Analysis of water velocity changes in time-lapse ocean bottom acquisitions - A synthetic 2D study in Santos Basin, offshore Brazil

Abstract

Time-lapse seismic is an important tool for reservoir management. As seismic data quality improves, there is a drive for moving towards quantitative interpretation, which demands data of higher repeatability. In deep water time-lapse projects, the changes in the speed of sound of the water layer between baseline and monitor acquisitions play a significant role in the data processing. In this work, we study the effects of these water velocity changes, in a scenario of ocean bottom nodes (OBN) acquisition. We perform 2D elastic seismic modeling with high-resolution property models for two distinct survey dates. In each date, different water velocity profiles, measured in field experiments in a Brazilian Pre-salt area, are considered. To incorporate source and receiver non-repeatability, some differences in the acquisition geometries are also tested for each vintage. The synthetic data are compared in terms of time-lapse amplitudes, and repeatability is quantified via the normalized root mean squared attribute (NRMS). For the parameters considered in our study, the deterioration of NRMS seems to be dominated by the non-repeatability of the receivers and by water velocity variations, with source non-repeatability having a lower contribution. We believe the results presented in this work are a first step towards a more robust methodology for time-lapse feasibility studies, which incorporates imaging uncertainties and allows us to understand the contribution of each element that could affect the 4D seismic image quality.