This article discusses non-invasive methods to identify resources and underground structures using gravity, magnetism, elastic waves, and electrical resistivity exploration without digging into the Earth’s interior.
When we want to see the inside of our bodies without digging, we use imaging techniques such as MRI and CT. We can observe and photograph the inside of the body by projecting the inside from the outside without damaging the body. This allows us to get quality information about the inside of the body. What if we could see the inside of the Earth without digging into it? It would be great if we could take pictures of the inside of the Earth. It would be great to be able to see exactly how many kilometers below an area there is oil, or how many kilometers below an area there are diamond mines, without having to dig.
There are three main technologies for exploring the underground from the surface. The first is gravity and magnetic surveys, the second is elastic wave surveys, and the third is electrical resistivity surveys. These technologies are important tools for non-invasively understanding the structure and resources of the Earth’s interior. They all work on the principle that the physical properties of underground rocks or specific structures vary from one type of rock to another, and that these physical properties can be measured to infer the underlying rocks. Based on these measurements and inferences, the internal structure of the underground can be modeled.
These techniques for studying the Earth’s interior have many implications for our lives and industries. For example, underground exploration techniques are essential for discovering and mining natural resources like oil and gas. Without them, we’d have to dig around in the ground, which would waste a lot of time and resources. Moreover, these technologies also play an important role in seismic studies and the exploration of groundwater resources.
Let’s take a closer look at each of them. First, gravity and magnetic surveys take advantage of the different gravitational and magnetic fields that different types of rocks receive. First, gravity probes measure changes in the gravitational field, or gravitational acceleration, caused by differences in the density of the rocks. By measuring the change in gravitational acceleration, it is possible to infer the density of the rocks in the subsurface, which in turn reveals the type and distribution of rocks. In fact, this technique can be used to map the distribution of rocks over large areas. Therefore, it’s the first and primary method used in exploration. Gravity surveys can also be carried out from the air to discover economically viable oil fields in inaccessible areas.
Magnetic surveys are widely available, from small to large scale. Basically, it has a similar principle to gravity prospecting. By measuring changes in the magnetic field, the magnetic properties of the rocks underground can be determined, which can then be used to predict the internal rock structure. Especially in large-scale exploration, it is often carried out simultaneously with gravity exploration. In this case, gravity and magnetic surveys complement each other and provide useful data that cannot be obtained by either survey alone. Therefore, gravity and magnetic surveys are often considered together because of their ability to rapidly cover large areas, which is the primary basis for all physical surveys and allows for the primary exploration of significant areas.
Second, and perhaps the most important and precise, is elastic wave surveys. When an elastic medium is shocked, waves are generated. As these waves travel through the ground and encounter different types of rock, they are reflected or refracted at the interface. By listening to the returning wave after these reflections and refractions, the subsurface image can be modeled from its speed, time, and waveform. The ability to measure precise boundaries allows for more precise measurements compared to gravity and magnetism. To derive the actual subsurface structure, a method called inverse calculation is used. If you send an elastic wave into a formation, it will return with a certain shape when the formation has a certain structure. This is called backcalculation, which is a way to figure out the number of such cases and use programming to fit them together one by one. After gravity and magnetic surveys have identified the approximate location of resources and different structures in the rock and interior, elastic wave surveys are useful when we want to more precisely locate the resources we want.
Third, there is electrical resistivity. All materials resist the flow of electricity to different degrees, or in other words, have different electrical resistivities. By sticking electrodes into the ground from above and running a current through them, you can measure the electrical resistivity to determine the type and distribution of rocks in the subsurface. The main advantage of electrical resistivity exploration is that there are many ways to measure this physical property. If the results obtained from different methods are the same, the data is more reliable and can be considered accurate, and a model that closely approximates the actual subsurface structure can be obtained with a high degree of confidence.
“Drill one well” doesn’t work in resource engineering. There is no point in just going to a random location and digging a well if there is no useful resource. From energy resources such as oil and gas to mineral resources such as gold, diamonds, and iron, underground resources are indispensable to our daily lives and national economic development. And securing these resources by intensively drilling and developing high-potential areas is an area that has attracted global attention. Accurately identifying underground structures with advanced technology and making development plans based on them is of utmost importance in the coming future.
If you can understand and analyze the technology and principles of physical exploration, the added value and development possibilities are very promising. Underground exploration is not just about finding resources, but also about understanding the history of the earth and predicting natural disasters. For example, it is used to study the mechanisms of earthquakes or predict volcanic activity. They also play an important role in the exploration and management of groundwater resources. Earth’s resources are finite, and efficient management and utilization of these resources is essential for our sustainable future.
Therefore, advances in subsurface exploration technology can enrich our lives and contribute to protecting the global environment. With advances in technology, we will gain a deeper understanding of the Earth’s mysteries, which will help us to prosper and protect the health of our planet.
