Improving Land Seismic Repeatability Using Buried Geophones: CO2CRC Field Study

Abstract

Stage 2C of the CO2CRC Otway project involves an injection of a small amount (around 15,000 tonnes) of CO₂/CH4 gas mixture into a saline aquifer at a depth of approximately 1.5 km. Previous studies at this site showed that seismic repeatability is relatively low due to variations in weather conditions, near surface geology and farming activities. In order to improve time-lapse seismic monitoring capabilities, a permanent receiver array will be utilised to improve signal to noise ratio and hence repeatability. To understand the repeatability improvement that can be achieved by such a deployment, and to optimize the acquisition parameters, a small-scale trial of a buried receiver array was conducted at the Otway site in June 2012. A set of 25 geophones was installed in 3m deep boreholes in parallel to the same number of surface geophones. In addition, four geophones were placed in boreholes of 1–12 m depth. In order to assess the gain in the signal-to-noise ratio and repeatability, both active and passive seismic surveys were carried out. The surveys were conducted in relatively poor weather conditions, with rain, strong wind and thunderstorms. With such an amplified background noise level, we found that the noise level for buried geophones is on average 20 dB lower than for the surface geophones. The levels of repeatability for borehole geophones estimated around direct wave, reflected wave and ground roll are twice as high as for the surface geophones. Both borehole and surface geophones produce the best repeatability in the 30–90 Hz frequency range. The influence of burying depth on S/N ratio and repeatability shows that significant improvement in repeatability can be reached at a depth of 3 m. The level of repeatability remains relatively constant in the depth interval of 3 to 12 m. Based on the results of the field tests, previous seismic surveys and modelling, a buried geophone array was designed for the Stage 2C of the Otway Project. The array includes 929 geophones buried at a depth of 4 m along eleven receiver lines covering an area of approximately 1 sq.km. The array is to be deployed in February-March 2015 and the baseline acquisition carried out in March-April 2015.

AAPG Datapages/Search and Discovery Article #90217 © 2015 International Conference & Exhibition, Melbourne, Australia, September 13-16, 2015