Seismic surveys are a sophisticated, non-invasive method used to create detailed images of the Earth’s subsurface. This technology operates much like medical ultrasound or sonar, utilizing controlled acoustic energy to map geological structures. The process involves generating a sound wave and then recording the returning echoes, which are reflections from various rock layers. By analyzing the time it takes for these sound waves to travel down and return, scientists can construct a picture of the underground environment.
The Basic Science of Seismic Imaging
The process begins with an energy source that initiates a compressional wave, often called a P-wave, which travels through the ground or water. As this wave encounters boundaries between different rock types, a portion of the energy is reflected back toward the surface, while the rest is transmitted deeper into the Earth.
The strength of the reflection is directly related to the contrast in a property known as acoustic impedance (AI) between the two layers. Acoustic impedance is calculated as the product of the rock’s density and the velocity at which the seismic wave travels through it. When the wave hits an interface where this AI changes significantly, such as the boundary between soft shale and hard sandstone, a strong reflection occurs.
Receivers placed along the surface, called geophones on land or hydrophones in water, record the arrival time of these reflected waves. The time it takes for the wave to travel down to a layer and return, known as two-way travel time, is the primary data used for interpretation. Converting this time measurement into an accurate depth requires a precise understanding of the seismic wave velocity, as wave speed varies based on material composition and density.
Seismic waves also undergo refraction, which is the bending of the wave as it passes into a medium with a different velocity. Both reflected waves, which provide high-resolution images of layer boundaries, and refracted waves, which determine the velocities of deeper layers, are analyzed to build a comprehensive subsurface model. The resulting seismic profile is a cross-sectional image that resembles a geological cross-section, revealing faults, folds, and horizontal bedding planes.
Different Methods of Survey Acquisition
The physical environment dictates the specific methods and equipment used for seismic data acquisition, broadly divided into land and marine operations. Land surveys typically use specialized vehicles called vibrator trucks, which generate controlled, low-frequency seismic waves by vibrating a heavy plate against the ground. For deeper penetration, surveys may still employ small explosive charges to create a strong acoustic impulse.
The returning echoes on land are captured by thousands of small sensors called geophones, which convert the ground motion into electrical signals. Marine surveys, conducted from specialized vessels, use arrays of airguns that release air into the water to produce a powerful acoustic pulse. The reflected pressure waves from the seabed and sub-bottom layers are then detected by streamers, which are long cables towed behind the vessel and equipped with hydrophones.
Seismic data is also categorized by the dimensionality of the subsurface image it creates. A two-dimensional (2D) survey involves the source and receivers moving along a single straight line, producing a vertical cross-section or slice of the Earth beneath that line. Three-dimensional (3D) surveys involve densely covering an area with both sources and receivers, resulting in a cube or volume of data. This 3D volume offers a significantly higher resolution and a more accurate representation of complex geological features, as it captures reflections from all directions.
Primary Applications and Uses
Seismic surveys serve a wide range of industries and scientific disciplines by providing a subsurface view. The most widely known application is in resource exploration, specifically for identifying underground traps likely to contain oil, natural gas, and geothermal energy reserves. By mapping geological faults and structural features, companies can significantly reduce the risk and cost associated with exploratory drilling.
Civil engineering projects rely on seismic data for site investigations before construction begins. This information is used to assess the stability of the near-surface geology for large infrastructure, such as dams, bridges, tunnels, and nuclear power plants. Engineers use these surveys to determine the depth to bedrock, identify unstable soil layers, and understand potential earthquake hazards.
In scientific research, these surveys are a fundamental tool for understanding the Earth’s deep interior and tectonic processes. Geophysicists use seismic images to map the structure of the Earth’s crust and mantle, study active fault lines, and gain insights into the movement of tectonic plates. These applications extend beyond resource development, providing a foundation for global geological knowledge.
Environmental Considerations
The use of powerful acoustic energy in seismic surveys, particularly in marine environments, raises concerns about the potential impact on marine ecosystems. The loud, low-frequency pulses generated by airguns can travel long distances underwater, which can interfere with marine mammals that rely on sound for navigation, communication, and foraging. Whales, dolphins, and seals may exhibit avoidance behavior, causing them to abandon feeding or breeding grounds in the vicinity of the survey.
While direct physical injury to marine mammals is thought to be low probability except at very close ranges, the noise can cause temporary or permanent hearing impairment. Fish are also affected, with studies showing that seismic noise can cause physical damage to their ears and swim bladders, and cause commercially important species to flee the area. Fish eggs and larvae, which are highly vulnerable, may also experience adverse effects, potentially impacting population levels.
To minimize these environmental effects, strict mitigation measures are routinely implemented during marine seismic operations. A “soft start” procedure is a common technique where the sound intensity of the airguns is gradually increased, allowing marine life to move away from the source. Marine Mammal Observers (MMOs) are stationed on the survey vessel to visually scan for protected species within a defined exclusion zone. If a marine mammal is sighted entering the exclusion zone, the seismic source must be immediately shut down and only restarted once the animal has moved safely away. Passive Acoustic Monitoring (PAM) systems are often used in conjunction with observers, employing towed hydrophones to detect the vocalizations, particularly in low visibility or at night.