In this blog post, we’ll explore the principles, processes, types, and applications of geophysical exploration—a method for identifying underground geological structures and resources without actually digging into the ground.
These days, the term “shale gas” often catches the eye in the news. Shale gas refers to natural gas trapped in the microscopic pores and fractures of shale, a clay-rich sedimentary rock, and has been utilized as an important energy source alongside existing natural gas resources. However, producing it requires a drilling process to bore holes into the ground, and the costs associated with this process are significant. Therefore, before drilling, it is essential to determine as accurately as possible the location and shape of the shale layer, as well as the geological structures where gas is most likely to be present. This helps minimize drilling in the wrong locations and reduce costs. Geophysical exploration technology plays a key role here by identifying underground geological structures and physical properties to provide the information necessary for drilling and development. Let’s now take a closer look at geophysical exploration technology.
Geophysical exploration is a branch of geophysics and is a distinct concept from pure geophysics, another branch of the discipline. Both share the common goal of identifying subsurface structures or locating specific targets by utilizing differences in the physical properties of subsurface materials or geological structures. However, while pure geophysics aims to understand and investigate the Earth’s own structure and phenomena, applied geophysics differs in that it applies geophysical methods to achieve specific objectives, such as resource exploration or geotechnical surveys. Furthermore, the depth and scope of an applied geophysical survey are determined by the specific exploration objective, which distinguishes it from pure geophysics, which studies the Earth’s structure and phenomena on a global scale. Geophysical exploration is also referred to as applied geophysics because it applies geophysics for economic and engineering purposes.
In addition to the resource exploration mentioned above, geophysical exploration is used in a variety of fields. Areas where geophysical exploration is utilized include bedrock and soil investigations in civil engineering and architecture; surveys of underground contamination; preventive surveys to prepare for natural disasters such as landslides; groundwater exploration; mineral resource exploration; and exploration for energy resources such as oil and shale gas. In particular, when geophysical exploration is used in the resource sector, it serves two purposes. The first is to identify economically viable resources or areas with high development potential and designate them as targets for drilling and detailed exploration; the second is to identify areas with low economic viability and exclude them from detailed exploration and drilling. In this way, geophysical exploration helps inform the decision-making process for exploration and development by gathering information about the subsurface before direct drilling begins.
So, what process does geophysical exploration follow? Geophysical exploration generally consists of three stages: data acquisition, data processing, and data interpretation.
The first stage is data acquisition, which involves measuring physical phenomena that occur naturally or are artificially induced. Specifically, it is the process of capturing phenomena resulting from the physical properties of subsurface materials using instruments installed at various locations on the ground surface. Among the measured values at various locations, values that differ from the general values in the surrounding area are called anomalies, and the representation of these values by location is referred to as a response curve. The presence of an anomaly implies the possibility of an object or geological structure with physical properties different from those of the surrounding area; therefore, it becomes a key focus in the interpretation of geophysical survey data.
The second stage is the data processing stage, which involves using computers to convert the measurement data obtained during data acquisition into a format that can be utilized more effectively for data interpretation. During this stage, preprocessing is performed prior to data interpretation; this includes removing noise—unwanted signals mixed in with the measured signals—improving signal quality through numerical processing, and performing corrections specific to each survey method. Thus, the data processing stage is a crucial step in transforming raw data collected in the field into more reliable and easily interpretable information.
Finally, the data interpretation stage involves using the processed exploration data to determine the size, location, shape, and geological structure of subsurface features. In the past, engineers predicted subsurface structures by comparing theoretical response curves based on assumed geological models with the actual measured response curves. In other words, when performing data interpretation, it was necessary to establish a hypothetical geological model that roughly matched the actual subsurface structure from the outset to reduce the time and effort required for calculations; therefore, the experience and judgment of the geoscientist were of great importance.
In recent years, methods utilizing inverse modeling (inverse analysis) have become widely adopted alongside these traditional data interpretation approaches. Inverse modeling is a method in which the physical properties of the subsurface we wish to determine are treated as variables; by varying these variables to calculate various response curves and then iteratively adjusting them until the error relative to the actual measured response curve falls within an acceptable range. Since these calculations can be performed using computers, this method offers the advantage of systematizing the computational process and comparing various possibilities more effectively than traditional methods in many cases. However, because inverse modeling is also influenced by assumptions about the data and subsurface structure, data quality, and the non-uniqueness of solutions, the results should not be accepted as a single definitive answer; the geological and geophysical judgment of the interpreter remains crucial. Inverse modeling and methods for interpreting geophysical data are currently the subject of ongoing research, with studies continuing to quantify uncertainty and achieve more accurate subsurface imaging.
In this context, the choice of which physical properties to utilize for exploration is crucial; depending on the physical properties employed, geophysical survey methods are categorized into seismic surveys, gravity surveys, magnetic surveys, electrical surveys, electromagnetic surveys, and radioisotope surveys. Among these, seismic exploration has played a particularly important role in oil and gas exploration and is widely used to image subsurface geological structures. Seismic exploration is a method of determining subsurface structures by measuring the reflection or refraction of artificially generated seismic waves within the subsurface. Other exploration methods utilize differences in gravity, magnetism, electrical properties, electromagnetic properties, and radioactivity, among others, and the most suitable method varies depending on the specific target and geological environment. Therefore, in actual geophysical exploration, rather than relying on a single method, multiple methods are appropriately selected and combined based on the exploration objectives and geological conditions.
We have thus examined the various objectives of geophysical exploration, its implementation processes, and its classification based on the physical properties it addresses. While many people may think that the field of geophysical exploration is already a nearly complete discipline, there is actually room for continuous improvement in areas such as inversion, data interpretation, exploration equipment, and data processing technologies; such improvements could have a positive impact on the resource development industry. In particular, advances in technologies that enable more accurate imaging and interpretation of the subsurface will help enhance the efficiency and economic viability of resource exploration. By taking the lead in developing these technologies and securing a competitive edge, we can establish a significant technological advantage in the energy and resource development sectors. Therefore, I look forward to the day when South Korea achieves remarkable results in research and technological development in the field of geophysical exploration and, based on this, gains world-class competitiveness in energy and resource technology.
