118 Well seismic surveying and acoustic logging One such Hydrogeological Experimental Site (HES) was built by the University of Poitiers (France) near its campus, for the purpose of providing facilities to develop long-term monitoring and experiments for a better understanding of flow and transfers in fractured rocks [2], [3], [4], [5]. After a brief review of the geological context, this chapter shows the contribution of seismic and acoustic methods to the characterization of a karstic carbonate reservoir. The different geophysical methods studied are: • 3D surface seismic method • Well seismic methods: ambient noise measurements and Vertical Seismic Profiles (VSP) survey • Acoustic method: full waveform acoustic logging This chapter shows the value of combining different geophysical methods to describe geological formations at different scales to detect karstic bodies and to quantify flows. 5.2 Geological context The aquifer studied is 20 to 130 m in depth and consists of tight karstic carbonates of Middle Jurassic age. It lies on the borderline, called the “Poitou threshold”, between the Paris and the Aquitaine sedimentary basins (Figure 5.1). The Hydrogeological Experimental Site (HES) covers an area of 12 hectares over which 35 wells were drilled to a depth of 120 m (Figure 5.1). The top of the reservoir was flat and horizontal 150 million years ago, before being eroded and weathered during the Cretaceous and Tertiary ages. Today the reservoir top is shaped with hollows and bumps with a magnitude reaching up to 20 m. The site construction phase started in 2002 and to date 35 wells have been bored over the whole thickness of the reservoir. Most wells are fully documented, with drilling records and logs of various types, including gamma ray, temperature, and acoustic logs. In addition, two wells were entirely cored. Hydrogeological investigations show that maximum pumping rates vary from well to well and range from 5 to 150 m3/h. However, the aquifer responds fairly evenly to the hydraulic stress of a pumped well. This is assumed to be the consequence of a local karstic flow in open conduits. The presence of karstic drains is supported by recent well logs using optic (OPTV) or acoustic (BHTV) imaging. Almost all wells have shown caves and conduits that were intersected by the walls of the boreholes, with average apertures sometimes in the range of 0.2 to 0.5 m. These conduits are mostly enclosed in three thin horizontal layers at depths of 35 m, 88 m and 110 m. Of course, these layers are intercepted by vertical wells and this potentially results in a good connection between wells and karstic drains. This connection is mainly controlled by the degree to which drains are re-opened
RkJQdWJsaXNoZXIy NjA3NzQ=