In this blog post, we’ll explore the principles of geophysical exploration—which reveals hidden subsurface structures and resources—the characteristics of seismic surveying, and how seismic surveying is utilized in oil and gas exploration.
Have you ever played the game StarCraft? For those who haven’t, it’s a strategy simulation game where you choose one of three races—Terran, Zerg, or Protoss—to battle against opponents. The game features various units for each race, some of which hide underground to launch attacks. For example, the Zerg have a unit called the Lurker. If you’re attacked by a Lurker during the game, it’s difficult to counterattack immediately because the Lurker is hidden underground, making its location impossible to pinpoint. Therefore, each race has its own methods for detecting units hiding underground. Take the Terran race, for example: by using communications between a Comsat Station—a facility equipped with a satellite antenna—and an artificial satellite, they can reveal units hidden underground in a specific area for a limited time. By utilizing this feature, players can locate Lurkers and attack them.
Just as players facing the Zerg race want to know the location of the Lurker—a unit hidden underground—people are curious about what lies beneath the ground and what its structure is like. The technology used for this is geophysical exploration. Geophysical exploration refers to the method of investigating geological structures, oil and gas reservoirs, the presence of minerals, the location of groundwater, and environmental pollution by observing physically phenomena—whether artificial or naturally occurring—at the Earth’s surface and then analyzing the resulting data. It is categorized into various methods depending on the physical properties of the rocks and minerals being explored; the method I am currently studying with particular interest is seismic exploration, which utilizes the elastic properties of rocks and minerals.
Seismic exploration is a method primarily used to locate subsurface resources such as oil and gas. Here, “seismic waves” refer to waves generated by vibrations caused by natural earthquakes or artificial energy sources as they propagate through rock. This exploration method relies on the fact that the speed, amplitude, and propagation path of seismic waves vary depending on the elastic properties of the rock. In actual field operations, elastic waves are generated using artificial energy sources such as dynamite or air guns and sent underground; the waves that return to the surface are then analyzed to determine the subsurface structure. Simply put, the principle is similar to when we buy a watermelon: we tap it with our hands to check if it’s ripe without cutting it open, and judge its condition by listening to the sound it makes. In this analogy, the watermelon’s flesh corresponds to rocks with different elastic properties; tapping the watermelon corresponds to the process of artificially generating seismic waves; and the resulting sound corresponds to the data obtained from seismic exploration. When viewed this way, it becomes clear that we experience principles similar to seismic exploration in our daily lives without even realizing it.
Seismic exploration involves sending seismic waves into the subsurface and analyzing the returning waves. Depending on the type of returning waves and the method of analysis, it is broadly divided into refraction surveys, which utilize refracted waves, and reflection surveys, which utilize reflected waves. First, a refraction survey is a method that focuses on recording refracted waves that enter at the critical angle. A refracted wave is one that enters a lower-speed upper layer at the critical angle, is refracted, travels along the boundary of the strata, and is then refracted back into the upper layer before reaching the surface. This method is particularly effective for geotechnical investigations at shallow depths of up to several tens of meters.
In contrast, reflection seismic surveying is a method that records waves reflected back from the boundaries of subsurface strata. Unlike refraction seismic surveying, reflection seismic surveying analyzes the waves reflected from stratigraphic boundaries to determine the subsurface structure. Because reflection seismic surveying can probe to greater depths than refraction seismic surveying, it is an important method used in oil and gas exploration, where information on stratigraphic structures extending several kilometers underground is required. In other words, when we read newspaper articles or watch the news and see that seismic exploration is being used in oil exploration, it often refers to reflection seismic surveys. Reflection seismic surveys have played a crucial role in oil and gas exploration and have advanced significantly, thanks to the early and active use of numerical analysis and computer technology in the data acquisition and processing stages. Even today, it is widely used to process and interpret 2D and 3D seismic data to identify subsurface structures and reservoirs.
In 2011, it was reported that South Korea had secured the right to participate in an oil field development project in Abu Dhabi, United Arab Emirates (UAE). Reports at the time estimated that South Korea would be able to secure crude oil and natural gas resources totaling at least 1.2 billion barrels. Seismic surveys are also utilized in the process of evaluating the size and reserves of oil fields. The application process is as follows: Since crude oil is primarily found in areas where sedimentary basins and sedimentary rock layers are distributed, the first step is to identify the distribution and structure of these sedimentary layers and pinpoint structures where oil and gas are likely to be present. Subsequently, the underground structure and the size of the reservoirs are analyzed to estimate the resource reserves. It is through this exploration and evaluation process that specific estimates, such as 1.2 billion barrels, are derived.
Competition among nations to secure energy resources like oil has existed in the past and continues today. Accordingly, research aimed at advancing seismic exploration—one of the technologies necessary for locating these resources and accurately mapping subsurface structures—will also continue. If you happen to come across an article or news report about seismic exploration in the future, I hope you won’t just pass it by but will take an interest in it. I look forward to the field of seismic exploration—which plays a crucial role in identifying geological structures and energy resources hidden underground and invisible to the naked eye—continuing to advance in the areas of energy resource development and underground environmental research.
