Evaluation of a Semi-Airborne Electromagnetic Survey Based on a Multicopter Aircraft System

The semi-airborne electromagnetic (EM) method has the potential to reach deeper exploration depths than purely airborne EM approaches. The concept of the method is to deploy high-power transmitters on the ground, which excite subsurface currents and induce strong magnetic fields, and to measure the corresponding EM fields with a passive airborne receiver instrument. Following recent conceptual developments of the semi-airborne EM technique deployed on helicopters, we performed a 10 km2 semi-airborne EM survey near Münster (Germany) based on a multicopter aircraft system. For this purpose, horizontal electric dipole (HED) transmitters were installed in the survey area and were surveyed individually. Magnetic transfer functions were determined and a model of the conductivity of the study area was derived. Despite restrictions such as low payload capacity and multicopter-related EM noise, we were able to estimate spatially and spectrally consistent transfer functions of high quality up to a distance of 2 km from the respective transmitter. Our results could be validated with independent results from a magnetotelluric and a direct current sounding. The study demonstrates that an unmanned aircraft system (UAS) is suitable for semi-airborne EM application and that such a system can be beneficial where ground-based methods and manned techniques become impractical.

Results of ground and UAV based EM investigations at the gold promising area (Altay Republic, Russia)

<p>The results of different scale EM investigations at the gold prospective area in the Altay republic are considered. The study was aimed at the first one to detect the prospective area of gold-deposits location and also to test the UAV based EM system. At the first stage the AMT surveys along 20 km length line were implemented. Their results and geological information allowed us to delineated the small area with size about 2x4 km for detailed survey. The ERT, IP, magnetometry and ground and UAV based EM surveys were implemented. EM surveys were carried out using VLF field and CS. The horizontal electric dipole of 1.6 km in length was used as a source of EM field and it produced the signals of rectangular wave form at 500 Hz. Three magnetic components of EM field (H<sub>x</sub>, H<sub>y</sub>, H<sub>z</sub>) were measured with sampling frequency 312 kHz. Data were obtained in the range 500 Hz – 100 kHz. There are the few VLF stations in the studied area and the general information were obtained from the measurements using CS technique. The comparison of ground-based EM soundings results with ERT data shows good correlation. In addition, the UAV based measurements possibility were shown and their results allow to map the general features of the area geological structure. The mineralized fault zones characterized by high conductivity and IP anomaly were delineated and they are the most promising for gold-deposits detection.</p>

Analytic formulas for complete and incomplete first-order TEM moments below ground in a conductive half-space

In the resistive limit, discrete conductors give rise to magnetic dipole fields. Magnetostatic modeling of time integrals, or moments, of transient electromagnetic (TEM) data therefore offers a means for fast approximate 3D modeling and inversion of TEM data sets. In our approximate inversion scheme, the net TEM moment response is represented as the combination of discrete conductor and uniform host responses. The inversion algorithm first estimates a homogeneous host conductivity, and then it subtracts the host response and fits the residual moment data by adjusting the conductivities of cells comprising a 3D rectangular mesh. To expedite calculation of the host response, we have derived analytic formulas for first-order TEM moments produced on and under the ground by a horizontal electric dipole on the surface of a homogeneous conductive half-space. We present analytic expressions for idealized all-time “complete” moments, or resistive limits, as well as for realizable finite-time “incomplete” moments. The moments produced by an arbitrary horizontal polygonal loop are determined by combining contributions from appropriately oriented electric dipoles. Downhole TEM moments computed with the new expressions reveal substantial differences between incomplete and complete moments when early time data are excluded and between step and impulse response incomplete moments. The role of the formulas in the first stage of our moment-based 3D inversion scheme is illustrated via analysis of downhole TEM data recorded at Santander, Peru. The host conductivity of best fit for early time B-field moments is 2.40 mS/m, consistent with apparent conductivities derived from ground TEM data recorded in the same area.

THE INFLUENCE OF THE IONOSPHERE ON THE VERTICAL FORCE EXCITED BY A HORIZONTAL ELECTRIC DIPOLE

An analytical expression for the vertical component of the magnetic field has been obtained, with the help of which calculations have been made showing the effect of the ionosphere on the low-frequency field in the Earth-ionosphere waveguide. At distances from the source that are less than the doubled waveguide height, in ELF, and a lower frequency range, noticeable changes in the field strength caused by the state of the ionosphere are found.