Abstract
A 3D seismic survey was done on a near surface karstic reservoir located at the hydrogeological experimental site (HES) of the University of Poitiers (France). The processing of the 3D data led to obtaining a 3D velocity block in depth. The velocity block was converted in pseudo porosity. The resulting 3D seismic pseudo-porosity block reveals three high-porosity, presumably-water-productive layers, at depths of 30–40, 85–87 and 110–115 m. This paper shows how full wave acoustic logging (FWAL) can be used to validate the results obtained from the 3D seismic survey if the karstic body has a lateral extension over several seismic. If karstic bodies have a small extension, FWAL in open hole can be fruitfully used to: detect highly permeable bodies, thanks to measurements of acoustic energy and attenuation; detect the presence of karstic bodies characterized by a very strong attenuation of the different wave trains and a loss of continuity of acoustic sections; confirm the results obtained by vertical seismic profile (VSP) data. The field example also shows that acoustic attenuation of the total wavefield as well as conversion of downward-going P-wave in Stoneley waves observed on VSP data are strongly correlated with the presence of flow.
Highlights
Electrical resistivity tomography (ERT) is a proven imaging technique well-suited to identifying karst features and partially characterizing karst aquifers [1]: “Karst features can be predominantly air-filled, making them highly resistive (e.g., >1000 Ω·m), or partially or completely water-filled (e.g., 60–1000 Ω·m); in the latter case, depending on the ionic concentration of the groundwater, karst features may have a bulk conductivity ranging from very conductive (e.g., 60–100 Ω·m) to relatively conductive (e.g., 100–250 Ω·m), compared to the host rock(e.g., >2000 Ω·m for not karst limestone)
The present paper illustrates the benefit of combining seismic methods (3D refraction tomography, 3D reflection imaging, vertical seismic profile (VSP) and full-wave acoustic logging (FWAL)) for the characterization of a near surface karstic aquifer
We show the interest of using both low resolution but large-scale investigation seismic method and very high resolution but small-scale investigation acoustic method to describe a reservoir at different scales
Summary
Many geophysical studies have been undertaken to investigate the presence of karstic structures, to detect conduits and cavities, and to characterize the surrounding host rock. The integration of field data from different geophysical methods is the most suitable approach for imaging karstic structures, detecting cavities, and characterizing the host rock [2]. The present paper illustrates the benefit of combining seismic methods (3D refraction tomography, 3D reflection imaging, vertical seismic profile (VSP) and full-wave acoustic logging (FWAL)) for the characterization of a near surface karstic aquifer. The 3D seismic imaging does not have enough resolution both in the vertical and horizontal directions to detect karstic bodies of small size, like those observed by BHTV in wells M04, MP7 and C1 in the 45–60 m depth interval. The results rendered by acoustic logging are confirmed by additional measurements such as flowmeters and VSP with hydrophones in wells
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