Thomas Kalscheuer, Uwe Zimmermann, Henrik Sparr, Anton Palm Ekspong, Alexander Lindblad
{"title":"An Improved Equivalent Circuit for Electric Field Sensors in Geophysical Exploration","authors":"Thomas Kalscheuer, Uwe Zimmermann, Henrik Sparr, Anton Palm Ekspong, Alexander Lindblad","doi":"10.1111/1365-2478.70047","DOIUrl":"https://doi.org/10.1111/1365-2478.70047","url":null,"abstract":"<p>In electromagnetic measurements, electric field sensors consist of two halves, with remote electrodes of negative and positive polarity coupled through wires and low pass filters to the differential inputs of an analogue-to-digital converter; the electrical ground of the analogue-to-digital converter is connected to the ground through a reference electrode. We present, analyse and evaluate improved equivalent circuits for such electric field sensors. This serves to identify the maximum contact resistances of the electrodes for which the recorded voltages are unaffected by system response effects over a given frequency range. In the first step, we verify a new equivalent circuit for one half of an electric field sensor by comparison to a previously published equivalent circuit. In contrast to the latter, our equivalent circuit accounts for the spatial variability of the electric field along an extended sensor cable, the finite impedance of the receiver input stage, the non-zero contact resistance of the reference electrode and residual cable on a winch. Furthermore, the cable is characterised by its resistance, self-inductance and capacitance to the ground and the ionosphere or the borehole fluid. Compared to the absolute value of the voltage, our results show that the system response affects the phase of the voltage at lower frequencies. In the next step, we develop an equivalent circuit for a complete electric field sensor connecting two sensor halves to an analogue-to-digital converter. We study both symmetric and asymmetric set-ups with identical and differing cable lengths, respectively, of the sensor halves. Over the whole frequency range, the amplitude gets the lower, the higher the sum of contact resistances of the remote electrodes is. In contrast, the phase is distorted only at higher frequencies. Generally, the contact resistance of the central reference electrode has little effect. For symmetric sensors, of the combinations of contact resistances of the remote electrodes that have the same sum, it is the combination of identical contact resistances that shows the lowest distortion. The distortion owing to different contact resistances of the remote electrodes is only slight and mostly in the amplitude at high frequencies. For asymmetric sensors, the benefits of using a differential analogue-to-digital converter input are no longer exploited. For instance, flipping the contact resistances of the remote electrodes leads to different responses at high frequencies. In borehole applications, it is of particular importance to account for the spatial variability of the electric field due to the skin effect, field propagation and the curvature of the borehole track. We consider an extended electric field sensor that is placed in a borehole at an inclination of <span></span><math>\u0000 <semantics>\u0000 <mrow>\u0000 <mn>45</mn>\u0000 <msup>\u0000 <mspace></mspace>\u0000 ","PeriodicalId":12793,"journal":{"name":"Geophysical Prospecting","volume":"73 8","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-10-03","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2478.70047","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145224080","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}
{"title":"Low-Frequency Extrapolation by Deep-Learning for Cross-Well Full-Waveform Inversion – Case Study From the Aquistore \u0000 \u0000 \u0000 CO\u0000 2\u0000 \u0000 $rm CO_{2}$\u0000 Storage Site","authors":"Amir Mardan, Don White","doi":"10.1111/1365-2478.70081","DOIUrl":"https://doi.org/10.1111/1365-2478.70081","url":null,"abstract":"<p>Full-waveform inversion (FWI) of seismic data is a powerful method for estimating high-resolution models of the subsurface. An accurate initial model and low-frequency data are necessary to avoid cycle skipping and perform a successful FWI. In the absence of this information, FWI is likely to fail due to convergence in local misfit minima. With the recent advancements in artificial intelligence, studies have shown that absent low-frequency data can be extrapolated using deep learning (DL). These studies have been mostly focused on surface seismic data whose frequency content is different from cross-well data. In this study, we assess the use of DL for low-frequency extrapolation for a cross-well survey that was done at the Aquistore <span></span><math>\u0000 <semantics>\u0000 <msub>\u0000 <mi>CO</mi>\u0000 <mn>2</mn>\u0000 </msub>\u0000 <annotation>$rm CO_{2}$</annotation>\u0000 </semantics></math> storage site in Saskatchewan. This assessment includes both numerical and field data examples. We extrapolate the low frequencies to increase the bandwidth of the acquired data at the Aquistore site and perform FWI. We evaluate the efficiency of this method by comparing the results with obtained velocity models from the conventional multiscale FWI. Our results for the Aquistore data show that the proposed strategy leads to an accuracy improvement of 39% and 20% in the model and data domains, respectively.</p>","PeriodicalId":12793,"journal":{"name":"Geophysical Prospecting","volume":"73 7","pages":""},"PeriodicalIF":1.8,"publicationDate":"2025-09-24","publicationTypes":"Journal Article","fieldsOfStudy":null,"isOpenAccess":false,"openAccessPdf":"https://onlinelibrary.wiley.com/doi/epdf/10.1111/1365-2478.70081","citationCount":null,"resultStr":null,"platform":"Semanticscholar","paperid":"145146338","PeriodicalName":null,"FirstCategoryId":null,"ListUrlMain":null,"RegionNum":3,"RegionCategory":"地球科学","ArticlePicture":[],"TitleCN":null,"AbstractTextCN":null,"PMCID":"OA","EPubDate":null,"PubModel":null,"JCR":null,"JCRName":null,"Score":null,"Total":0}