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Geophysical Surveys

As the essential tool used throughout the energy exploration industry, geophysical surveys have been used extensively around the world for more than five decades and are critical in meeting the world’s growing energy demand. Advancements in geophysical surveying technology have helped find, drill and produce oil and natural gas with the least risk and the least possible impact to the earth. As operators explore for energy resources, the use of geophysical technologies helps to reduce cost, increase safety and minimize impact to the environment. Energy Starts Here!

Colin Murdoch

Since its beginnings, geophysical industry technology has played a central role in reducing the environmental impact associated with exploration and production, and enabling global access to affordable energy. Over the decades this has transformed economies, communities and lives. Looking forward, I believe geophysics will be critical to bringing the innovations required to support the world’s efficient transition to renewable energy. This is one of the many reasons that I joined and continue to work within the industry.

Colin Murdoch
EVP Geoscience
CGG

Seismic Surveys:
The Power of Transformation

Over the last few decades, there have been significant advances and innovation in energy exploration and development. Seismic survey technology has been one of the greatest driving forces behind the energy exploration and development revolution, providing some of the greatest return on investment. From the exploration phase to the first well drilled and throughout the life of the asset, seismic survey technology is making its mark across the entire lifecycle of energy development.

For oil and gas exploration, images of the earth’s subsurface created from geophysical data can shed light on potential drilling hazards to ensure it is as safe, reliable and efficient as possible. Analysis of a subsurface in advance of drilling is only available through seismic techniques and assists in the design of well trajectories that can reach the resource reservoir while avoiding any hazardous zones that could cause potentially serious issues. Companies also use seismic to perform hazard surveys to look for geologic hazards on the sea bottom and in the shallow subsurface that could affect the drilling of a well. Once oil or gas is found, and a reservoir is being developed and produced, seismic images increase the understanding of the characteristics of the reservoir and optimize development plans. Use of seismic data leads to more efficient oil and gas extraction requiring fewer wells while increasing the production of hydrocarbons.

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Use of Geophysical Surveys:

So, what is the geophysical industry and why is it so important to the world? The geological & geophysical (G&G) industry is vast, and the people working in the industry are in the business of studying the earth by using different types of surveying techniques. The objective of G&G surveying is to use the data collected to produce maps or models that indicate the earth’s geography, stratigraphy, rock distribution, and geological structures. These surveys determine characteristics of the earth’s subsurface by measuring the physical differences between rock types or physical discontinuities without seeing them directly by digging, drilling or tunneling.

Geologists and geophysicists use their knowledge and interpretation of G&G survey data to help to determine safe locations to build homes, offices, and offshore renewable energy projects; find and develop oil and natural gas deposits onshore and offshore; locate mineral and precious metal mines; detect ground water; and to check foundations for roads and buildings to research purposes to determine environmental concerns and reduce hazard for issues such as landslides to assisting in archaeological activities.

Land and marine seismic surveys and other geophysical survey data provide critical information for the benefit of society and quality of life.

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Types of Geophysical Surveys
Geophysical surveys are categorized as non-seismic and seismic and provide various levels of detail, but all generate useful data based on the purpose. Non-seismic geophysical surveys include airborne gravity and gradiometry, magnetic, and electromagnetic techniques.

Seismic is perhaps the most well-known type of geophysical survey used to locate potential oil and natural gas deposits in the geologic structures within the earth. Seismic surveys can be conducted onshore (land) and offshore (marine). Land and marine surveys operate on the same basic principles of sound, but differ operationally. Similar to an ultrasound imaging using sound to generate images of the human body, seismic surveys use sound to generate images of the earth’s subsurface.

There are two main types of seismic survey techniques, refraction and reflection. Seismic refraction measures travel time of seismic waves at the interfaces with subsurface layers of the earth. Seismic reflection is based on the two-way travel time of seismic waves reflecting back from subsurface layers of the earth. Seismic reflection provides the greatest detailed information about the earth’s subsurface.

