Model of horizontal stress in the Aigion10 well (Corinth) calculated from acoustic body waves

Abstract

In this paper we try to deduce the in situ stresses from the monopole acoustic waves of the well AIG10 between 689 and 1004 meters in depth (Corinth Golf). This borehole crosses competent sedimentary formations (mainly limestone), and the active Aigion fault between 769 and 780 meters in depth. This study is the application of two methods previously described by the author who shows the relationships between in situ horizontal stresses, and (i) the presence or absence of double body waves, (ii) the amplitude ratios between S and P waves (Rousseau, 2005a,b). The full waveforms of this well exhibit two distinct domains separated by the Aigion fault. Within the upper area the three typical waves (P, S and Stoneley) may appear, but the S waves are not numerous, and there is no double body wave, whereas within the lower area there are sometimes double P waves, but no S waves. From those observations, we conclude that the stress domain is isotropic above the Aigion fault, and anisotropic below, which is consistent with conclusions drawn by authors from other kinds of data. The calculation applied to the characteristics of the double P waves suggests that this horizontal stress anisotropy is very high, up to 190 % with the maximum horizontal stress superior to 100 MPa and the minimum horizontal stress close to zero at 939 meters in depth. The amplitude ratios between the S and P wave indicate a very strong compression - which is isotropic - above the Aigion fault.