17 FEBRUARY 2024 WorldWide Drilling Resource® Types of Seismic Surveys for Gas and Oil Exploration Adapted from Information by Crown Exploration II, Ltd. and the Utah Geological Survey Seismic surveys play a vital role in the exploration of gas and oil. Although the world of seismic surveys is complex, there are three basic types of technology used: 2D, 3D, and 4D. Each has its advantages in assisting gas and oil exploration efforts. A seismic program is expensive and time consuming. Collecting and processing the data can take 12-18 months. However, sophisticated computer imaging of subsurface structures can enhance the likelihood of a successful well, especially after a play has shown some promise by basin analysis and aerial surveys. Seismic images are produced by generating, recording, and analyzing sound waves traveling through the earth (seismic waves). Explosives or vibrating plates generate the waves and a line or grid of geophones records them. Density changes between rock or soil layers reflect the waves back to the surface, and how quickly and strongly the waves are reflected back indicates what lies below. The amount of shaking associated with different seismic surveys varies, depending on site-specific factors such as soil and rock type, how deep the survey needs to image, and the required source. A steel plate struck with a sledgehammer generates enough energy for shallow (less than 60 feet) soil investigations used for engineering or environmental surveys. To “see” a little deeper, a trailer or pickup truck-mounted drop weight might suffice. To get a really deep picture (miles), as is needed for gas and oil exploration, dynamite charges or vehicle-mounted vibrator plates (called vibroseis trucks and buggies) are used to generate waves from multiple source points. Here’s a little more information about the three basic types of seismic technologies used for gas and oil exploration: 2D - 2D seismic surveys are recorded using straight lines of receivers crossing the surface. This type of seismic survey works well for imaging major structures. 2D surveying is still popular because data gathering and analysis of 2D seismic information is much quicker and cheaper than 3D or 4D. Plus, 2D data requires much less permitting, surveying, and processing time than even small 3D surveys. 3D - The late 1970s saw the development of the 3D seismic survey in which the data imaged was not just a vertical crosssection but an entire volume of earth. One of the most obvious differences between 2D and 3D seismic is 3D imaging provides information continuously through the subsurface whereas 2D seismic reveals only strips of information. Large 3D seismic shoots may take one to two years to acquire, and three to four months to process the information. Nevertheless, 3D seismic may not be cost-effective in many onshore applications, especially in the early stages of exploration. In onshore 3D seismic surveys, many lines of receivers are used and recorded across the earth’s surface - the area of receivers recorded is known as a “patch”. 3D operations are considerably more elaborate than 2D and the daily cost of the crew is substantially increased. However, 3D seismic data collection improves exploration performance by allowing for: fewer dry holes, more optimized well locations, guidance for horizontal drilling projects, more complete evaluation of mineral rights, and a better understanding of the nature of the prospects. 4D - 4D, or time-lapse, seismic surveys use 3D seismic data acquired at different times, over the same area. It is used to assess changes in a producing hydrocarbon reservoir over time (the fourth dimension). Changes may be observed in fluid location and saturation, pressure, and temperature. To maximize the value of a 4D seismic project, exploration and production assets are carefully screened because of the expense to acquire and analyze the data. Seismic technology has been used since the early 1900s to measure water depths and detect icebergs, and by 1924, crude seismic data was first used in the discovery of a Texas oil field. Vibroseis trucks and buggies come in a variety of designs and sizes, but all of them generally have a large pad that is lowered from the vehicle to the surface and then vibrated to generate seismic waves. Photo courtesy of the Utah Geological Survey. 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