A novel semiairborne frequency-domain controlled-source electromagnetic system: Three-dimensional inversion of semiairborne data from the flight experiment over an ancient mining area near Schleiz, Germany

Abstract

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.

2-D joint inversion of semi-airborne CSEM and LOTEM data in eastern Thuringia, Germany

“…Due to the moving receivers and the limited 5 s spatial averaging window length, the corresponding data errors derived from the standard deviations of the stacked semi-AEM data were higher than those of the LOTEM measurements. Moreover, the errors decrease with increasing frequency, ranging from 20% to 1% (Smirnova et al, 2019) for 10 Hz to 1000 Hz, respectively. For large offsets (> 2000 m), the signal-to-noise ratio reduced substantially, limiting the depth of investigation.…”

“…The gaps in data coverage over the vicinities of source positions can be filled in by the data from the other transmitter. The recorded magnetic fields were subsequently processed in the frequency-domain (Smirnova et al, 2019), resulting in CSEM transfer functions (T z ) between the vertical component of the magnetic field and the current. Where ( ) ( ) ( ), ( ) denotes the observed magnetic flux densities and the ( ) indicates the injection currents (Becken et al, 2020).…”

“…The semi-AEM concept is similar to the standard ground-based frequency-domain CSEM except that the receivers are positioned in the air (Smirnova et al, 2019). For all simulations, we assign the air layer a resistivity of 10 6 Ωm.…”

“…mineral detection (e.g., Mörbe et al, 2020), ground water investigation (e.g., Haroon et al, 2018b), hydrocarbon (e.g., Schwalenberg et al, 2017) and geothermal prospection (e.g., Commer et al, 2005). Recently, a novel semi-airborne controlled source electromagnetic (semi-AEM) system operating in frequency-domain has been developed and tested in the frame of the DESMEX (Deep Electromagnetic Sounding for Mineral Exploration) project (Becken et al, 2020;Smirnova et al, 2019). This DESMEX system aims to acquire dense CSEM data at relatively high survey speed (120 km/h) with an exploration depth of approximately 1 km.…”

“…Based on extracting the different resolution characteristics of semi-AEM and LOTEM data, we intend to optimize mineral exploration surveys via the joint inversion of these two EM methods. In a large-scale mineral exploration survey conducted in the DESMEX frame in eastern Thuringia, Germany (Liu et al, 2020;Mörbe et al, 2020;Steuer et al, 2020;Smirnova et al, 2019), one 8.5 km long LOTEM profile was measured along one of the semi-AEM flight lines. Many other geophysical experiments were also conducted during the DESMEX survey.…”