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

Abstract

S U M M A R YIn 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 wideangle 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.

Detection of sub-basaltic sediments by a multi-parametric joint inversion approach

“…To compute the refraction travel time, we take the following expression (Sain and Kaila 1996;Manglik and Verma 1998) for a horizontally layered, homogeneous, isotropic earth:…”

Crust and Lithospheric Structure - Active Source Studies of Crust and Lithospheric Structure

“…These inversion techniques assume a monotonically increasing velocity-depth model, which in most tectonic settings is reasonable, but in some cases may be too restrictive, such as in high heat flow regions Wenzel and Sandmeier, 1992). If LVZs are involved, wide-angle reflections from the top and bottom of the LVZ can be used to constrain them (e.g., Sain and Kaila, 1996). The 1-D 'reflectivity' method has been very popular because of its ability to provide elastic full-wavefield seismograms.…”

Delineation of sediments below flood basalts by joint inversion of seismic and magnetotelluric data

“…The refraction travel time for a horizontally layered, homogeneous, isotropic structure is given by [Sain and Kaila, 1996 We added a damping parameter (Marquardt lambda) in the diagonal elements of the Jacobian matrix to stabilize the solution in the initial runs. Addition of a large damping parameter stabilizes the matrix by enhancing small eigen-values but at a cost of losing some information.…”

“…The refraction travel time for a horizontally layered, homogeneous, isotropic structure is given by [Sain and Kaila, 1996] …”