104 A new concept of karst development based on hydrogeology and geophysics sections) into down-going and up-going Stoneley waves was observed at the level of the karstic bodies. This phenomenon occurs in highly permeable formations. On the VSP sections, the down-going P-wave is identified by a blue arrow, the downgoing Stoneley waves by red arrows, and the up-going Stoneley waves by green arrows. On VSP recorded at borehole C1, the phenomenon of conversion of a downgoing P-wave into a down-going Stoneley wave can be observed at a depth of 50 m (Fig. 13a, left). It is more difficult to identify the converted upgoing Stoneley wave. Indeed, it can be noted that the down-going P-wave is highly attenuated at the same depth. The acoustic section (Fig. 13a, right) clearly shows the karstic level where the conversion phenomenon occurs. On VSP recorded at borehole M20, the conversion of the down-going P-wave into down-going and up-going Stoneley waves is clearly visible at a depth of 80 m (Fig. 13b, left), which corresponds to the top of karstic layers identified on the acoustic section (Fig. 13b, right). On the VSP section below 60 ms, in the 40–80 m depth interval, a low-frequency – strong-amplitude down-going Stoneley wave can be seen. The Stoneley wave, generated in the borehole at the air-borehole fluid contact, is reflected at the top of karstic layers and strongly attenuated below. The VSP was processed to separate the different wave fields. Figure 14 shows the extraction of the down-going Stoneley waves (Fig. 14a) and the up-going Stoneley waves (Fig. 14b). The VSP sections clearly show the converted down-going and up-going Stoneley waves. The VSP sections are converted into instantaneous amplitude sections (Fig. 14a and b, central part). In each instantaneous amplitude VSP section, the instantaneous amplitudes of the Stoneley waves are stacked in a small corridor located after the arrival time of the down-going P-wave, to obtain body-wave to Stoneley-wave conversion factors (Fig. 14a and b, right part) used to detect the depth at which the conversion occurs (80–90 m depth interval). During VSP acquisition, at each sensor (hydrophone) position, several records of ambient noise were registered. Figure 15a left, is an example of an ambient noise VSP section. The seismic noise was analyzed to detect the presence of flows (Mari & Porel, 2016). For this purpose, an ambient noise factor is calculated, which is defined as the average-to-standard deviation ratio of the amplitude spectrum of each noise trace. We noted a significant increase in the ambient noise factor at the level of karstic bodies. Ambient noise analysis therefore, shows that variations of the ambient noise factor correlate with the conversion level of P-waves into Stoneley waves (Fig.15a right).
RkJQdWJsaXNoZXIy NjA3NzQ=