Ambiguity in the solution to the velocity inversion problem and a solution by joint inversion of seismic refraction and wide‐angle reflection times

SUMMARY In a multilayered earth system no refraction signal will be generated from the top of a layer that has a seismic velocity less than that of the layer immediately above. The interpretation of refraction data in such a situation cannot provide any information about the low‐speed layer (LSL), and its presence leads to the overestimation of depths to the bottom of the overlying or cap layer and all subsequent underlying layers. This is the velocity inversion (VI) problem of seismic refraction work. Other geological/geophysical information is required to solve the VI problem. In this respect, wide‐angle reflections from the bottom of the LSL may constitute the most reliable information, since they are observed as very strong arrivals on the seismogram after first arrivals. This paper shows that, even if reflections from the bottom of the LSL are used, the solution (i.e. the thickness of the cap layer, and the thickness and velocity of the low‐speed layer) to the VI problem is still ambiguous. However, use of the wide‐angle reflected phase limits the number of solutions. A method of joint inversion of traveltimes for refractions and wide‐angle reflections from the bottom of the LSL is proposed to calculate the thickness of the cap layer and that of the LSL, the velocity of the LSL having been constrained from other information. The reliability of the method has been tested on synthetic data using various initial models. Application to field data demonstrates the practical feasibility of the method by comparing the result with the available solution for the region.