Optimization of acquisition setup for cross-hole GPR full-waveform inversion using checkerboard analysis
M. Oberröhrmann, A. Klotzsche, H. Vereecken and J. van der Kruk
Issue: Vol 11, No 2, April 2013 pp. 197 - 209
Special Topic: Ground-Penetrating Radar
Info: Article, PDF ( 5.56Mb )
Tomographic inversions of cross-hole ground-penetrating radar provide images of electromagnetic properties of the shallow subsurface and are used in a wide range of applications. Whereas the resolutions of ray-based methods like first-arrival traveltime and first-cycle amplitude tomography are limited to the scale of the first Fresnel zone, full-waveform inversions incorporate precise forward modelling using the full recorded signal for a solution of Maxwell’s equation, which results in sub-wavelength resolutions. In practice, the method can be time-consuming in data acquisition and expensive in computational costs. To overcome these expenses, a semi-reciprocal acquisition setup with a reduced number of transmitters and an interchange of transmitter and receiver boreholes instead of a one-sided equidistant setup in either borehole yielded promising results. Here, this optimized, semi-reciprocal acquisition setup is compared to a dense, equidistant, one-sided acquisition setup measured at the field site Krauthausen, Germany. The full-waveform inversion results are evaluated using the checkerboard test as a capable resolution analysis tool to determine resolvabilities. We introduced also a new method of time-zero correction by a cross-correlation of a zerooffset profile with corresponding horizontal traces of each multi-offset gather. The obtained experimental results from Krauthausen combined with the checkerboard analysis indicate the main threepermittivity layers that correspond with different porosities. Also fine-layered structures within these main layers were reliably imaged. We conclude that the use of the semi-reciprocal setup is optimum for acquisition speed, inversion speed and obtained permittivity inversion results. Our results indicate that conductivity results are better for denser transmitter-receiver setups.