Ground Roll Repeatability Analysis - CO2CRC Otway Project Case Study

Time-lapse (TL) seismic is an essential tool for the monitoring of changes in reservoir conditions induced by reservoir production, reservoir stimulation and more recently CO2 sequestration. The Cooperative Research Centre for Greenhouse Gas Technologies (CO2CRC) is currently conducting a carbon dioxide geo-sequestration pilot study in the Otway Basin, onshore Victoria, Australia. An extensive TL seismic monitoring is a part of monitoring and verification program of the project. Being a well-developed methodology for offshore applications, time-lapse seismic is also gaining popularity as an onshore reservoir monitoring technique. However its onshore use is limited by issues of seismic repeatability. This is primarily attributed to the higher near-surface variability as compared to marine seismic. Analysis of several repeated Otway 2D surveys show that different frequency bands have different repeatability. Low frequencies (< 30 Hz), contaminated by the ground roll noise, are characterised by particularly poor repeatability. In this paper we analyse three repeated 2D surveys obtained with same geometry but different sources and near-surface conditions in order to investigate ground roll repeatability by evaluation of surface waves phase velocity spectra.