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.
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