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Summary In order to improve the seismic repeatability required for weak 4D signal measurement, we bury the sensors and the sources below the weathering layer. Nevertheless a part of the emitted signal is transmitted to the weathering layer. After a reflection or a conversion in the vicinity of the earth surface, the downgoing waves are recorded by the buried sensors and may affect the repeatability of the primary 4D signal. The reduction of these unwanted waves is necessary to ensure a good reliability of the seismic monitoring results. In an industrial noise context (which is usually the case for reservoir production), it has been observed that geophones are much noisier than hydrophones (Forgues and Schissele, 2010). In these circumstances, we show that the use of dual-depth hydrophones as an alternative to P-Z summation allows us to handle the receiver ghost and significantly improve the signal repeatability and stabilize the signal variation above the reservoir.

Summary A series of land surface vibrator repeatability tests were conducted in Saudi Arabia as a part of a feasibility experiment for permanent monitoring. While post-stack repeatability of 15% to 20% was achieved, pre-stack seismic repeatability is difficult to accurately quantify. If we can understand the main controlling factors behind prestack vibrator repeatability, we stand a chance to improve upon these results. This study focuses on pre-stack repeatability metrics for field data acquired using a surface vibrator and a combination of surface and cemented buried geophones. A series of six 2D surveys were repeated as well as daily and hourly sweep tests. Observations suggest that the main factors affecting seismic repeatability include vibrator geometry errors (as small as 0.5 m), and how the vibrator interacts with the near surface. It was observed that the initial sweeps acquired with a vibrator show significant time and amplitude variability as measured by both the surface and deep cemented sensors, whereas data recorded from later sweeps appears more repeatable. This initial “warming up stage” followed by a more stable sweep was observed on all repeat acquisition tests, even when sweep sequences were only one hour apart. This effect may be caused by ground compaction, with some partial rebound within a short time following termination of the sweep sequence. Due to all these factors, it is clear that land seismic data acquired using a surface vibrator has some inherent non-repeatability, even when the source positioning errors are minimal.

Summary A permanent reservoir monitoring system has been installed on an onshore heavy oil field in the context of redevelopment planned for the coming years. The challenge was to continuously monitor with seismic reflection the lateral and vertical expansion of the steam chest injected in the reservoir during production over a year. We show that the very high sensitivity of our buried acquisition system allows for the detection and mapping of tiny changes within the reservoir on a daily basis: we measure a 6 s time shift and a 0.1% amplitude variation per day. The values obtained from seismic monitoring fit the pressure variations measured at observation and production wells. These results confirm that the steam does not follow the expected path to the producer wells.