Both land and marine surveys include a source and a receiver. They may differ in the pattern of the receiver system, concentration of measurements taken within a given area, and the type of receiver or sensor used.

Land Surveys:

Land seismic operations are similar to marine operations in that the energy sources are acoustic energy generated using vibrators mounted on trucks or low-impact charges placed in shot-holes which have been drilled by truck-mounted or portable drills.  The receivers are typically geophones, which are like small microphones pushed into the soil to measure the ground motion.  Onshore seismic are used in sensitive locations without damaging buildings or the environment.

Seismic data is not only vital in identification of potential hydrocarbon resources, but are also an important source for planning exploration and development of complex geological structures containing unconventional hydrocarbon resources.  The term “unconventional” refers to the methods used and the types of rock from which the oil and natural gas are produced.  Regardless of the production process or the where they come from, unconventional oil and natural gas are essentially the same as their conventional counterparts and depend on seismic survey data.

As technological advances have enabled extraction of oil and natural gas in complex geological structures, advancements in seismic survey technology have allowed for the discovery and identification of “tight gas” and other “sweet spot” formations which are more difficult to discover with 2D land seismic.

Land seismic data acquisition uses primarily two types of seismic sources, non-impulsive vibroseis vehicles or an impulsive energy source such as a low-impact charge—that generate acoustic waves which propagate deep into the earth.  Each time an acoustic wave encounters a change in the rock formation, part of the wave is reflected back to the surface where an array of sensors records the returning signals.

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Non-Impulsive Energy Sources
Non-impulsive energy sources include the use of vibrators or vibroseis trucks.  A line of trucks with vibrating plates stop at regular intervals as the vibrator plates are compressed against the ground and vibrated to send sound waves into the surface one at a time. The sound waves are reflected off the rocks below and picked up by grids of receivers, called geophones, which are like small microphones pushed into the soil to measure the ground motion.  These reflected sound waves are then analyzed to give a picture of where hydrocarbon resources are.  Because vibrator trucks can be used in urban areas and equipped with special tires or tracks for deployment in environmentally sensitive areas, hydraulic seismic vibrators are the predominant source used in land seismic exploration today.

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Impulsive Energy Sources
Impulsive energy sources are used in some areas where vibroseis trucks are not an option. In these operations a single shot hole is drilled and a small charge specifically designed for geophysical surveys is buried with strict and careful adherence to set back requirements and local regulations and under carefully controlled and safe conditions.  A radio-controlled unit from a nearby recording truck controls the detonation of the buried charge, resulting in a seismic sound wave.

The typical seismic shot hole ranges from 10 to 200 feet in depth and 2.5 to 4 inches in diameter.  Detonations are contained within the hole to force the energy generated downward through various rock strata, and the only sound heard above ground is a dull thud.

This operation involves a drilling crew, motorized vehicles, a crew for detonation, a shot hole plugging crew, and clean-up personnel.  In inaccessible areas, the drilling crew moves a portable drilling system from point to point by helicopter operations.  Shot holes are properly plugged as specified by state and/or federal government agencies.

Marine Surveys:

Marine seismic surveying has evolved by leaps and bounds over the last decades.  In addition to 2D (dimensional) surveys, the industry introduced 3D and 4D surveys, ocean bottom seismic (OBS) and 3C and 4C OBS has significantly impacted by improving productivity and efficiency and providing flexibility in mobilization and operations.

By determining geological features below the sea floor, marine seismic surveys are a vital part of exploring for oil and gas. The survey is conducted by sending acoustic pulses into the various buried rock layers beneath the sea floor and then recording the time it takes for each wave to bounce back while measuring the various characteristics of each returning wave.

In water, the energy source is typically an array of different sized air-chambers, filled with compressed air.  The source is towed behind a seismic survey vessel and releases bursts of high-pressure energy into the water.  The returning sound waves are detected and recorded by hydrophones that are spaced out along a series of cables.

Marine Survey