The geologic medium is a multiphase structure and measured induction process (that we can interpret very well) is usually complicated with processes of induced polarization (IP), superparamagnetism effects (SPM), piezoelectric disturbance and other factors.
Electromagnetic excitation of the medium creates a multi-parameter field which defines the medium’s multiphase “portrait”. The development of the elements of the general electrodynamics theory and methodology of field decomposition creates qualitatively new background for exploration tasks solving. It is especially important on the modern stage of experimental geophysics development and obtained phenomenological knowledge.
To obtain the data of near-surface layer properties with high performance, the technology of EM-scanning and the system of separation of induction fields from polarization fields are created.
Brief description of the EM-scanning technology.
Brief description of the EM-scanning technology
To separate TEM-processes from IP- and SPM-processes during the study of cross-section on the depths up to 100 meters, multichannel registration is used in the “Impulse-Auto” measuring system. The system is based on distance-separated detectors. The central detector registrates TEM+IP+SPM signal. Remote detector registers TEM-signal due to fast decay of IP+SPM processes.
Depending on conditions of performed experiment, the data on studied geological medium, can be formed whether from fixed source of EM-field or from the moving group of transmitter and receiver.
During the scanning, the source of EM-field is configured by differential or integral scheme using spatial or time differentiation of the signal. Fixing of the generator can provide a significant increase in depth of investigation and dense measurement grid, but with the slight loss in details of research. The areal data on studied area is acquired from the moving moving receiver unit, synchronized with the transmitter.
The effectiveness of dense spatial-time system of observation can be estimated on the example of scanning of the conductive disk, which is overlapped with inhomogeneous layer.
The model: a conductive object located under the horizontally inhomogeneous inclined layer. An object to find is a brass disk with with the next parameters: resistivity ρ = 8⋅10-8 Ohm⋅m, diameter D = 50 mm, thickness h = 3.9 mm.
The layer: a plate with 2-side copper coating (ρ = 1.75⋅10-8 Ohm⋅m, h = 35 micron). Inhomogeneity is created by a special ring that is made from the same material as a layer. External diameter of the ring = 54.5 mm, internal diameter = 40.5 mm. The depth of the disk relating to the layer – 20 mm. “Impulse-Auto” equipment was used in the experiment: coaxial transmitter-receiver installation with round transmitter and receiver loops. Transmitter parameters: Radius = 4.8 cm, number of coils = 5. Receiver parameters: Radius = 1 cm, Current = 10 A, The height above the layer = 1 cm. The results are represented in the form of areal distribution of measured signal for fixed time cuts (Fig.1).
Fig.1. Areal distribution of Ez(t) signal over horizontally inhomogeneous medium with local object at different time intervals
Depending on the time cut, signal distribution shows relaxing trend. Disk appears on inversed stage of transient response in the time range of 40-80 mks. In the late stage of transient response, the sign of an anomaly over the disk is positive and exactly coincides with its horizontal projection. The experiment shows that there is the capability to obtain almost photographic precision in searching of objects using spatial-time domain surveys.
- Technology of electromagnetic scanning of surface layer for solving engineering geological problems. G.M. Trigubovich, M.I. Epov, V.V. Voevoda. Report’s theses for International Geophysical Conference and Exhibition EAGE, 15-18 of September 1997. Moscow, 1997.
- Transient geo-electromagnetics. F.M. Kamenetsky, E.H. Stettler, G.M. Trigubovich. Munich, 2010.