La méthode de polarisation spontanée en hydrogéologie des terrains volcaniques Self-Potential Method in Hydrogeological Exploration of Volcanic Areas

Résumé

En terrain volcanique, les eaux de pluie s'infiltrent jusqu'à leur rencontre avec un niveau imperméable qui correspond le plus souvent au socle cristallin. Ce sont les talwegs et les lignes de crête des paléo- reliefs de ce socle dont la profondeur peut dépasser la centaine de mètres qu'il convient de détecter, parfois avec une précision décamétrique.

La méthode géophysique la plus utilisée en hydrogéologie des terrains volcaniques est la prospection électrique qui fournit des coupes verticales des résistivités électriques. La morphologie du substratum imperméable ou saturé peut aussi être obtenue en mesurant en surface les potentiels électriques de polarisation spontanée (en abrégé PS) qui se forment par la percolation de l'eau infiltrée dans le terrain poreux. La base de la zone non saturée, appelée surface SPS, est calculée par une relation faisant intervenir les données PS, les altitudes et deux coefficients définis à partir des données géologiques. Cette surface indique directement les axes de circulation et les lignes de partage des eaux.

Deux exemples pris sur des sites bien documentés montrent la validité de la méthode pour localiser les axes de circulation de l'eau souterraine et les limites entre bassins versants. Un troisième exemple montre les résultats PS comparés à ceux des méthodes électromagnétiques VLF et AMT. La méthode PS est légère et offre une bonne précision horizontale, mais elle demande au moins un forage d'étalonnage pour préciser la profondeur des interfaces.

Mots-clés:

• Polarisation spontanée,
• géophysique,
• axe de drainage,
• ligne de partage des eaux

Abstract

Hydrogeological prospecting poses great problems in volcanic areas, particularly on volcanic islands. Despite a generally high rainfall in these areas, the high mean permeability of volcanic rocks results in only a few easily exploited resources. Volcanic rocks form a particular case of geophysical prospecting with respect to hydrogeological research. Indeed these formations are characterised by specific hydrological and geophysical parameters, which generally possess two features. First, there are heterogeneous systems composed of non-stratified layers. Acceptable modelling of these structures requires an extensive coverage of data. Generally only light equipment and simple methods can be used. Also, the electrical parameters of volcanoclastics are often very different from those of their surrounding rocks, so geoelectrical methods are generally used in volcanic areas, namely the electrical resistivity and self-potential methods.

The Self-Potential (SP) method gives very good results because SP effects in volcanoclastics are strong. In volcanic areas it has been used for the last twenty years for geothermal and hydrogeological research. It is a measure at the Earth's surface of the natural electric potentials generated in the ground by streaming potential, due to ground water flowing through the porous rock medium. This medium becomes polarised. The principles of data interpretation can be summarised as follows. There is a linear correlation between the range of negative SP anomaly and the thickness of the vadose zone when two conditions are satisfied. The first condition is a high ratio between the resistivity of the vadose zone, and the resistivities of the substratum and the water-saturated zone. The second condition is the homogeneity of the vadose zone. If these two conditions are met, it is possible to define a geophysical surface called SPS, calculated from the SP and topographic data. The SPS is both an equipotential SP surface and the interface between the vadose zone and the saturated medium below. Drainage courses and watersheds are located exactly on the valleys and the ridge lines of the SPS respectively. The survey should be carried out using a high density of measurements, because underground water circulation can sometimes be confined to narrow talwegs. Nevertheless, the analysis of profiles across a valley does not always accurately indicate a narrow flow within the valley, and it is then necessary to run several mechanical probe-tests to locate the underground channel.

Two well-documented examples focus on the groundwater flow below a basaltic lava flow, recognised partly by boreholes (Chaîne des Puys, France). In the first example, the water is channelled along a gallery where the contact between volcanic rocks and granite can be clearly observed. The valley of SPS is narrow and corresponds to the end of the gallery to within 10 m, at 75 m below the surface. In the second example, a SP map with a surface of 5 km2 shows the boundary between two basins, the second being intersected by an impermeable zone. A third example concerns the shield volcano of Piton de la Fournaise (Réunion Island, Indian Ocean), considered as a pile of permeable lavas. Four electrical methods, Self-Potential (SP), Audio-Magneto-Telluric (AMT), Very-Low-Frequency (VLF), Electrical Sounding (ES), were used to provide some information on the groundwater system. The comparison among these methods shows that the SP method is the best method to identify in detail the upper vadose zone, whereas the AMT method is able to identify deeper layers, but without much precision. The comparison between electrical resistivity and SP methods was studied from a practical point of view, in terms of horizontal and vertical precision and accessibility.

Finally, the paper describes practical aspects of the SP method, including SPS calculation methods, specific equipment, field measurement procedures, and disruptions due to anthropogenic factors.

Keywords:

• Self-Potential method,
• groundwater flow path,
• watershed