GNGTS 2014 - Atti del 33° Convegno Nazionale

GNGTS 2014 S essione 3.2 177 to the pole. Interestingly, the central part of the map, corresponding to the of the summit of the island, shows a very large negative anomaly which is extended to the eastern part of the island. A preliminary DEXP analysis of magnetic data (Fedi, 2007) pointed out the presence of a low magnetization volume, which may be connected to: may be connected to: a high temperature (above the Curie temperature) in the deeper part of the volcano, a low total remanent magnetization of pyroclastic caldera sediments due to non-aligned sedimentation, and/or the influence of hydrothermal alteration within the caldera. In the latter case the caldera structure would extend further to the southern part of the island than assumed before. Gamma-ray spectroscopy. This methodology determines the natural and artificial radioactivity, which depends on the content of radioactive minerals within the first decimeters of the subsurface. Natural gamma radiation is essentially derived from three sources: the radioactive elements thorium (energy peak: 2.62 MeV), uranium (energy peak: 1.76 MeV) and potassium (energy peak: 1.46 MeV). These elements occur in different rocks and soils at various concentration levels. The measuring sensor consists of several sodium-iodide crystals, which convert gamma radiation into flashes of light. Appropriate survey instruments determine their energies. With conventional measuring systems the energy spectrum between 0.2 to 6.0 MeV is resolved in 256 channels. Since the air layer between helicopter and ground is absorbing gamma radiation (depending on the physical condition of the air), the exact flight altitude, air pressure, air temperature as well as air moisture have to be taken into consideration when correcting survey data. Prior to processing, data were inspected for incorrect values and in particular all samples which were taken at altitude higher than 400 m were deleted. The following calibration constants and coefficients were used for the processing of gamma- ray data of Socorro Island (Tab. 1): Processing was performed both with and without the use of PCA smoothed spectra. Radon removal was first run with the full-spectrum method, but this seemed to remove too much and so radon removal was run for a second time with the spectral-ratio method. The best gridding results could be achieved with the gridding-method kriging. Since flight line spacing at Socorro was approximately100 m, the grid-node spacing was also chosen to be 100 m, with a search radius of 150 m. ��� ������� ��� ���������� ����� �� ���� �� ������� The results for potassium, shown in Fig. 3, provide valuable information for geological and volcanological research. They show a good correlation between the maxima of the potassium map and the peralkaline trachytes dated approximately at 15 ka �������� (Bohrson et al. , 1996) ��������� �� ��� ������ ������ ����� ��� belonging to the Felipe member (Fig. 1). Interpretation of gamma-ray data of Socorro Island is being performed with the combined use of radioelement maps, ternary maps, a detailed satellite image and several pre-existing geological maps. Conclusions. Through this work we aim at retrieving information about the surface and subsurface geology of the volcanic island of Socorro based on the analysis and interpretation Tab. 1 - Calibration constants and coefficients used for the gamma-ray processing. K40 U238 Th232 TC Aircraft Background 28.32 13.408 7.078 40.02 Cosmic Background 0.083 0.043 0.059 1.9 Attenuation by air -0.0057333 -0.0058056 -0.0070921 -0.0061463 Sensitivity 47.2 7.3 3.4 29.5 Stripping Coefficients 0.2 0.26 0.74 0.05 SC altitude increase 0.00049 0.00065 0.00069 0