The transient electromagnetic (TEM) method is a commonly used, nonintrusive, geophysical method, but inherent mutual induction between the transmitter (TX) and receiver (RX) coils strongly influences the measurements. We have developed an opposing-coils configuration to greatly reduce this effect. Three coils are used in this system. The upper opposing coil is physically the same as the lower TX coil, and they are concentric and parallel to the middle RX coil. A pair of currents with equal amplitudes but reverse directions is injected into the opposing and TX coils. Theoretical calculations in free space show that the received magnetic field by the RX coil is zero, which indicates that the mutual induction effect could be largely reduced. Physical experiments prove that an almost-pure secondary field could be acquired using this system. We have studied an optimal separation between the TX and opposing coils to guarantee that the primary magnetic field is powerful and the instrument is compacted for field work. Then, the efficient exploration depth of this system for typical geoelectric models was simulated to be approximately 15–50 m. Comparisons of simulated responses over highly conductive thick plates in free space and a field test over a culvert structure between this system and EM-47 showed that the system has enhanced sensitivity and lateral resolution. This system can be used in near-surface investigations, e.g., groundwater, environmental, and engineering investigations.

An induction coil sensor (ICS) is important for measuring low-frequency (DC-1kHz) geomagnetic field. The accuracy of the preamplifier is one key factor determining the performance of the sensor. But the preamplifier is susceptible to 1/ f noise, offset voltage and drift. In order to eliminate these influences, a preamplifier circuit with three amplifier stages based on chopper technology has been designed, and its performance has been tested. The results show that: 1) The 1/ f noise corner frequency is 3 mHz, the equivalent input voltage noise (EIVN) level of the circuit is [Formula: see text] and [Formula: see text]; 2) The equivalent input current noise (EICN) level of the circuit is [Formula: see text]; 3) The offset voltage is about 600 nV, and the time drift performance is excellent. In conclusion, the preamplifier circuit has characteristics of ultralow noise, low offset voltage and low time drift. It can effectively amplify low-frequency weak geomagnetic signals from 1 mHz to 1 kHz and provides excellent performance for low-frequency ICS.

Central-loop TEM sounding data are often interpretead with apparent resistivity as a function of depth. This can quickly give a qualitative understanding of the subsurface resistivity variation. However, when the resistivity contrast between anomalies and background is weak, especially when there is a conductive overburden, the apparent resistivity image is inadequate to quickly and visually identify local anomalies. As an alternative, the apparent resistivity ratios of each measured station to a reference station is proposed as a fast way to identify local conductive anomalies qualitatively. Modeling analyses are carried out on 1D layer model, 2D fault models, and a 3D prism model with conductive overburdens. The results show that the apparent resistivity ratio outperforms the apparent resistivity on intuitively reflecting the target anomaly. As an image enhancement technique, the apparent resistivity ratio performs data transformation on the original apparent resistivity data, normalizing the background response, and highlighting the anomaly. At last, the apparent resistivity ratio method is successfully applied on interpreting a small central-loop TEM survey on investigating karst structure beneath a freshwater river. It provides an alternative simple fast way for interpretation of central loop TEM measurements.

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Hierarchical defects are defined as adjacent defects at different length scales. Involved are the two scales where the stress field distribution is interrelated. Based on the complex variable method and conformal mapping, a multiscale framework for solving the problems of hierarchical defects is formulated. The separated representations of mapping function, the governing equations of potentials, and the stress field are subsequently obtained. The proposed multiscale framework can be used to solve a variety of simplified engineering problems. The case in point is the analytical solution of a macroscopic elliptic hole with a microscopic circular edge defect. The results indicate that the microscopic defect aggregates the stress concentration on the macroscopic defect and likely leads to global propagation and rupture. Multiple micro-defects have interactive effects on the distribution of the stress field. The level of stress concentration may be reduced by the coalescence of micro-defects. This work provides a unified method to analytically investigate the influence of edge micro-defects within the scope of multiscale hierarchy. The formulated multiscale approach can also be potentially applied to materials with hierarchical defects, such as additive manufacturing and bio-inspired materials.