Introduction

We present a robust reverse-time-migration approach for imaging vertical seismic profiles. With this method it is possible to calculate time-shift, depth-shift, and angle gathers for immediate validation of the propagation parameters in the vicinity of the borehole. It makes full waveform inversion of vertical seismic profiles (VSP) straightforward. Through a densely sampled 2D walkaway synthetic, we show that full waveform inversion is a viable process for extraction of near borehole model properties. Applying our process to synthetic anisotropic data verifies the extensibility of the process to complex media. The ability to produce angle gathers means that we can utilize prestack inversion methods to enhance extraction of reservoir properties. We demonstrate that our method produces higher quality results then traditional wavefield separation approaches even in cases where the borehole is poorly sampled.

Traditional VSP Processing

Modern processing of VSP's has remained static for many years. Wave field separation is applied to estimate upward and downward traveling waves at the borehole which are then used to obtain subsurface images. The utility of this approximate separation is best for "rig-source" non-deviated wells, but loses meaning rapidly in the presence of deviated wells and offset VSPs. We concluded that this methodology did not provide anything close to the kind of image quality that should be possible.

Panorama VSP Processing

Our imaging process achieves a more complete separation of the wavefields into directional components, and handles correlations between any two components of "source" and "receiver" wavefields to produce a subsurface VSP image consistent with and highly correlated with surface reflection measurements.

History of Panorama RTM/VSP Processing

In 2007, Panorama developed a new RTM imaging condition motivated by the desire to improve amplitude response at high opening angles while avoiding well known RTM imaging artifacts. Such artifacts are usually attenuated during post processing with a low wavelength filter. The two problems are really one. An angle domain imaging adjustment turned out to be the solution. Ultimately we discovered that our new approach also showed promise for imaging seismic borehole data. Some modifications for the unique VSP geometry resulted in a hands-free VSP imaging method that we call VSPRS. Application of VSPRS to several full waveform inversion projects resulted in superior results and thereby validated the approach.

Success

High angle responses are much more prevalent in VSP data then those in surface measurements. As evident in the following examples, large features such as water bottoms, tops of salt, and others, are entirely imaged in the high angle domain, so it is crucial to accurately handle those amplitudes. We consider our success with VSP imaging to be a complete validation of our FWI approach.

The overall workflow is quite simple. The VSP is sorted into either borehole receiver gathers or surface shot gathers and then imaged using our version of reverse time migration. This is equivalent to the migration of traditional surface shot records which is quite simple in concept. Moreover, as shown in the following examples it is possible, but computationally costly to produce time/depth shift and angle gathers. However, these gathers are extremely useful because they can directly impact subsurface parameter estimation and further enhance the imaging process.

Summary

Our proprietary imaging algorithm gives more accurate amplitudes and fewer artifacts, and has been extended to VSP processing since 2013.