Seismic Overview

The words seismic and geophysics are often associated with earthquakes. But seismic data are also a valuable technology used extensively by the oil and gas industry in its exploration, development and reservoir management operations.

The Purpose of Seismic

The main purpose of seismic exploration is to render the most accurate possible graphic representation of specific portions of the Earth's subsurface geologic structure.

The images produced allow exploration companies to accurately and cost-effectively evaluate a promising target (prospect) for its oil and gas yielding potential.

Seismic Fundamentals

Seismic imaging is simple. But it takes knowledge, experience and advanced technology to do it right.

Acquisition of seismic data involves the transmission of controlled acoustic energy into the Earth, and recording the energy that is reflected back from geologic boundaries in the subsurface.

Information regarding the structure and nature of the reflecting strata can be derived from the two-way travel time, and other attributes, of the returning energy. Processing these reflections produces a synthetic image of the Earth's subsurface geologic structure.

Acquiring Seismic Data at Sea

3D seismic data are displayed as a three-dimensional cube that may be sliced into numerous planes or cross-sections.

At sea, the procedure is essentially the same except that our instruments are continuously moving!

The seismic (energy) source is usually an array of airguns towed behind the survey vessel and just below the sea surface. The airguns are fired at regular intervals as the vessel moves along pre-determined survey lines.

Energy reflected from beneath the seafloor is detected by numerous 'hydrophones' contained inside long, neutrally buoyant 'streamers' - often almost 5 miles long - also towed behind the vessel.

2D Seismic Data

Two types of seismic surveys are available to the geophysicist: two-dimensional (2D) surveys, or three-dimensional (3D) surveys.

2D seismic data are displayed as a single vertical plane or cross-section sliced into the Earth beneath the seismic line's location.

2D is generally used for regional reconnaissance, or for detailed exploration work where economics may not support the greater cost of 3D . . .

3D Seismic Data

More expensive than 2D data, 3D produces spatially continuous results which reduce uncertainty in areas of structurally complex geology and/or small stratigraphic targets.

4D Seismic Data

Two or more 3D seismic surveys acquired at different times can be compared in order to search for changes in the fluids within the rock formations.

This type of survey is known as 4D, where elapsed TIME is the fourth dimension of information.

The Five Key Ingredients

There are five key ingredients to acquiring useful seismic data:

1. Positioning / Surveying

2. Seismic Energy Source

3. Data Recording

4. Data Processing

5. Data Interpretation

Key #1: Positioning / Surveying

Accurate positioning is fundamental and vital to acquiring seismic data.

We must know PRECISELY where all our instruments are on the Earth's surface.

Otherwise, however good the quality of the recorded seismic data . . . the data are worthless if we don't know where they came from.

In both marine (left) and land (right) environments, energy source and receiver layout patterns are pre-planned, and their positions pre-determined, so that we can calculate precisely where our recorded seismic data originate.

Positioning Technology

Today we are in the 'space age' of GPS - the Global Positioning System - which offers unprecedented accuracy.

GPS is a constellation of 24 satellites in orbit about 20,200 kms above the Earth. The satellites act as precise reference points in space and transmit radio signals that allow a GPS receiver on Earth to triangulate its position to within about 10 meters.

While 10-meter accuracy is adequate for many purposes, for seismic we use Differential GPS (DGPS) correction techniques to bring our levels of accuracy to between 2 meters and 30 centimeters!

Positioning at Sea

At sea, positioning is more difficult than on land because our vessel - and all its towed equipment - is continuously in motion.

Nevertheless, the precise locations of the energy source(s) and the streamer(s) MUST be known at all times.

In such a dynamic environment, real-time positioning is extremely complex and highly computer-intensive.

We use an integrated combination of multiple reference site DGPS, Relative GPS, laser measurements of ranges and angles, underwater acoustic ranging and digital compasses along the streamer(s).

Literally hundreds of complex mathematical position calculations are carried out every few seconds, enabling the precise positions of the vessel, the seismic source(s) and the individual hydrophone groups in the streamer(s) to be calculated in real-time as the vessel continuously moves along.

Key #2: Energy Source

At Sea: AirgunsTo gather seismic data, we must first generate and transmit controlled acoustic energy into the ground.

In the past, dynamite was the preferred seismic energy source both on land and at sea. Dynamite is still used on land, particularly in areas of soft, unconsolidated or weathered surface layers. When buried and detonated in safely plugged shot holes below the surface layer, dynamite produces a sharp, acoustically clean energy pulse.

However, in urban and/or populous areas, dynamite is obviously not practical! There are several other energy source technologies used for acquiring seismic data, but the main one is 'vibroseis'.

On Land: Vibroseis

Large servo-hydraulic vibrators on vibroseis trucks are safer, faster, more adaptable and more environmentally friendly than dynamite, and can yield equal (or sometimes better) data quality.

How Vibroseis Works

A vibroseis truck generates a controlled vibratory force of up to 70,000 lbs through a baseplate that is placed in contact with the ground.

At Sea: Airguns

In the marine environment, and sometimes in swamp or marsh, dynamite has been almost completely replaced by airguns.

In an airgun，high pressure air is stored in a firing chamber and explosively released through portholes by the action of a sliding shuttle with pistons at each end.

Seismic energy is generated by the rapid, explosive release of compressed air through the airgun's ports

How Airguns Are Deployed

into the surrounding water. This produces a primary energy pulse and an oscillating bubble.

Typically, multiple airguns are towed behind the vessel, several meters below the sea surface in a pre-determined combination, or 'array' of different chamber volumes designed to generate an optimally tuned energy output of desirable sound frequencies.