A. Dupis

SEDIMENT YIELD DURING LATE GLACIAL AND HOLOCENE PERIODS IN THE LAC CHAMBON WATERSHED, MASSIF CENTRAL, FRANCE

A sediment budget for the Late Glacial and Holocene periods was calculated for the Lac Chambon watershed which is located in a formerly glaciated temperate crystalline mountain area. It appears that over 15 500 years: (1) 69 per cent of eroded particles have been displaced by gravity processes and then stored within the watershed, compared to 31 per cent that have been displaced by running water and evacuated outward; (2) the mean mechanical erosion due to gravity processes on the slopes amounted to 16·1± 6 m and only developed on a quarter of the watershed surface, whereas the mean mechanical erosion due to running water amounted 1·24± 0·37 m and involved the whole watershed surface. The mean sediment yields due to gravity processes on slopes were 2300 ±1360, 1770± 960 and 380±100 m 3 km -3 a -1 , respectively, for basalts, and basic and acidic trachyandesites. Values of sediment yield due to running water were 49 ±15, 120 ± 36 and 79± 24 m 3 km -2 a -1 , respectively, during the Bolling‐Allerod, the Younger Dryas and the Pre-Boreal‐Boreal periods. They were 56±17 and 166± 50 m 3 km -2 a -1 during the Sub-Atlantic period before and after 1360 a BP, respectively. These values reflect variations in the natural environment and the impact of human-induced deforestation.

2D joint inversion of dc and scalar audio-magnetotelluric data in the evaluation of low enthalpy geothermal fields

Audio-magnetotelluric (AMT) and resistivity (dc) surveys are often used in environmental, hydrological and geothermal evaluation. The separate interpretation of those geophysical data sets assuming two-dimensional models frequently produces ambiguous results. The joint inversion of AMT and dc data is advocated by several authors as an efficient method for reducing the ambiguity inherent to each of those methods. This paper presents results obtained from the two-dimensional joint inversion of dipole?dipole and scalar AMT data acquired in a low enthalpy geothermal field situated in a graben. The joint inverted models show a better definition of shallow and deep structures. The results show that the extension of the benefits using joint inversion depends on the number and spacing of the AMT sites. The models obtained from experimental data display a low resistivity zone (<20 ? m) in the central part of the graben that was correlated with the geothermal reservoir. The resistivity distribution models were used to estimate the distribution of the porosity in the geothermal reservoir applying two different approaches and considering the clay minerals effect. The results suggest that the maximum porosity of the reservoir is not uniform and might be in the range of 12% to 24%.

1D joint inversion of AMT and resistivity data acquired over a graben

Abstract Schlumberger and AMT surveys over 2D resistivity structures are often interpreted using 1D automatic inversion. However, the models obtained using such approach have inherent limitations which must be known. Therefore, the study of model limitations and of alternative interpretations is a very important issue. Using synthetic data sets constructed from a relatively complex conductive graben, we examined the advantage of joint inversion over inversion using individual data sets. The data sets consist of apparent resistivity curves generated by arrays extended parallel and perpendicular to strike. In this case the AMT survey appears to have an advantage over the resistivity survey, since the dc data are more affected by lateral effects. As might be expected, neither of the methods is effective in resolving deep structures, located beneath the conductive target, when applied separately. The use of a 1D joint inversion of the data yields a better estimate imaging of the true resistivity distribution. 1D joint inversion of the averaged AMT and dc apparent resistivity curves, seems to be the most effective method to determine the resistivity structure. Some of the conclusions derived from the synthetic example are used as guidelines in the interpretation of real field data acquired over a graben, northern Portugal.

An audiomagnetotelluric survey over the chaves geothermal field (NE Portugal)

Abstract In an attempt to define the resistivity model of the Chaves geothermal field in NE Portugal, a detailed survey with scalar audiomagnetotelluric measurements was performed. The soundings were made in the frequency range from 2300 to 4.1 Hz. Electrical resistivity models were derived from the application of 1-D inversion, 2-D trial and error modeling and 2-D inversion procedures. The resistivities inside the geothermal field are low, reaching not more than 30 Ωm and increasing up to 60–150 Ωm beneath the shallow geothermal reservoir. The conductance in the low resistivity zone (20–35 S) suggests that the geothermal field is behaving as a local conductivity anomaly, with predominant 2-D features. The low resistivity structure is associated with sedimentary and metamorphic rocks saturated with highly mineralized water and with fractured zones allowing upward movements of hydrothermal fluids. The range of porosity (13–27%) of the formations associated with the geothermal field was estimated using the modified Archie law.

Magnetotelluric observations over the Chaves geothermal field (NE Portugal)—Preliminary results

Abstract On the Portuguese mainland the most important low enthalpy field occurs in the graben of the Chaves region, where magnetotelluric measurements have been carried out to determine the crustal electrical conductivity distribution. From the impedance tensor decomposition two distinct regional directions (N-S and N65° ± 10°E) have been found. These directions are consistent with the geology and related to the main faults. The apparent resistivity and phase curves derived from the determinant of the impedance tensor were used to obtain 1-D and 2-D interpretations. The regional models indicate the existence of a conductive layer at a depth between 7 and 12 km (resistivity in the range of 100–300 Ω m, from 1-D modelling and 60 Ω m from 2-D model). The 2-D interpretation provides a first estimate of the regional geothermal gradient (28°C km −1 ) in the Hercynian granite. The 1-D models from the soundings located within the graben suggest that local tectonics could play a significant role in the CO 2 extraction and in migration of the deep CO 2 , as indicated from isotopic data.