The Ground Penetrating Radar (GPR) method is comparable to the reflection seismic method.

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  Step 1: Acquisition from the surface
  Figure shows both a constant offset section and a common mid-point. Measurements were taken with a Pulse-Ekko-3 radar emitting an electromagnetic pulse with a central frequency of 100 MHz, the source and the receiver antennas being moved on the surface on the second level with respect to the quarry floor. The distance between 2 traces of the radar section is 1 m. On a constant offset section, the reflected arrivals have an infinite apparent velocity for electromagnetic impedance limits parallel to the surface of the ground. On the common mid-point gather, inserted in the constant offset section, various waves can be identified:
  
  • Wave (1) is the electromagnetic wave travelling in the air. It is the fastest wave.
  • Waves (2) are the reflected refracted modes created by wave (1). They have high apparent velocities.
  • Wave (3) is the direct wave transmitted in the formation
  • Waves (4) are reflected waves. Their time distance curves are hyperbolic. Waves (4) are composed of primaries and multiples.
  Waves 3 and 4 are analogous to seismic waves. Processing of radar data is similar to seismic data processing for these waves.


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  Step 2: Imaging in a highly-deviated well
  Figure shows a radar section recorded in the very strongly deviated drain well of the quarry. The dip of the well is 10° with respect to a horizontal line. The image was obtained from a constant offset section recorded with an ABEM system with a RAMAC radar emitting an electromagnetic pulse with a central frequency of 100 MHz. The source antenna – receiver antenna distance is 4.6 m, the spread sampling interval is 25 cm. Acquisition, analysis and data processing are described by Jean Claude Dubois in a paper published in 1995. The radar velocity of the limestone is 7.5 cm/ns on average. Processing includes amplitude recovery, spiking deconvolution, separation of reflected events coming from markers located above and below the drain in two sections according to their apparent velocities (positive and negative). Each section is processed independently. Their processing includes NMO corrections and migration. The sections are reconstructed to provide an image of the quarry above and below the drain. It is correlated in depth with a natural gamma ray log recorded in the vertical well. The image shows more or less discontinuous reflected events. The events S20 and S74 correspond to reflections from markers located respectively at 20 and 74 m from the top of the quarry. Two types of reflections can be identified on the radar section. The continuous events of type A represent reflections associated to an electromagnetic impedance contrast. The events of types B and C are associated with reflections on fractures.

Reference

Dubois J. C., 1995, Borehole radar experiment in limestone: analysis and data processing. First Break, 13, 2, 57-67