Frequency-domain Electromagnetic System Research Articles

Design of Motion Attitude Measurement System and Correction Method in Semi‐Airborne Frequency Domain Electromagnetic Detection

In the semi‐airborne frequency domain electromagnetic detection, strong noise will be generated due to the attitude change of the receiving coil sensor. However, the existing attitude measurement system cannot be directly used in the semi‐airborne frequency domain electromagnetic system because of the requirement on the volume and accuracy of the instrument. In order to reduce the effect of attitude noise, this paper analyzes the cause and influence of attitude noise in the semi‐airborne frequency domain electromagnetic system, and presents the attitude noise correction method based on coefficient rotation matrix and Kalman algorithm. A high‐performance attitude sensor based on (Micro‐Electro‐Mechanical System (MEMS) technology is used to design an attitude measurement system for the semi‐airborne frequency domain electromagnetic system. The calculated result shows that the required accuracy of the roll angle and pitch angle of the attitude measurement system is less than 0.1°, whereas the required accuracy of the yaw angle is lower. In the field application, the apparent resistivity results show that the imaging profile corrected by attitude error is more consistent with local geological data than that without correction. Consequently, the attitude measurement system and correction method proposed in this paper can improve the detection accuracy of the semi‐airborne frequency domain electromagnetic system significantly. © 2024 Institute of Electrical Engineer of Japan and Wiley Periodicals LLC.

Sensitive characteristics of contributed region and detectability analysis for semi-airborne frequency-domain electromagnetic system

There is restricted research on the sensitive characteristic and detection ability analysis of semi-airborne frequency domain electromagnetic system at present. However, it is critical to the optimal design of the observation scheme, and it may be beneficial for the optimal selection of the record region and increasing the resolution of apparent resistivity imaging. So it is necessary to analyze the sensitive characteristic of the semi-airborne frequency domain system. In this paper, the primary electric field based on the closed-form expression of the electromagnetic field in the underground excited by a horizontal electric dipole is calculated, each element of the underground will be regarded as a current source, and the secondary magnetic field can be obtained based on tensor Green's function. The sensitivity is defined by the contribution of each element to the total secondary magnetic field at the receiving point, and the sensitivity distribution characteristic of the related region is analyzed. Numerical experiments reveal that the region with high sensitivity can produce a strong relative anomaly, and the contribution of the region with sensitivity near zero is almost negligible. To verify the analysis for sensitive characteristics and detectability, we have conducted a field test, after the design of the observation scheme, the data obtained reflects the underground fault zone well. Thus, the relationship between detectability and sensitivity is worth studying.

Design of an airborne concentric frequency-domain electromagnetic induction system for shallow subsurface geophysical exploration

For the exploration of shallow and small subsurface targets, rigid-beam helicopter-borne electromagnetic systems are often inconvenient or costly, whereas man-portable or ground-based systems are difficult to operate in complex terrain. An airborne concentric frequency-domain electromagnetic system with an unmanned rotorcraft as a flying platform is proposed to avoid the problems of low security in conventional helicopter-borne electromagnetic method and low efficiency in ground-based electromagnetic method for the exploration of shallow subsurface targets. Compared with the series mode of man-portable system, the currents of transmitting and bucking coils are assumed to be separately and continuously adjustable, which can reduce sensitivity of the small errors of the system. In this paper, the principle of canceling the primary magnetic field is analyzed. The design index of the physical parameters of the sensor is determined. The simulation experiment of layered earth model is carried out to verify the feasibility of the proposed system.

High-Resolution Surveying With Small-Loop Frequency Domain Electromagnetic Systems: Efficient Survey Design and Adaptive Processing

The range of near-surface applications whereby the electrical and magnetic properties of the subsurface are investigated expands continually [1], and, particularly for terrestrial applications, increasingly higher spatial sampling densities are deployed [2]. Within the broad range of geophysical methods, small-loop frequency-domain electromagnetic (FDEM) instrumentation is often used to record variations of multiple EM properties simultaneously [3]-[7]. Such instruments typically incorporate at least one transmitter in combination with one or more receivers. The resulting transmitter-receiver pairs are commonly configured in either a horizontal coplanar (HCP), vertical coplanar (VCP), vertical coaxial (VCA), or perpendicular (PRP) orientation [8], [9]. A distinction can be made between symmetrical (e.g., HCP, VCP, and VCA) and asymmetrical (e.g., PRP) coil configurations [10], [11] in relation to survey heading. Although the former render an isotropic response unaffected by the acquisition direction, an asymmetrical configuration will produce different responses based on survey heading.

Dual Transmitter–Receiver Electromagnetic System for Lateral Boundary Detection of Subsurface Formations

A new frequency domain electromagnetic system, based on different working principle has been designed and its efficacy tested over the existing systems through laboratory-scale-model studies. In this system, two transmitter coils have been employed to generate a magnetic null plane at their geometric centre. The receiver coil is placed in the null plane to record the induced secondary field. The interaction of the primary field is almost negligible on the secondary field recorded by the receiver. We present the theory and physical model results describing the system parameters and efficacy. The testing through physical model studies suggests an increased depth of detection in this new configuration compared to the existing systems. In terms of secondary field, the strength of the anomaly reflects the magnetic permeability/susceptibility difference of the subsurface medium on either side of the receiver. The study concludes that there is significant increase in depth of investigation and secondary field strength in this system over the existing conventional frequency domain systems and also more robust for boundary detection.

Electromagnetic mapping of saline contamination at an active brine pit

Frequency-domain and time-domain electromagnetic methods were used to investigate groundwater contamination at an active brine pit in southwestern Manitoba, Canada. The objectives of the survey were to delineate contamination suspected to be occuring at the site and to compare frequency-domain electromagnetic (FDEM) and time-domain electromagnetic (TDEM) measurements in a survey area containing pipelines, fences, and power lines. The survey successfully delineated a region of high conductivity around brine pit, confirming that leakage is occurring from the pit. Modelling of the FDEM results suggests the contamination is spreading within a series of shallow sand units. Comparison of FDEM and TDEM survey results indicate that small-separation FDEM systems are much more useful for mapping in a developed area containing sources of cultural noise. The FDEM systems permit rapid mapping of spatial variations in conductivity, are affected to only a limited degree by cultural features, and provide some resolution of the depth variation of conductivity at shallow depth. It was not possible to obtain useful TDEM measurements anywhere near the active brine pit because of the signal distortion in the late-time response. Key words: geophysics, electromagnetic, brine pit, saline contamination.

Exploration of mid-ocean ridges with a frequency-domain electromagnetic system

SUMMARY Electromagnetic-survey methods using controlled sources have been applied to ocean floor and mid-ocean-ridge surveys and have shed new light on the distribution of fluids within these structures. The present state of knowledge of the electrical nature of oceanic crust is reviewed and some alternative mid-ocean-ridge electromagnetic surveys are simulated using a numerical-modelling algorithm to see what further information they could provide. This simulation considers the response of both large-scale crustal targets such as magma bodies and zones of hydrothermal circulation, and also the effect of near-surface heterogeneity that may spatially alias the data. The magnitude of this heterogeneity is estimated and it is shown that it could be large enough to prevent useful structural constraints being obtained. If the effect spatial aliasing was negligible, or could be removed, a trans-ridge experiment could delinate zones of partial melting and hydrothermal circulation within the oceanic crust.