Meaning and Purpose
A seismic survey is a geophysical technique used to study underground rock formations by generating controlled sound or seismic waves and recording how those waves travel through the subsurface.
It is widely used in:
- oil and natural-gas exploration;
- offshore hydrocarbon exploration;
- geological mapping;
- groundwater studies;
- engineering and infrastructure planning.
In hydrocarbon exploration, seismic surveys help identify underground structures that may contain commercially viable oil or gas reserves before expensive drilling begins.
Working Principle
A seismic survey involves three basic stages.
Generation of seismic waves
On land, vibrations may be produced using specialised vehicles or controlled explosive charges.
At sea, compressed-air devices called air guns release bursts of sound energy into the water.
Reflection from rock layers
The waves travel downward through water and rock. When they encounter boundaries between different geological layers, part of the energy is reflected back towards the surface.
Recording and interpretation
Sensitive instruments record the returning waves.
On land, these instruments are called geophones. Offshore surveys commonly use long cables containing hydrophones.
Computers then analyse the travel time and strength of reflected waves to construct images of underground geological structures.
Types of Seismic Surveys
Two-dimensional seismic survey
It produces cross-sectional images along individual survey lines and is useful for preliminary exploration.
Three-dimensional seismic survey
It collects data across a wider area and creates a detailed three-dimensional representation of subsurface formations. It is widely used before drilling expensive offshore wells.
Four-dimensional seismic survey
It involves repeating three-dimensional surveys over time to observe changes within an oil or gas reservoir during production.
Modern surveys may also use ocean-bottom sensors, advanced computing and artificial intelligence for improved geological interpretation.
Importance in Hydrocarbon Exploration
Seismic surveys help identify features such as:
- folds and faults;
- sedimentary layers;
- structural traps;
- salt formations;
- potential hydrocarbon reservoirs.
They reduce exploration risk because companies can select drilling locations based on detailed geological information.
In deepwater regions, seismic surveys are especially important because direct geological observation of the seabed and underlying rocks is difficult and drilling costs are extremely high.
However, seismic data can indicate favourable geological structures but cannot confirm the presence of hydrocarbons. Only exploratory drilling can establish whether commercially recoverable oil or gas actually exists.
Environmental Concerns
Offshore seismic surveys generate intense underwater sound, particularly through air-gun arrays.
Potential impacts include:
- disturbance to whales and dolphins;
- disruption of communication and navigation among marine mammals;
- behavioural changes in fish;
- temporary displacement of marine species;
- possible effects on fisheries.
Environmental safeguards may include:
- avoiding sensitive breeding and migration periods;
- maintaining exclusion zones around marine mammals;
- gradually increasing sound intensity before full operation;
- using trained marine-wildlife observers;
- stopping operations when protected species approach the survey area;
- developing quieter seismic technologies.
Conclusion
Seismic surveys are essential tools for mapping underground geological structures and reducing uncertainty in oil and gas exploration. Their economic value is substantial, but offshore surveys require careful regulation to minimise disturbance to marine ecosystems.
