132 Well seismic surveying and acoustic logging are correlated with the conversion level of P-waves into Stoneley waves. An attribute, the VSP flow index, was calculated to detect both karstic levels and flows. Full waveform acoustic data were recorded both to locally validate the results obtained by the seismic methods (3D and VSP) and to evaluate the potential of the acoustic method to detect karstic bodies with a very high resolution (0.25 m deep). A methodology has also been developed to process the acoustic data generated by the flows in the karst conduits. The methodology is based on Singular Value Decomposition processing, which is used to estimate simultaneously the velocity of the formation, the amplitude of the acoustic signal, and the level of noise. An attribute, called the Noise/Signal detector, was calculated to detect karstic levels. The seismic (3D, VSP) and acoustic data were compared with OPTV logging data and PLT data. The flows predicted by seismic methods were confirmed by PLT data. The conclusions of the study are: • The 3D seismic block can be used to build a 3D model of karst aquifers, • Conversion of P-waves into Stoneley waves was observed in the VSP data at the top of the producing levels, • The variations of the spectral characteristics of the ambient noise are strongly correlated with flow measurements (PLT) and the conversion of P-waves to Stoneley waves, • Analysis of ambient noise and conversion of body waves into Stoneley waves could also be used to detect flow circulation. Therefore, we can conclude that 3D seismic, full waveform acoustic logging and VSP enable the description of karstic formations at different scales. 5.7 Acknowledgements We thank the University of Poiters for granting us their permission to use the data. We thank Benoît Nauleau and Denis Paquet for their technical support. We thank Patrick Meynier for his support in the acquisition of the VSP data. We thank Pierre Gaudiani for the acquisition of full waveform acoustic data. References [1] Mari J.L., Porel G., 2007, 3D seismic imaging of a near – surface heterogeneous aquifer: a case study, Oil and Gas Science and Technology, Rev IFP 63, 179-201. doi: 10.2516/ogst/2007077. [2] Bernard S., Delay F., Porel G., 2006, A new method of data inversion for the identification of fractal characteristics and homogenization scale from hydraulic pumping tests in fractured aquifers. Journal of Hydrology 328, 647-658.
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