All-time Apparent Resistivity Research Articles

Airborne transient electromagnetic imaging method based on wavelet neural network

The airborne transient electromagnetic method is a crucial technique for near-surface exploration in rugged terrains. In this study, we introduce a multi-input, single-output double-hidden wavelet neural network design for airborne transient electromagnetic pseudo-resistivity imaging. We construct uniform half-space, stratified stratum, and three-dimensional geoelectric models. By evaluating various performance indicators of neural networks that employ different wavelet basis functions as activation functions, we identify the most suitable wavelet basis functions. The quasi-resistivity is computed using both the wavelet neural network and the backpropagation neural network and then juxtaposed with traditional apparent resistivity. Our findings indicate that the wavelet neural network's quasi-resistivity aligns more closely with the resistivity of the real model. It is also more responsive to low resistivity anomalies than the conventional apparent resistivity translation algorithm. The wavelet approach moderates the undershoot or overshoot occurrences during abrupt stratum changes, offering a more accurate representation of subterranean electrical properties. When compared to the backpropagation neural network, the wavelet neural network provides a superior fit for the model, rendering a smoother quasi-resistivity depth curve. Therefore, it stands out as an improved method for pseudo-resistivity imaging. By processing survey data via the trained wavelet neural network, we find that the all-time apparent resistivity and quasi-resistivity align well with real-world situations. The wavelet neural network prominently showcases the low-resistance calculation results, offering a broader resistivity range. This clarity enhances anomaly detection, confirming the wavelet neural network's applicability and practicality for airborne transient electromagnetic imaging.

Transient electromagnetic perspective technology in the ultra-long coal mining face

The ultra-long coal mining face has the advantages of high recovery efficiency and low production cost; however, the widening of the coal face poses the difficulty of detecting the hidden geological bodies inside the coal face, and using conventional geophysical methods gives rise to imprecise detection results. Therefore, this paper proposes a transient electromagnetic perspective technology based on the conventional mining transient electromagnetic method. A long rectangular loop is used as the transmitting source in one coal mining tunnel of the coal mining face, and it is received in the opposite coal mining tunnel. This method is characterized by a long detection distance and high construction efficiency. Firstly, the paper used numerical simulation to investigate the spread of the transient electromagnetic field from the long rectangular loop source in both a uniform full-space model and a full-space laminated model. This showed the transient electromagnetic field propagation law and the effect of the coal seam thickness. Then, we analyzed the transient electromagnetic perspective signal characteristics of typical geological structures and gave the all-time apparent resistivity calculation method. Conclusively, the feasibility of this method is ultimately confirmed by the analysis of the measured data. Also, the research results provide a new technological approach to detecting hidden geological structures in the ultra-long coal mining faces.

Resistivity-depth Imaging with the Airborne Transient Electromagnetic Method Based on an Artificial Neural Network

We developed an artificial neural network to map the distribution of geologic conductivity in the earth subsurface using the airborne transient electromagnetic method. The artificial neural network avoids the need for complex derivations of electromagnetic field formulas and requires only input and transfer functions to obtain a quasi-resistivity image. First, training sample set from the airborne transient electromagnetic response of homogeneous half-space models with different resistivities was formed, and network model parameters, including the flight altitude, time constant, and response amplitude, were determined. Then, a double-hidden-layer back-propagation (BP) neural network was established based on the mapping relationship between quasi-resistivity and airborne transient electromagnetic response. By analyzing the mean square error curve, the training termination criterion of the BP neural network was determined. Next, the trained BP neural network was used to interpret the airborne transient electromagnetic responses of various typical layered geo-electric models, and the results were compared with that from the all-time apparent resistivity algorithm. The comparison indicated that the resistivity imaging from the BP neural network approach was much closer to the true resistivity of the model, and the response to anomalous bodies was better than that from an all-time apparent resistivity. Finally, this imaging technique was used to process field data acquired by employing the airborne transient method from the HuaYin survey area. Quasi-resistivity depth sections calculated with the BP neural network and the actual geological situation were in good.

Small multi-turn coils based on transient electromagnetic method for coal mine detection

To improve the detection of underground anomalies, a transient electromagnetic method with coincident loop is devised and the derivation of late-time and all-time apparent resistivity formula for a coincident loop is presented. The derivation takes the induced electromotive force generated from the coincident loop in a homogeneous whole space into consideration. A translation algorithm for the all-time apparent resistivity in half space is extended into the whole space. The current turn-off affect the detection ability of the instrument and the interpretation of the data, so a general procedure to correct this effect is introduced to compute the transient electromagnetic response of the whole-space considering ramp turn-off time. In addition, unlike the conventional measurement method, the sector detection technique is used to improve the accuracy of the underground transient electromagnetic method. The results from two field examples show good consistence with hydrological and geological information, indicating that the robustness and reliability of our theory and technical scheme.

