Significant advances in the technology surrounding 3-D seismic data through the years no doubt have encouraged many observers to view this major component of E&P to be just a routine piece of doing business.
It’s a highly complex routine, both at sea and on land.
Three-dimensional seismic data acquisition survey design, techniques and data processing are crucial aspects of the process overall. Even when this all works, a survey might provide unsatisfactory results.
For example, the target to be imaged could lie deep within the ground, but complexity in the near surface geology can interfere with the seismic signal. With land surveys, the surface itself can provide obstacles in the form of buried soft layers of sediment, surface dunes and irregular topography, among other hindrances to acquisition of usable data.
Saudi Arabia is known to be a challenging region for land seismic.
But challenges are meant to be overcome.
“Several broadband, high density, full azimuth 3-D land seismic surveys have been acquired recently in Saudi Arabia, over producing fields and exploration areas,” said Luis Giroldi, geophysical consultant at Saudi Aramco.
“In those surveys, increased bandwidth, improved signal-to-noise ratio and better subsurface sampling have had a beneficial impact on both qualitative and quantitative interpretation of the data,” Giroldi noted.
He emphasized that recording of lower frequencies significantly improved the interpretation of deep levels, enhancing both the structural clarity and the resolution of the seismic data along with faster, more accurate performance.
In addition, improved, more-data-driven seismic inversion turned out to be a reliable indicator of subsurface heterogeneity both laterally and vertically, away from well control.
Eastern Province 3-D Land Survey
Giroldi and colleagues have provided an overview of a broadband, high density, full azimuth 3-D land survey, which was acquired in 2011 over a producing oil and gas field in the Eastern Province of Saudi Arabia.
The survey was laid out to test a combination of point receiver sensors and a distinctively designed low frequency sweep for utilization in improved reservoir property prediction.
For the uninitiated, point receiver acquisition entails recording traces from individual receivers instead of tallying the responses of a group or array of receivers prior to recording the aggregate trace.
Once processing was complete, analysis of the new 3-D seismic data volume indicated a bandwidth increase with improved signal-to-noise ratio when compared to legacy 3-D data, according to Giroldi.
He noted specifically that the interval of the deep main clastic reservoirs of Permian age displays a 3 to 45 Hz frequency range at -12 dB in the new data compared to 8 to 25/30 Hz frequency range at -12 db with the legacy data.
The new dataset garnered high marks in the interpretation process, proving to be less complex, more accurate and faster than the legacy package.
In fact, large parts of the new survey enabled a more continuous and coherent interpretation to be made while preserving structural detail. In contrast, the legacy 3-D has poorer event continuity and tracking overall.
“The new dataset is fully interpretable and is able to clearly image the subsurface below 3 seconds two-way time,” Giroldi said. “Structural attributes applied on the band-limited new data delivered sharp definition of the deep seated tectonic elements affecting the area.”
He noted that at the end of the day, the interpretation of the new broadband, high density, full azimuth 3-D land survey demonstrates the beneficial impact of the increased bandwidth and improved signal-to-noise ratio of the seismic data.
“The recovery of low frequency signals has dramatic implications for seismic inversion as less reliance can be placed on the initial model,” Giroldi explained, “and in the interpretation of deep levels, improved structural clarity and resolution can be achieved.
“This dataset has richer information on the low end of the frequency spectrum and, as a result, there is less uncertainty on the initial model when performing model-based acoustic impedance inversion, resulting in improved fidelity for determination of reservoir properties.”