The traditional technique for processing and interpretation of refraction seismic data (see Standard Refraction Seismics) often results in geological models which show disturbed subsoil areas (deformations, weak zones, discontinuities, faults, etc.) as a thin sharp low velocity zone surrounded by fresh rock. The real case is most likely much more complex. Deformation zones may have a narrow core of heavily crushed rocks, but they are often surrounded by a wider zone (influence area) constituting altered, fractured rocks with deteriorated mechanical properties as compared to the fresh rock. In order to extract more information from the seismic data as compared to the traditional processing technique, to overcome the limitations of the standard refraction seismic technique and to be able to create more advanced geological models (more diverse bedrock velocity models) that come closer to the complex reality, the data are processed by use of modern computerized tools for analyzing refraction seismic data with tomography inversion techniques.
The tomography inversion is performed with the software RAYFRACT. The program uses so called WET or Wave Eikonal Traveltime Tomography processing. The software RAYFRACT is a windows based 32-bits software package for processing of refraction seismic data. The program basically offers two different ways of processing the data:
1. DELTA-TV method, Gebrande and Miller (1985)
The Delta TV method is a pseudo 2D Inversion method that delivers a continuous 1D velocity versus depth model for all geophone stations. The method handles real life geological situations such as velocity gradients / linear increasing of velocity with depth / velocity inversions / pinching out layers and outcrops / faults and local velocity anomalies gracefully. It is especially a very strong tool for structural interpretation of the geologic situation.
2. WET or Wave Eikonal Traveltime Tomography Processing
Wave propagation is modeled in a physically meaningful way with ray paths, using the output from the Delta-TV inversion as starting model. It handles several real life geological situations, such as discontinuities velocity distributions and sharp vertical or horizontal velocity gradients caused by e.g. deformation zones. Quality control of geological models is performed by direct graphical comparison of the measured travel time data to those calculated from the model solution.
The tomography inversion is performed with the software RAYFRACT. The program uses so called WET or Wave Eikonal Traveltime tomography processing (formula for the back projection of traveltime residuals in traveltime tomography). The special case of the WET formula leads to a computationally efficient inversion scheme in the space-time domain that is, in principle, almost as effective as WT inversion yet is an order of magnitude faster. It also leads to an analytic formula for the fast computation of wavepaths. Wave Eikonal Tomography models multiple signal propagation paths contributing to one first break. Conventional ray tracing tomography is limited to the modeling of just one ray per first break. The Eikonal solver (Lecomte, Gjoystdal et al. Geophysical Prospecting May 2000) used for traveltime field computation explicitly models diffraction besides refraction and transmission of acoustic waves. As a consequence the velocity anomaly imaging capability is enhanced with the WET tomographic inversion compared to conventional ray tomography and or standard refraction techniques. WET Tomography is a non-intrusive method that is performed on the ground surface with a conventional refraction seismic geophone spread and seismic source (e.g. sledgehammer, drop weight, etc.). WET Tomography supports any surface based 2D recording geometry, including roll-along seismic reflection lines.
... More than 400.000 m of seismic refraction tomography surveys carried out and processed for exploration and engineering projects between 2006 and 2016 ...