Near Surface Geophysics

Through field case studies, we show how an optimum choice of signal processing  tools (filters and transformations) allows to process near surface geophysical data.

We present two integrated field cases associated with two sites:

The site of Poitiers

The site of Vesdun

The integrated field case of Poitiers

All the near surface data set have been recorded on the Hydrogeological Experimental Site of the University of Poitiers (France). The site lies on the borderline, named the "Poitou threshold", between the Paris and the Aquitaine sedimentary basins. The site has been developed, since 2002 thanks to Gilles Porel and Bernard Legube, for the sole purpose of providing facilities to do experiments for a better understanding of flow and transfers in fractured rocks. The aquifer studied, 20 to 130 m in depth, consists of tight karstic carbonates of Middle Jurassic age. The top of the reservoir is shaped as hollows and bumps with a magnitude reaching up to 20 m. 35 wells have been bored over the whole thickness of the reservoir. The presence of pervasive karstic drains is supported by recent logs in the wells using optic imaging. Almost all the wells have shown caves and conduits cut by the walls of the boreholes with sometimes mean apertures of 0.2 – 0.5 m.  These conduits are mostly enclosed in three thin horizontal layers at depths of 35, 88 and 110 m. It was considered crucial to better image the geometry of the reservoir with a resolution compatible with, on the one hand, the scale of a well and, on the other hand, the scale of the entire experimental disposal.

High resolution geophysical tools seem well designed to undertake that kind of investigation. Different geophysical experiments have been conducted: a very high resolution 3D seismic survey, vertical seismic profiles VSP, and full wave form acoustic logging.

We present 7 field examples:

3D seismic survey
3C VSP at well C1
VSP with an hydrophone at well C1
VSP with an hydrophone at well M03
VSP with an hydrophone at well M20
Acoustic logging at well M20

For each example, we describe the processing methods which have been applied and geophysical information which could be extracted.

For more information, download the document.

The integrated fieldcase of Vesdun

The Experimental Site of Vesdun (situated in the Cher region, central part of France) has been developed both for the training of students of IFP School and universities, and for professionals. The training in geophysics concerns the acquisition and processing of surface seismic data in 2D or 3D.

On the site, a borehole has been drilled. It allows the acquisition of well seismic data such as vertical seismic profiles (VSP) and logging data such as full wave form acoustic data. The site is also used for experimental studies in near surface geophysics.

We present 4 field examples:

3D Imaging
VSP and depth conversion
Acoustic logging and cementing
Calibration and Synthetic seismograms

For each example, we describe the signal processing methods which have been applied to the data and show the contribution of the geophysical method to the understanding of the experimental site.

For more information, download the document.