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Seismic Operations: a Backgrounder

Technicians set off blasts and use sensitive instruments to record the echoes from deep underground. The pattern of the echoes can show the shape of rock formations and possibly reveal a likely hiding place of heavy oil.

A seismic survey is usually the last exploration step before a prospective site is actually drilled. Seismic surveys give precise details on the structures and stratigraphy beneath the surface. Data is collected by creating vibrations, usually by setting off an explosion, and detecting the sound waves reflected back with a seismometer, which records them on a seismograph and depicts them in a seismogram. Seismograms are used to generate a seismic section, which is much like a cross sectional view of the subsurface.

The first seismometer, as inventor David Milne called it, was used in 1841 to record the vibrations of the ground caused by earthquakes. Dr. L. Mintrop, a German scientist, developed a use for seismic data during World War I when he used a portable seismograph machine to locate the position of Allied heavy artillery pieces. He found errors in some of his measurements which he concluded were caused by the variation of sound waves as they encountered different geological formations. After the war, Mintrop reversed the process, setting off explosions a known distance from the seismograph and measuring the return time of subsurface shock wave reflections to estimate the depths of formations.

This is still the basic technique today. The theory behind the seismic survey is that subsurface structures can be deduced by measuring the transit time of sound waves generated by an explosion. First, the geophysicist decides on an area to be surveyed. Listening devices, called geophones, are spread out over an area of several hundred square meters. Each geophone is attached by electric cable to recording instruments in a seismograph truck.

Explosions, typically dynamite charges, are set off in a shot hole. The sensitive geophones pick up the sonic reflections from the sound waves. Computerized analysis of the recorded data, such as the elapsed time from explosion to reception, and variations in the angles of reflection from various returned sound waves allow geophysicists to "map" the underground structures and locate traps and other formations usually associated with oil collections. Some claim that seismic data can produce direct indications of oil and gas formations, but most conclude interpreting seismograms is as much an art as a science.

There was an extensive seismic survey done in the Lloydminster area in the early 1950's.  Under the auspices of Husky Oil, an area approximately 50 kilometers in every direction from Lloydminster was surveyed.  Shot holes were dug to the unusually deep level of 500+ feet.  The result showed formations suggestive of oil deposits.   The seismic survey was followed by a "slim hole" drilling program in which many bore holes were dug to the level of the oil deposits.  Core samples from these confirmed the existence of oil and helped more accurately confirm the results of the seismic surveys.  However, partly due to the low price of crude and the high costs of production in those days, actual development of producing wells did not occur for more than 25 years in some cases.

During that time, seismic was not used extensively in the Lloydminster region.  In the late 1990's however, with the increasing use of high powered computers to analyze seismic data, and other improvements, 3 dimensional seismic readings are being obtained.  

These seem uniquely suited to the meandering, on-again off-again nature of the oil deposits in our area and they are beginning to be used more frequently to improve the accuracy of drilling to productive deposits.

Note: Illustration below shows "vibrator truck" originating sound wave rather than the commonly used small, truck mounted drilling rig which digs "shot holes".  Otherwise the process is the same as described above.

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