Ultrasonic travel-time tomography in core plugs

Abstract

This paper applies ultrasonic travel-time tomography in order to investigate the resolution (i.e. definition of dimension and shape) of velocity heterogeneities in tomograms from core plugs. Designing the tomographic algorithm involves two main steps, namely, modelling and inversion. In the modelling step, we use straight and curved ray-tracing schemes to calculate raypaths and travel times corresponding to the source-receiver configuration. The inversion step then submits a predefined norm of the residuals between calculated and measured travel times to a minimization process. The conjugate gradient method works to iteratively minimize the ℓ1 norm of travel-time residuals. Numerical experiments show that, in addition to the dominant frequency of the probing ultrasonic pulse, the resolution of tomograms from core plugs with high-contrast heterogeneities depends on the chosen ray-tracing scheme. In summary, if the rock sample exhibits low to moderate velocity contrasts, the tomographic algorithm using straight rays in the modelling step yields tomograms with good resolution. However, in the presence of high velocity contrasts, plausibly resolved tomograms can be obtained only by adopting a curved ray-tracing scheme. The application of ultrasonic tomography using travel-time data from real core plugs corroborates the results obtained with numerical data. © 2007 Nanjing Institute of Geophysical Prospecting.