Interpretation of Very Low Frequency Measurements Carried Out with an Unmanned Aerial System by 2D Conductivity Models

Abstract

Very Low Frequency (VLF) measurements were successfully acquired using an Unmanned Aircraft System (UAS) as the measurement platform at two sites. The first site was characterized by strong anthropogenic anomalies and was located in Switzerland. The second site was a saltwater-freshwater transition zone in Northern Germany. The induction coils were used to measure the three components of the magnetic field of VLF transmitters. The magnetic field sensors and the corresponding data logger were suspended below a 50 kg rotary wing UAS. Prior to the test survey, noise measurements were carried out to establish a proper distance for the logger and the sensor, which were found at 2 m and 4 m from the body of the UAS, respectively. A suspension system was developed to enable stable flight characteristics in combination with the VLF system. The test flights were carried out with low speed (1 m/s) and at low altitude (approximately 3 m) above ground. A processing algorithm was developed to analyze the time series of the measured magnetic fields. Transfer functions for the vertical magnetic field were derived using both a scalar and a bivariate analysis approach. After rotating the transfer functions into the strike direction of the 2D subsurface anomaly, a two dimensional inversion method was applied for the interpretation of the conductivity structure to a depth of 15 m. In addition, radiomagnetotelluric (RMT) measurements were carried out on the same UAS-VLF profiles. The RMT data were interpreted by 2D inversion for comparison and are used here to derive the background resistivity, which cannot be achieved by VLF measurements. As a result, good correlation was found between the UAS-VLF and RMT conductivity models.