## 3 Results
### Acoustic Monitoring of Fluid Substitution and Weakening Processes  
During the injection tests, the P wave velocity and amplitude were recorded at three different heights using three pairs of ultrasonic sensors. For Obourg Chalk, such data were analyzed and thoroughly interpreted in a previous publication [13]; here we want to highlight the differences with a new data set obtained on the Ciply Chalk, and link these differences to possible water weakening effects. In Figure 13, we plotted the evolution of the P wave velocity and amplitude normalized to the amplitude at the start of the experiment vs. time, with time t = 0 corresponding to the beginning of the water injection into the dry samples. The Obourg Chalk sample, referenced as OB15-73 in our previous works [9; 13], was tested at 1.5 MPa confining pressure and 7.3 MPa differential stress. The Ciply Chalk sample was also tested at 1.5 MPa confining pressure and at 4.0 MPa differential stress. For the Obourg Chalk, the observed behavior was very similar to that of sample OB15-68, which was analyzed by [PERSON] and [PERSON] [13]. Two different stages can be distinguished: in stage #1, the P wave velocity and amplitude are impacted by the rising of the water injected from bottom to top into the dry sample. The P wave velocity decreases in sequence, first on the bottom plane, then the middle one and finally the top plane. During the rise of the water, the P wave amplitude decreases systematically before the velocity does, in good agreement with our previous observations, then increases and stabilizes at a constant value. Figure 12: (**a**) Injected water volume normalized to pore volume vs. applied differential stress normalized to the strength of the dry rock at fixed confining pressure (1.5 MPa). (**b**) Samples after failure induced by water injection at constant stress level. After some time, a different kind of evolution was observed, referred to as stage #2 in Figure 13, which was interpreted as a relaxation transition induced by an increase in water saturation once the water front had reached the top surface of the sample. The P wave velocity increased because of the transition from a relaxed to an unrelaxed regime; this was also manifested by a peak-like shape of amplitude, where the minimum amplitude corresponds with the inflection point on the velocity plot. It was shown [13] that this behavior can be successfully modelled using the patchy saturation model proposed by [PERSON] [24]. Note that near the velocity inflection point on the bottom and middle planes, a sharp velocity drop occurred: this was due to failure in the lower part of the sample, induced by water weakening processes as discussed in [9]. In sample OB15-68 analyzed in [13], the sample failure occurred later, after completion of the relaxed-unrelaxed transition. For the test on Ciply Chalk shown on the right-hand-side in Figure 13, our results are qualitatively in agreement with the Obourg Chalk behavior, with some differences. Firstly, the initial P wave velocity for the dry rock is significantly lower for the Ciply Chalk (average V\({}_{\text{P}}\) = 1763 m/s) compared to Obourg Chalk (average V\({}_{\text{P}}\) = 2438 m/s), which leads to a large contrast in the P wave elastic modulus M = \(\rho\)V\({}_{\text{P}}\)\({}^{2}\) where \(\rho\) is the dry rock density. We found M = 5.2 GPa for Ciply Chalk and M = 9.2 GPa for Obourg Chalk, so that from a dynamic elasticity viewpoint, Obourg Chalk appears to be much stiffer than Ciply Chalk under dry conditions. In contrast, the relative drop in velocity when the water front reaches the P wave measurement planes is almost the same for Obourg Chalk (\(-\)12.7%) and Ciply Chalk (\(-\)12.3%), meaning that water has a comparable effect on the P wave velocity for both rocks. However, Figure 13 clearly shows that the corresponding amplitude loss is much larger in Ciply Chalk, but it is difficult to relate this observation to the magnitude of water weakening effects.