For nearly a century, energy companies have relied on seismic surveys as crucial information sources in the quest to discover subsurface oil and gas reservoirs. Advances in computing power have dramatically improved the fidelity of reservoir simulations, driving the need for more complex and precise seismic surveys. Increasingly, these are planned with satellite imagery.
Geophysicists use seismic survey data to create two- or three-dimensional views of the geologic structures beneath the Earth’s surface. During hydrocarbon exploration, these subsurface perspectives help identify formations that could serve as traps for oil and natural gas. Once potential reservoirs are located, sophisticated computer simulations combine seismic and other geophysical data to model the best methods of extracting the hydrocarbons.
Seismic data is collected by applying sound waves to the surface, either in the form of vibrations or explosions. These waves travel through the underlying strata where they are reflected and refracted according to characteristics of the geologic layers. A multitude of receivers on the surface, known as geophones, captures and measures these returned waves. The strength, location and timing of the signals are processed to reveal the depth, position and composition of geologic structures.
Gary Crews, vice president of customer service and operations manager for Spatial Energy, a Boulder, Colo.-based company that provides geospatial imagery for energy companies, says the use of satellite imagery for seismic surveying is growing.
“The earlier satellite image products can be introduced to the design process, the better the results will be in terms of survey quality, project cost and risk mitigation,” says Crews.
Spatial Energy helps clients apply high-resolution imagery and DEMs throughout the planning and operations phases of a seismic survey. When conditions in the field differ from what was anticipated during the planning stage, the survey may have to be redesigned at significant cost of time and money for the operator and vendor. In the worst cases, shoddy planning work can result in damage to the environment or equipment, and the risk of injury to field crews can be greater.
“Using high quality satellite imagery and terrain data is all about risk mitigation,” says Bob Brook, CEO of Salamanca Energy. “The overall cost is usually less than 1 percent of a typical seismic survey–a small expense to significantly reduce the risk.”
Spatial Energy utilizes submeter imagery and derived DEMs from the twin Pléiades optical satellites. Launched in December 2011 and December 2012, the Pléiades 1A and 1B satellites operated by Astrium, a space technology company, capture panchromatic and four-band multispectral data processed to generate 50-centimeter, orthorectified, color imagery.
Each satellite can pivot in orbit to acquire fore- and aft-looking stereo images in the same pass. The Pléiades satellites can even roll fast enough to capture a third nadir-looking dataset for tri-stereo imaging, a necessity for accurate DEM generation in extremely steep terrain. Same-pass stereo acquisition means that all images are captured under identical ground conditions, which leads to higher elevation data quality overall.
“The Pléiades stereo images can be processed to two levels of DEM post-spacing: 1 and 4 meters,” says Crews. “We have found the Pléiades 4-meter DEM product, called Elevation4, offers the best combination of cost and accuracy for seismic survey planning.”
Post-spacing refers to the distance between elevation points in the DEM. Elevation4, therefore, has a measured elevation point every 4 meters. The vertical accuracy of this elevation model is 3 meters. Spatial Energy sells the Elevation4 DEMs and the images from which they are generated in a package that ensures consistency in both products because they are derived from the same source data.
As is true with most phases of hydrocarbon operations, satellite imagery plays multiple roles in seismic survey planning. The first is as a logistics tool. Seismic crews usually include hundreds of people, dozens of large vehicles and tons of equipment, all of which must be transported to the site. During planning, a seismic company views the imagery to find existing access roads, water sources, modern infrastructure and towns to support the endeavor.
Next, the imagery is used for planning the actual layout of the survey lines and geophone locations, and this is where the multispectral content and high-resolution of the Pléiades imagery makes a difference. Planners use the imagery, often classified by land-cover and land-use type, to evaluate the ground surfaces in the project area to determine the most and least conducive to seismic operations.
The centerpieces of most seismic surveys are vibration trucks, or large vehicles equipped with vibrators on their undersides. The trucks move along the survey line stopping at prescribed intervals where they lower the vibrator to the ground and apply high-frequency sound waves. These vibrators need flat surfaces to achieve sufficient contact with the ground to transmit quality signals.
As the imagery is being used to evaluate land cover, the satellite-derived DEMs are analyzed for consideration of the most suitable topography for operations. The vibration trucks can only be operated safely on slopes of 20 degrees or less. The four-meter Pléiades elevation models offer just the right detail for planners to determine where the terrain is safe.
Some topographies are too steep and rugged for vibration truck operations even with the assistance of earth moving vehicles. When these slopes coincide with where the seismic lines must be run, the planners opt to use explosives instead. Knowing ahead of time when and where vibration trucks versus explosives will be used is crucial to the detailed planning process because the explosives are considerably more expensive.
“Between the land-cover information in the high-resolution imagery and the ground slope detail in the DEMs, the seismic planner in the office can design a grid of survey lines and geophone locations that is both safe and effective,” says Crews. “Precise advance planning reduces the risk of having to delay a 100-person crew onsite because something unexpected is encountered in the field.”
The same geospatial information is valuable in other areas, too, because surveys will be required for subsequent planning for well drilling and other operations.
“We use the data over and over again, for everything from refining our geological mapping and modeling through the production and development phase,” Brook says. “It’s well worth the investment.”
The sharing and reuse of datasets by several vendors has never been easier thanks to cloud-based platforms. Spatial Energy offers Spatial on Demand, a cloud-based data management service that provides delivery of data into oil and gas computer applications, enabling internal and external collaboration throughout the exploration and production lifecycle.
“Just as the seismic planners are increasingly analyzing imagery and elevation data to design their survey grids, geologists and petroleum engineers have long applied the same datasets to drilling, production and environmental remediation phases,” Crews says. “The applications are almost identical, just further downstream in the operation. Certain other activities, such as selection of pipeline and processing plant locations, may require use of the higher resolution Elevation 1 DEMs.”