Exploiting the potential of 3D borehole seismic data for high-resolution imaging and velocity estimation, a full wavefield approach

Abstract

Geophysics is a branch of physics that is mainly concerned about under- standing and describing the physical behaviour and activities of the earth’s geological system. Usually, seismic data is acquired at the surface and the corresponding signals go through a sequence of preprocessing steps to filter out the noise and enhance the quality of the measurements. These mea- surements are then transformed into a so-called reflectivity image (snapshot in time) of the subsurface via the deployment of the so called migration al- gorithms. Extensive studies and great effort is usually made to determine a suitable acquisition geometry design for optimal illumination of every imaging grid point in the subsurface in the studied area. However, within the geo- energy industry it has always been an undisputed believe that improved and better results can be obtained only if more data is acquired with denser sam- pling at both the source’s and receiver’s side. This ”linear” way of thinking is also consistent with the conceptual assumptions of most current migration algorithms.
The challenge is of course to achieve the same, or maybe even better, results with less data. This is in accordance with the currently ongoing en- ergy transition, which is forcing the geophysical scientific community to shift their focus from the acquisition and processing of large and expensive surface seismic surveys toward optimized management, in terms of data acquisition, processing and production, of the existing hydrocarbon-based reservoirs. Es- pecially datasets with sparse acquisition geometry like 3D Ocean Bottom Node (OBN) and 3D Borehole Seismic Data (BSD) surveys, where we have measurements at a limited number of sensors along the ocean bottom or in the borehole but usually with dense sources sampling at the surface, can greatly benefit from such a development.
3D borehole geophysics, which is the main subject of this thesis, has for a long time been an underdeveloped and, therefore, an unappreciated compo- nent within most geophysical organisations. This is mainly because accurate results are usually obtained only in the immediate vicinity of the borehole and
their quality decays rapidly in the lateral extent. However, and especially in the marine case, 3D BSD surveys are rich in higher-order scatterings that can have significant added value when combined with unconventional and non- linear inversion-imaging algorithms like Full Wavefield Migration (FWM) and Joint Migration Inversion (JMI). Furthermore, in combination with modern measurements techniques (like Distributed Acoustic Sensing (DAS) technol- ogy), continuous and permanent monitoring of existing and new reservoirs – whether hydrocarbon-based or geothermal – can easily be realised.
In this thesis the recently developed inversion-imaging algorithms FWM and JMI are extended to the 3D case and further engineered to properly han- dle the special acquisition geometry of 3D BSD surveys and exploit the full potential of the total wavefield available in 3D borehole seismic data. First, a more complete and comprehensive derivation of the involved gradients, for the reflectivity image and velocity model update, is presented. This makes the combination of one-way tomography of the direct wavefield with reflec- tion tomography of the other energy modes (primary reflections, higher-order scatterings of the up- and down-going wavefield) a straightforward process. Then, an effective strategy of the application of the 3D JMI algorithm to 3D BSD is developed and, with the presented examples, it will be demonstrated that, for instance, the standard and conventional separation of the up- and down-going wavefield of 3D BSD becomes an obsolete process. Along the same lines, we will show that integration of surface seismic data and 3D BSD, or even multi 3D BSD surveys, in one inversion process produces more accurate and geologically consistent solutions. Next, the capability of the 3D FWM algorithm together with 3D BSD surveys for solid reservoir monitoring will be demonstrated. After that the challenges of the current acoustic imple- mentation of the FWM JMI algorithms will be discussed, especially the effect of the mode converted waves on the velocity model gradient. Finally, some suggestions are made for further enhancement of the JMI algorithm, partic- ularly at the side of the migration velocities update. This can be achieved by the combination of complementary and effective objective-functions, which makes the JMI algorithm more robust especially in the case of geological environment with high velocity contrast.