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Seismic data on deck: detecting the Cascadia Megathrust fault


Seismic data on deck: detecting the Cascadia Megathrust fault

Brian Boston
|July 1, 2021

Graph showing R/V Marcus G. LancesThe sound transmitter and sound waves bounce off the different layers below the ocean floor. The hydrophone streamers towed behind the ship listened for these echoes.

Help us in our Looking for a high thrust fault In the Cascadia subduction zone, R/V Lances Behind it is a seismic sound source and a 12-kilometer (approximately 7.5-mile) streamer with a hydrophone. With this device, we can collect a lot of data. But what do we actually collect?

Every time we release a sound wave from the source, we record the streamer energy that enters the earth, bounces off the rocky layers of the ocean floor to the mantle, and then returns to the surface of the streamer.This Lances The long streamer has 960 channels and records 15 seconds of energy at a sampling rate of 2 milliseconds. This is enough to allow sound waves to penetrate deep into the earth’s crust and return to the surface, recorded by our seismic streamers, and then repeat the process. However, because our streamers are very sensitive and can help listen to the weak geological signals reflected back, we not only recorded the signals from the earth, but also recorded a series of noises that can be carefully removed to help us understand the situation below us. .

If we just want to see what is below us, why do we need such a long seismic streamer? There are two main reasons. First, we can use multiple recordings of the sound reflected from each geological layer to help enhance the reflected signal and reduce the amount of noise. Future work will focus on increasing the signal from the geological features we are interested in, while reducing noise that may hide important information underground. Second, the long streamer helps us observe how sound waves pass through the earth’s crust, which provides us with speed information. After mastering the time and speed, we can place the geological layer deep below the sea surface and allow true geological interpretation of the data.

Acoustic reflection chart

In our experiment, an example recorded after a single pulse from a seismic sound source. Apply light filtering to help eliminate random unwanted noise and help increase the signal from the bottom and lower layers. Additional processing will capture these raw data and create geological images of the seafloor. This will allow us to find and map the giant thrust faults along the Pacific Northwest.

Since the giant thrust fault is not only located near Cascadia, but also a few kilometers below the sea floor, we need all the available data to sort out what the fault looks like.

Brian Boston He is an associate research scientist at Columbia University’s Lamont-Dougherty Earth Observatory.




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