineering investigations. Downhole electrical conductivity logging can assist environmental and geotechnical engineers by increasing the quantity of extractable information from a pre-established borehole location.
Draig Geoscience has access to a suite of downhole conductivity logging systems able to provide measured borehole interval conductivity readings. The borehole interval conductivity results can be correlated and interpreted alongside the observed geological borehole information. Downhole conductivity provides quantitative measurements of conductivity (mS/m) or resistivity (Ω.m), with measurements relating to changes in porosity, clay content and water quality.
Downhole Conductivity probes are frequency-domain electromagnetic instruments with a continuous sinusoidal transmitter waveform. In the frequency-domain, the secondary magnetic field has components both in-phase and out-of-phase with the primary magnetic field of the transmitter. Borehole conductivity probes typically measure only the out-of-phase component, and the instrument parameters (Transmitter-Receiver spacing, frequency and geometry) are chosen so that the measured response satisfies the so-called Low Induction Number (LIN) approximation. Subject to the LIN approximation, the measured secondary magnetic field is linearly proportional to the conductivity of the material surrounding the borehole. Subject to suitable calibration, the measure data can therefore be transformed to conductivity (or resistivity). Commercially available probes perform this transformation automatically. An example of a downhole electricity curve can be viewed in Figure 1.
Figure 1. An example of a downhole electrical conductivity monitoring borehole location over increasing time.
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