Joint Transmission and Reflection Traveltime Tomography in Imaging Permafrost and Thermokarst lakes at Northwest Territories of Canada

Abstract
We propose an application of joint tomographic inversion using both transmitted and reflected arrivals to map permafrost velocity structures at Mackenzie Delta, Northwest Territories of Canada. Our tomography algorithm is based on a grid based solver of Eikonal equation, Huygens’ Principle and the adjoint method. The grid based solver assigning a traveltime to each grid avoids the shadow zone problem in classical ray tracing and is well adapted for parallelization. The adjoint method provides the gradient of a given objective functional without explicitly estimating Fréchet derivative matrix. Combined with Huygens’ Principle, the tomographic inversion can simultaneously use first and later arrivals to optimize the final model. We first demonstrate the performance of the joint tomography algorithm on a two-dimensional (2D) synthetic model with velocity increasing with depth and decreasing afterwards mimicking a typical permafrost environment. The field 2D seismic survey, covering over 20 kilometers with up to 4 kilometer offset, was acquired from Mackenzie delta, Northwest Territories of Canada, where the shallow subsurface is characterized by high velocity permafrost and low velocity thermokarst lakes. Our results show the potential of the joint tomography in characterizing multi-scale heterogeneous velocity structures in the presence of permafrost.

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