GNGTS 2014 - Atti del 33° Convegno Nazionale

Siebe et al. (1995) carried out a geochemical study on the 1993 submarine eruption near Socorro Island. Further work was done to investigate the genesis of silicic peralkaline magmas (Bohrson et al. , 1996; Bohrson and Reid, 1998). In 1999, a seismic survey was carried out in the south-eastern part of Socorro Island with the use of five portable broadband seismometers (Valenzuela et al. , 2005). The intention was to install a seismic T-phase station on Socorro, as part of the International Monitoring System (IMS) of the Comprehensive Nuclear-Test- Ban Treaty Organization (CTBTO). The geochemistry of hydrothermal fluids on Socorro was investigated by Taran et al. (2010). The airborne geophysical and ground geoelectric survey of the Geological Survey of Austria (GSA) in 2009 was the first detailed geophysical survey on Socorro Island. Airborne and Gamma-ray Survey at Socorro. Airborne geophysics has been intensively applied for exploration of raw materials within the last decades, however so far it was hardly ever used to investigate the groundwater resources of volcanic islands. One of the big advantages of airborne geophysical measurements is that large areas can be surveyed within relatively short survey times. Furthermore, airborne geophysics is the only remote sensing method yielding information not only about the surface but also about the subsurface structures. To perform the airborne measurements we used the complex airborne system of the Austrian Geological Survey (Motschka, 2001). Its instrumentation consists of a frequency domain electromagnetic bird operating at four different frequencies, a magnetic sensor and a gamma spectrometer. Positioning was performed using the data of a laser altimeter, two differential GPS sensors and flight path recordings from a downward looking digital video camera. For correction of altitude raw values for variations in vegetation thickness we employed advanced algorithms. The survey covered an area of 200 km 2 , with an average line spacing of 100 m. The use of the electromagnetic system required the distance between terrain and sensor (“bird”) to be less than 100 m, in order to yield a reliable subsurface resistivity mapping. Airborne Magnetics. With this methodology the total intensity of the Earth’s magnetic field is measured. Deviations from a reference earth magnetic field (IGRF) are considered as anomalies and assist e.g. in the discovery of differently magnetized bodies (i.e. ore bodies, young volcanic rocks, metallic contents of waste repositories) or fracture zones. The results are given in Fig. 2, which shows a high resolution map of the magnetic field anomalies reduced Fig. 2 – Preliminary interpretation of the pole-reduced magnetic data: magnetic lineaments and structures. 176 GNGTS 2014 S essione 3.2