We send sound into the ground and listen what comes back to the surface.
There are a lot of sources we can use to make this sound. A very basic but effective approach is to use a sledgehammer and bang it on a steel cap on the ground. This works well to get some acoustic waves into the top layers of the subsurface, but there are even some ways to get images from the boundary between Earth’s crust and the mantle. We need more sophisticated ways that get this amount of energy into the ground.
But once we start to use a charge of explosives that is big enough something like this might happen.
We also have to consider that using explosives isn’t exactly the green way to go. The environment will get damaged and it will obviously be harder to justify the use of explosives. However, if we can’t use brute force there is another way to go. Instead of sending a big amount of energy into the ground at once we can send less energy over a sustained time.
This is a method called Vibroseis. A hydraulic system mounted on a truck sends vibrations into the ground. This vibration will usually last twelve seconds and change in frequency. I have attached a video of one of these trucks below. It’s not too spectacular but don’t underestimate that truck it weighs around 30 tons.
How does this give us any information about the subsurface?
All the time I have been talking about sound in the subsurface, but when we look at sound it is basically an acoustic wave. This wave travels through the ground just like our favorite piece of music travels from the speaker to our eardrum. The air that transports the sound of music is fairly homogenous, however the ground beneath us had 4.54 ? 0.05 billion years to form and a doesn’t mix quite as well as air. There is quite a lot of history hidden under our feet. Over time the depositional patterns change and additionally we have processes like tectonics that cause unconformities in the subsurface. When an acoustic wave encounters a change in the subsurface part of it will be reflected and another part will be transmitted. The part that was transmitted will be reflected at another change in the subsurface and so on. On the surface we record the reflected waves with something called geophone and through thorough analysis, we may obtain an image of the subsurface. Just take a look at the picture of the grand canyon. You can easily identify different layers of rock just like seismics would in the subsurface.
Marine Seismics Introduction
This works very well on land, but 71% of the Earth are covered by water and those trucks don’t swim too well.
Fortunately water can transport acoustic waves as well, so we just need to find a suitable source to send sound into the water that travels down into the subsurface. This works quite well with something we call airgun. It injects a bubble of air into the water that collapses. This collapse will create the acoustic wave we need.
Listening for that wave is the next thing we have to solve. Ships create a lot of vibration, so we have to record the acoustic waves somewhere else. The listening devices are stored in something we call streamer that gets dragged behind the ship.
Seismics is pretty easy, when we boil it down to its core.
You send sound into the ground with some sort of source and then have devices that will record the reflected waves back at the surface.