INTEGRATION OF HYDROCHEMICAL, GEOLOGICAL AND GEOPHYSICAL METHODS FOR THE EXPLORATION OF A NEW THERMAL WATER-RESOURCE - CASE OF YVERDON-LES-BAINS, FOOT OF THE JURA RANGE

The major goal of this study is to evaluate the potential of warmer an d deeper groundwater than the one presently produced by the well F4 at the Centre thermal of Yverdon-les-Bains. Numerous data originating fr om seismic lines and boreholes allow to obtain a good understanding of the structural geology in the Yverdon area. However, these data are l imited in the faulted zone of Pipechat-Chamblon-Chevressy (PCC) crossi ng the city of Yverdon, and make difficult the detailed structural int erpretation on the site of the old thermal spring and well F4. The lat ter drains the Malm limestones near the southern fault plane, which sh ows the importance of the hydraulic role played by the main fault or b y a network of associated faults. A vibro-seismic survey carried out i n the urban area of Yverdon, close enough to the Centre thermal, shows the precise location of the anticlinal axis along the PCC fault zone. Moreover, individual reflectors have been deciphered and allow to eva luate the thickness and the structure of the quaternary and molassic s ediments on both sides of the fault zone. Hydrogeological investigatio ns allow to detail the different roles of the PCC fault. On the one ha nd it allows a slow water flow rising from depth to surface and a rapi d circulation from the Jura to the Plateau. On the other hand, the PCC fault forms a hydraulic barrier in the north-south direction. Only th e oil exploration borehole at Hermringen in the Seeland gave indicatio ns on deeper fluids, namely those in the Dogger formation. Springs and water wells of the Yverdon area allowed to identify groundwaters from different horizons: the Molasse, the Cretaceous and the Malm. Thoroug h observations of the old thermal spring and of the well F4 reveal the interaction existing between the two groundwaters tapped. Indeed, the artesian exploitation of the well F4 results in the partial dilution and cooling of the old thermal spring. Integrated interpretation of th e chemical composition of the groundwaters from the Centre thermal, th eir evolution with timer as well as the comparison with the borehole L a Greve-1 from Arkina credit the hypothesis of a fluid warmer than in the Maim aquifer. Moreover, the geothermal gradient calculated for the well F4 (3.7 degrees C/100 m) is slightly above the one found in the borehole La Greve-1. which indicates the effect of an upflow of warmer fluids in the PCC fault zone. Consequently, the Dogger limestones are considered as the probable aquifer below the Maim, between 1000 and 1 400 m below surface. The hypothesis of a fluid at even greater depths, namely in the Triassic sediments, seems to be excluded for hydrochemi cal reasons. The groundwaters tapped at the Centre thermal (thermal sp ring and well F4) are the result of a mixing of three principal compon ents. A recent groundwater, relatively cold and weakly mineralized: si milar to the subthermal spring Le Cossaux (14 degrees C and 0.4 g/l). An old groundwater, warm and weakly mineralized originating from the M alm: similar to the well La Greve-1 from Arkina (31 degrees C and 0.34 g/l). An old, warmer and more mineralized groundwater than in the Mal m: represented by a Na-Cl fluid of the Dogger aquifer. From calculatio ns and extrapolations. the fluid from the Dogger should have a reservo ir temperature above 50 degrees C and a maximum mineralization of 5 g/ l, including some H2S. In case of exploitation of the Dogger aquifer t hrough a deep borehole, it is possible that the temperature and minera lization of the well F4 and the old thermal spring will decrease sligh tly. The drilling of a vertical 1400 m-deep borehole in the geological environment of Yverdon-les-Bains will not cause a priori technical pr oblems. Several drilling methods can be envisaged, but the choice will be done mainly for cost reasons. The well site choosen on the indicat ions of the structural geology is compatible with the proximity of the Centre thermal. A tentative geological section has been realized, but it includes some uncertainties, which will be removed after the first 250 m of drilling (Cretaceous and top of the Maim). The final target is represented by the Dogger limestones, but the borehole will have to closely follow the PCC fault zone, because this fault is certainly re sponsible for the good permeability for the regional deep ground water flows.