Seismic Surveys: A look into the technology behind oil and gas exploration
Seismic surveys have become an integral part of exploration activities for oil and gas companies around the world. Using sophisticated seismic technology, companies are able to map underground geological structures to locate potential hydrocarbon reserves. In this article, we take a closer look at the seismic survey process and the various techniques used.
What is a seismic survey?
An seismic survey is a non-invasive geophysical method used to image the subsurface of the Earth by creating and studying the propagation of elastic waves in deep geologic formations. Energy sources like dynamite charges or vibrator trucks are used on the surface to generate seismic waves. These waves travel downward into the earth, reflecting off subsurface rock layers and structures. The reflected seismic waves are detected by sensitive geophones or hydrophones laid out on the surface in patterns called spreads. By analyzing the travel times and strength of the returning seismic pulses, geophysicists can map, with some degree of certainty, the subsurface geological structures, possibly indicating traps for oil and natural gas accumulation.
Seismic survey techniques
There are several techniques used in seismic surveys depending on the type and scale of exploration:
- 2D seismic: 2D seismic surveys involve gathering seismic data along predefined seismic lines. This provides interpretative images of vertically elongated subsurface features along the survey lines. While cheaper, 2D surveys have limitations in mapping complex subsurface structures.
- 3D seismic: 3D surveys acquire and process seismic data in three physical dimensions - two spatial directions and one depth dimension. Using a grid of sensor receivers, 3D surveys build up a detailed 3D image or 'cube' of the subsurface. This provides a more accurate picture of the complex geological structures and stratigraphy below. Most surveys today are 3D as they allow for better resource estimation.
- Ocean bottom cable (OBC) surveys: For offshore seismic surveying, OBC techniques involve laying long seismic cables directly on the seafloor rather than towing hydrophone streamers behind a seismic vessel. This provides higher data quality especially in areas with complex geology or shallow water depths.
- Permanent reservoir monitoring (PRM): PRM involves installing seismic sensor arrays permanently in oilfields to monitor changes in producing reservoirs over time. This helps optimize production and reservoir management.
Data acquisition techniques
The various techniques used for collecting raw seismic data include:
- Vibroseis: Vibroseis uses large vibrating plates or "swept frequency" vibrator trucks as seismic energy sources. They sweep through a range of frequencies to generate seismic waves traveling into the earth.
- Dynamite: Explosive charges laid out in patterns are detonated to generate seismic pulses. Dynamite has more energy but strict safety and environmental regulations limit its usage today.
- Airgun arrays: Offshore, airguns fitted in arrays and towed behind survey vessels are commonly used as seismic sources. They rapidly release high-pressure air to generate sound waves penetrating the seafloor.
- Nodal acquisitions: "Nodal" surveys involve deploying self-contained seismic recorders called "nodes" passively on land or seafloor. Unlike the towed streamer method, nodes don't need surface vessels near the receivers for real-time data relay.
Processing and analysis
The raw seismic data recorded by thousands of receivers undergoes advanced processing using specialized seismic processing software:
- Data editing removes bad traces and noise caused by equipment or environment.
- Deconvolution and migration improve resolution to flatten reflecting interfaces and focus scattered energy back to reflectors.
- Velocity analysis identifies seismic velocities of subsurface rock layers critical for depth conversion.
- Attributes analysis extracts quantified properties of seismic reflections, enhancing hydrocarbon indicators.
- Interpretation by geophysicists and geologists analyzes the final processed seismic volume to map subsurface structures and identify potential reservoirs and traps.
Seismic surveys are crucial exploration tools employed by oil and gas companies globally. With continuous technological advancements, the seismic imaging of underground geological formations is becoming more refined, aiding discoveries and improved reservoir management. Careful planning, data acquisition and advanced processing come together to extract geological insights from seismic surveys.
Regulatory and environmental issues
While seismic surveys provide vital insight to the industry, they also face closer scrutiny due to environmental concerns:
- Marine mammal protection: Regulations limit seismic shooting periods and monitor exclusion zones to reduce impacts on marine species like whales from intense sound pulses.
- Habitat protection: Onshore surveys require careful planning to avoid sensitive areas like wetlands or bird nesting sites. Routes use existing access roads where possible.
- Cultural heritage: Surveys screen for potential sites of archaeological or indigenous cultural significance to avoid disturbance.
- Waste disposal: Strict disposal norms govern handling and burying of seismic acquisition equipment, streamers, sensors and waste explosives after surveys.
With growing awareness, seismic contractors implement passive acoustic monitoring, pre-survey baseline studies and post-survey impact assessments to ensure minimum environmental footprint from their exploration activities. Regulators worldwide are also strengthening surveillance and compliance enforcement. The search for energy requires a careful balance of economic progress and environmental protection.
In summary, seismic surveys employ sophisticated non-invasive techniques to map the geology beneath the earth’s surface. The insights derived through careful seismic data acquisition, advanced processing and expert geological interpretation guide exploration decisions in the oil and gas industry and beyond. Continuous improvement in the technology alongside prudent regulation aims to maximize rewards from beneath the earth while minimizing surface footprint.
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