How Underground Water Is Detected
I recently had a question about determining the level or depth of groundwater. The question came to me after recalling childhood cartoons walking around with a stick, supposedly to find water wells. I wondered: How exactly is groundwater detection actually done?
In my search for answers, I discovered that petroleum is often located using seismological waves. I also read that water doesn’t have specific P- and S-wave velocities, which makes it difficult to detect in a given area. This led me to a deeper understanding of how people search for underground water, and how these methods work in practice.
The Role of Seismic Waves
In geophysics, seismic waves - specifically P-waves and S-waves - are crucial for understanding subsurface structures. P-waves, or primary waves, are compressional waves that travel through both solids and liquids. S-waves, or secondary waves, are shear waves that only move through solids. This difference in wave propagation is key when detecting underground materials.
When seismologists use seismic waves to detect oil or gas deposits, they generate a shockwave (often through controlled explosions) to study how waves travel through the Earth’s subsurface. The speed of these waves depends on the density and properties of the different layers they pass through. In the case of water, it causes a decrease in seismic wave speeds due to its liquid nature. This difference in wave behavior can help identify the presence of water in the soil.
If you’re intrested in more of the math side of things, you can click here to download some slides from Prof. David Van Rooij at UGent, to which I turned for more clarity on this question.
For a more visual explanation of how seismic waves propagate, you can view this animation: Seismic Wave Behavior: A Single Boundary Refracts & Reflects.
Detecting Groundwater with Seismic Methods
Although seismic methods are commonly used in oil exploration, they can also be applied to detect groundwater.
Liquids, such as petroleum and water, cause a relative decrease in the propagation speed of seismic waves. P-waves remain intact, but S-waves cannot propagate in a liquid due to the nature of their motion (which is only possible in a solid).
This can indeed be used for detecting the groundwater level in shallow layers. Imagine that your soil consists of sand. Between the surface and the groundwater level, the pores in the sand are filled with air. Below the groundwater level, the pores are saturated with water. In other words, there will be a difference in density, and this change is detected by seismic refraction as a change in the speed of the seismic waves.
In practice, however, using seismic methods to detect groundwater is not as common. In Belgium, for instance, the government has a well-developed network of monitoring wells, where groundwater levels are tracked regularly - sometimes even automatically. These wells are more commonly used in construction projects to determine the stability of the ground, especially for large-scale building projects where groundwater levels may impact the foundation.
Conclusion
While the use of sticks to detect groundwater - something often seen in cartoons - may not have scientific backing, the detection of underground water is a real and important task. Methods like seismic refraction, which measure the speed of seismic waves through different layers of soil, are used for locating groundwater. However, in practice, more direct methods, such as monitoring wells, are typically employed for measuring groundwater levels, especially for large projects where accuracy and safety are paramount.