Apparent resistivity for transient electromagnetic induction logging and its correction in radial layer identification

We propose an algorithm for calculating all-time apparent resistivity from transient electromagnetic induction logging. The algorithm is based on the whole-space transient electric field expression of the uniform model and Halley's optimisation. In trial calculations for uniform models, the all-time algorithm is shown to have high accuracy. We use the finite-difference time-domain method to simulate the transient electromagnetic field in radial two-layer models without wall rock and convert the simulation results to apparent resistivity using the all-time algorithm. The time-varying apparent resistivity reflects the radially layered geoelectrical structure of the models and the apparent resistivity of the earliest time channel follows the true resistivity of the inner layer; however, the apparent resistivity at larger times reflects the comprehensive electrical characteristics of the inner and outer layers. To accurately identify the outer layer resistivity based on the series relationship model of the layered resistance, the apparent resistivity and diffusion depth of the different time channels are approximately replaced by related model parameters; that is, we propose an apparent resistivity correction algorithm. By correcting the time-varying apparent resistivity of radial two-layer models, we show that the correction results reflect the radially layered electrical structure and the corrected resistivities of the larger time channels follow the outer layer resistivity. The transient electromagnetic fields of radially layered models with wall rock are simulated to obtain the 2D time-varying profiles of the apparent resistivity and corrections. The results suggest that the time-varying apparent resistivity and correction results reflect the vertical and radial geoelectrical structures. For models with small wall-rock effect, the correction removes the effect of the low-resistance inner layer on the apparent resistivity of the larger time channels.

Analysis of Long-offset Transient Electromagnetic (LOTEM) Data in Time, Frequency, and Pseudo-seismic Domain

Long-offset transient electromagnetic (LOTEM) has received great attention in mineral, hydrocarbon and hydrogeological investigations for the last several years. Conventionally, TEM soundings have been presented as apparent resistivity curves as function of time. With development of sophisticated computational algorithms, it became possible to extract more realistic geoelectric information by applying inversion programs to 1-D and 3-D problems. Here, we analyze LOTEM data by carrying out analysis in time, frequency and pseudo-seismic domain supported by borehole information. At first, H, K, A & Q type geoelectric models are processed using a proven inversion program (1-D Occam inversion). Second, time-to-frequency transformation is conducted from TEM ρa( f) curves for the same models based on all-time apparent resistivity curves. Third, 1-D Bostick's algorithm was applied to the transformed resistivity. Finally, EM diffusion field is transformed into propagating wave field and constructed pseudo-seismic section. The transformed seismic-like wave indicates that some reflection and refraction phenomena appear when the EM wave field interacts with a geoelectric interface at different depth intervals due to contrast in resistivity. In all three cases, synthetic tests showed that conductive anomaly in resistive host environment can be retrieved more clearly than resistive target. A case study illustrates the successful application of proposed approach in recovering a water-filled mined-out area in a coal field located in Ye county, Henan province, China. The results support the introduction of pseudo-seismic imaging technology in long-offset version of TEM which can also be an useful aid if integrated with seismic reflection technique to explore possibilities for high resolution EM imaging in future. [Figure: see text]

Research on Application of all Time Apparent Resistivity Translation Algorithm for Large Fixed Loop TEM

In order to calculate the all-time apparent resistivity for large fixed loop more quickly, the transient electromagnetic field analytic expression for lager fixed loop in homogeneous half space is analyzed in this paper. The facts that the curve has a characteristic of translational expansion and contraction with the conductivity, the test point coordinates and observation time are discovered. A new method using translation algorithm to calculate all-time apparent resistivity is proposed for the large fixed loop. A new calculation equation of all-time apparent resistivity for large fixed loop is derived which shows that the method is feasible and effective.

Discussion on Method of Eliminating Large Loop Source TEM Border Effect

Large loop source TEM method has played an important role in minerals and engineering exploration. But there is a strong border effect when one explains by central loop later-time apparent resistivity. In this paper, a method was used to calculate all-time apparent resistivity by magnetic dipoles composing loop. For 1D forward data, all-time apparent resistivity was computed and compared with later-time apparent resistivity. Conclusions have been drawn that all-time apparent resistivity has no border effect on homogeneous ground model; but for complex layered model, all-time apparent resistivity border effect is reflected in vicinity of false extreme point of apparent resistivity curve, and is much weaker than later-time apparent resistivity border effect. The conclusions are conducive to a correct understanding of using all-time apparent resistivity to eliminate border effect and guide correct interpretation of measurement data.

Calculation of all-time apparent resistivity of large loop transient electromagnetic method with very fast simulated annealing

In large loop transient electromagnetic method (TEM), the late time apparent resistivity formula cannot truly reflect the geoelectric model, thus it needs to define the all-time apparent resistivity with the position information of measuring point. Utilizing very fast simulated annealing (VFSA) to fit the theoretical electromagnetic force (EMF) and measured EMF could obtain the all-time apparent resistivity of the measuring points in rectangular transmitting loop. The selective cope of initial model of VFSA could be confirmed by taking the late time apparent resistivity of transient electromagnetic method as the prior information. For verifying the correctness, the all-time apparent resistivities of the geoelectric models were calculated by VFSA and dichotomy, respectively. The results indicate that the relative differences of apparent resistivities calculated by these two methods are within 3%. The change of measuring point position has little influence on the tracing pattern of all-time apparent resistivity. The first branch of the curve of all-time apparent resistivity is close to the resistivity of the first layer medium and the last branch is close to the resistivity of the last layer medium, which proves the correctness of the arithmetics proposed.