Abstract

Understanding subsurface flow, especially in fractured rocks only housing water through a few preferential pathways, is still challenging. The point is mainly associated with the poor accessibility of the subsurface and the lack of accurate representations for both heterogeneity and spatial distribution of water bearing bodies. This notwithstanding, highly-resolved geophysical investigations bring new images of the subsurface. This is exemplified over a fractured limestone aquifer at the site scale (for example, that of the radius of influence of an extraction well). On an experimental site, situated in the Cher region (France), two boreholes have been drilled for field experiments. Full Waveform Acoustic Logging (FWAL) and seismic experiments were conducted. Hybrid seismic imaging, which consists in combining refraction and reflection seismic results, has been carried out. Based on a four-step procedure, the processing of refracted and reflected waves provided two sections. After assemblage, these sections produced in a first step an extended time reflectivity section starting from the surface and, in a second step, a section over depth after calibration with Vertical Seismic Profile (VSP) and acoustic data. However, even the Very High Resolution (VHR) seismic methods do not have a sufficient vertical resolution to describe accurately the geological formation. The acoustic sections were processed to separate the different wave fields, to extract the criss-cross events and to build a criss-cross index log. A log of fracturation index, based on both criss-cross index and P-wave velocity measurements, was computed to detect the presence of fractures. After calibration, and under the assumption that the slower the P-wave velocity, the higher the permeability – porosity, a 3D seismic block of reflection can inform on preferential areas where flow should occur. At the scale of an open wellbore, acoustic loggings that measure wave velocities over a short distance within the well also inform on open features crosscut by the well. Finally, flow log measurements confirm the occurrence of flowing horizons that were previously marked by both seismic and acoustic data. Seismic and acoustic data are therefore suited to image contrasted hydraulic properties over fractured subsurface systems usually poorly documented.

Highlights

  • The need for images of the shallow subsurface at high resolution is claimed by various applications including geotechnics, hydrology, reservoir engineering, etc

  • This solution is clearly associated with investigations crossing and inter-comparing the results from diverse geological and geophysical techniques: 2D–3D Very High Resolution (VHR) seismic imaging including refraction tomography, acoustic logging, and flow measurements

  • Seismic imaging of the site is of limited extension over space

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Summary

Introduction

The need for images of the shallow subsurface at high resolution is claimed by various applications including geotechnics, hydrology, reservoir engineering, etc. FWI is an advanced seismic imaging method [8] with the objective to determine a model (velocity, density, and possibly anisotropy and attenuation) of the subsurface in which the synthetic shots gather the best fit of. Through an integrated field case obtained on an experimental site, this work shows that a solution exists to improve our knowledge regarding the structure and the properties of the subsurface This solution is clearly associated with investigations crossing and inter-comparing the results from diverse geological and geophysical techniques: 2D–3D Very High Resolution (VHR) seismic imaging including refraction tomography, acoustic logging, and flow measurements. After a short description of the geological context and a review of the seismic imaging carried out at the site, this work shows how acoustic logging and refraction tomography can be merged to form a high-resolution continuous velocity model from the surface up to the terminal depth of the boreholes. The Permian substratum could be reached at 230–250 m depth

Geological context
Seismic imaging
VSP and 3D seismic imaging
Acoustic logging
Calibration of seismic section by full waveform acoustic logging and VSP
Fracture detection
Flow-log measurements
Discussion
Findings
Example of a near surface karstic reservoir

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