A method for determining the complete regional stress field is presented. It is based on measurements of the normal stress supported by preexisting fracture planes (with various dip and strike) by means of hydraulic tests. The solution does not involve the stress field in the immediate vicinity of the well. Two examples concerning granitic rocks are presented. The first one involves fourteen tests between 100m and 973m, the second one sixteen tests between 50m and 620m. For the second case, a stress discontinuity is observed around 300m Deep results are coherent with focal mechanisms of microseismic events.
Une méthode de me sure de contrainte, basée sur la détermination de la contrainte normale supportée par des fractures naturelles (de pendages et azimuts variés) est présentée. La solution ne fait pas intervenir le champ de contrainte au voisinage des forages utilisés. Cette méthode est illustrée par les résultats de deux campagnes d''essai en massif granitique. La première concerne quatorze mesures entre 100m et 973m, la seconde seize essais entre 50m et 620m. Dans le deuxième cas, une discontinuité du champ de contrainte est mise en évidence autour de 300 m. Les résultats des essais profonds sont cohérents avec les mécanismes au foyer d''évènements microsismiques.
The hydraulic fracturing stress determination technique is now well established (see e.g. Hubbert and Willis.1957.: Scheidegger, 1962, Fairhust, 1964· Haimson and Fairhurst, 1969 / Haimson 1978; Haimson, 1983, Hickman and Zoback 1983). This method is based on analysis of pressure time records obtained in packed off zones of boreholes where the rock is homogeneous, linearly elastic and isotropic with respect to both its elastic characteristics and its "strength". When the borehole axis is parallel to one of the principal stress directions it provides ways to evaluate the magnitude and the direction of the two principal stresses which are perpendicular to the borehole axis. In order to alleviate the constraining assumptions underlying the classical theory of hydraulic fracturing, Cornet (1980, 1982) and Cornet and Va1ette (1984) have proposed conducting hydraulic tests on preexisting fractures with various dip and strike for determining the complete regional stress tensor (H.T.P.F stress determination method). Bertrand et al, (1983) and Rummel and Baumgartner (1984) have reported successful stress measurements obtained with the H.T P.F. method. Their computational procedures are somewhat different from that proposed by Cornet and Valette (1984), but they also rest on the same hypothesis that the regional stress field is continuous and is a linear function of depth. In the present paper the H.T.P.F. method described by Cornet and Valette is illustrated by field results obtained in two granite area of central France, namely Auriat and Le Mayet de Montagne. In the first case a solution is presented for the stress field, which fits measurements taken between 150m and 980m. In the second (Le Mayet de Montagne), it is found that the stress field is not continuous in the depth range where measurements were conducted (50m, 620m).
