In the framework of the Deep Electromagnetic Soundings for Mineral Exploration project, we conducted ground‐based long‐offset transient‐electromagnetic measurements in a former mining area in eastern Thuringia, Germany. The large‐scale survey resulted in an extensive dataset acquired with multiple high‐power transmitters and a high number of electric and magnetic field receivers. The recorded data exhibit a high data quality over several decades of time and orders of magnitude. Although the obtained subsurface models indicate a strong multi‐dimensional subsurface with variations in resistivity over three orders of magnitude, the electrical field step‐on transients are well fitted using a conventional one‐dimensional inversion. Due to superimposed induced polarization effects, the transient step‐off data are not interpretable with conventional electromagnetic inversion. For further interpretation in one and two dimensions, a new approach to evaluate the long‐offset transient‐electromagnetic data in frequency domain is realized. We present a detailed workflow for data processing in both domains and give an overview of technical obstructions that can occur in one domain or the other. The derived one‐dimensional inversion models of frequency‐domain data show strong multi‐dimensional effects and are well comparable with the conventional time domain inversion results. To adequately interpret the data, a 2.5D frequency‐domain inversion using the open source algorithm MARE2DEM (Modeling with Adaptively Refined Elements for 2‐D EM) is carried out. The inversion leads to a consistent subsurface model with shallow and deep conductive structures, which are confirmed by geology and additional geophysical surveys.
We have developed a novel semiairborne frequency-domain electromagnetic (EM) system and successfully tested it within the DESMEX project. The semiairborne approach relies on the fact that part of the system is positioned on the ground and the rest is airborne. This allows us to take advantage of ground and airborne techniques. In particular, a high-moment transmitter can be installed on the earth’s surface, which enables us to inject and induce strong EM fields in the subsurface. Moreover, galvanic coupling is possible, which is an advantage if additional ground stations are deployed. The airborne receivers allow easier, significantly faster, and more uniform spatial coverage of the study area than the ground receivers. In our implementation, transmitters and electric field receivers are installed on the ground. Magnetic field sensors, such as commercially available fluxgate, total field magnetometers, and newly developed induction coils, are installed on a helicopter-towed bird. First, we describe the results of a semiairborne survey performed in a selected area with ancient mining located in the Saxothuringian zone near Schleiz, Germany. A 3D semiairborne inversion model represents several conductive anomalies, which agree well with the outcrop of alum shale formations at the surface. In addition, the shallow parts of the semiairborne model are compared with the result of an independent helicopter-borne survey, which consists of stepwise 1D models.
There is a cleardemand to increase detection depths in the context of raw materialexploration programs. Semi-airborne electromagnetic (semi-AEM) methodscan adress these demands by combining the advantages of powerful transmitters deployed on the ground with efficienthelicopter-borne mapping of the magnetic field response in the air.The penetration depth can exceed those of classical airborne EM systems,since low frequencies and large transmitter-receiver offsets can berealized in practice. Anovel system has been developed that combines high-moment horizontalelectric bipoletransmitters on the ground with low-noise three-axis induction coilmagnetometers, a three-axis fluxgate magnetometer and a laser gyroinertial measurement unit integrated within a helicopter-towed airborneplatform. The attitude data are used to correct the time series formotional noise and subsequently to rotate into an Earth-fixed referenceframe. In a second processing step, and as opposed to existing semi-airbornesystems, we transform the data into the frequency domain and estimatethe complex-valued transfer functions between the received magneticfield components and the synchronously recorded injection currentby regression analysis. This approach is similar to the procedureemployed in controlled-source EM. For typical source bipole momentsof 20-40 kAm and for rectangular current waveforms witha fundamental frequency of about 10 Hz, we can estimate reliable three-componenttransfer functions in the frequency range from 10-5000 Hzover a measurement area of 4 x 5 km2 for a singlesource installation. The system has the potential to be used for focusedexploration of deep targets.
SUMMARY Electrical anisotropy of formations has been long recognized by field and laboratory evidence. However, most interpretations of long-offset transient electromagnetic (LOTEM) data are based on the assumption of an electrical isotropic earth. Neglecting electrical anisotropy of formations may cause severe misleading interpretations in regions with strong electrical anisotropy. During a large scale LOTEM survey in a former mining area in Eastern Germany, data was acquired over black shale formations. These black shales are expected to produce a pronounced bulk anisotropy. Here, we investigate the effects of electrical anisotropy on LOTEM responses through numerical simulation using a finite-volume time-domain (FVTD) algorithm. On the basis of isotropic models obtained from LOTEM field data, various anisotropic models are developed and analysed. Numerical results demonstrate that the presence of electrical anisotropy has a significant influence on LOTEM responses. Based on the numerical modelling results, an isolated deep conductive anomaly presented in the 2-D isotropic LOTEM electric field data inversion result is identified as a possible artifact introduced by using an isotropic inversion scheme. Trial-and-error forward modelling of the LOTEM electric field data using an anisotropic conductivity model can explain the data and results in a reasonable quantitative data fit. The derived anisotropic 2-D model is consistent with the prior geological information.
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