Fourier Transform > Velocity correction: applied to a georadar (GPR) section

A georadar section was recorded by ANDRA in an inclined well which traverses a granitic formation in presence of fractures. The emitting antenna is situated near the well head, the receiving antenna is moved within the well every 1 m. The entire georadar section (140 sensors) shows different wave types:

  • Linear arrivals with an apparent positive velocity associated with the incident wave,
  • Arrival waves with an apparent negative velocity which are roughly symmetric in relation to the incident wave corresponding to reflections from fractures which dip contrastingly to the well axis,
  • Hyperbolic arrival forms associated with diffractions coming from discontinuities situated several metres laterally from the well.

Firstly we propose to determine the formation's velocity and secondly to estimate the lateral offset of a major diffracting point in relation to the well, whose apex is situated at 43 m on the shadowed side. The formation velocity and the lateral offset can be determined by velocity correction carried out in the frequency domain by applying delay distributions to the georadar data (Convolution product : application to the translation of a wavelet and Convolution).