What are these OBS things anyway?

With an improvement in the weather the task of deploying is now well underway. Also, thanks to a training session about monitoring the bathymetry equipment and more work to do on deck preparing the instruments, the science team is starting to feel more useful! However, theres only so much we can write about being on site and lowering objects into the sea before even the most committed readers would get bored. Instead, lets take a moment to look in more detail about these objects and how they work.

 

Cruise_region
Our current location (site LT11). 5 down, 43 to go. The next site is LT14, which is a TRM with an Absolute Pressure Gauge (APG) and a pop-up buoy. Read on for a description of what this means!

 

We’ll be deploying two types of OBS during this cruise. The first, the TRM, is designed for shallow water (less than 1000m). In this design the sensors are housed in a steel casing to protect them from trawler nets. The whole package weighs about 650kg. There are two sub-types of TRM: Those that are recovered by a cable attached to a buoy that comes up to the surface and those that have to be recovered by a cable attached by a remotely operated submersible. We’ll be deploying the former in water shallower than 150m and the latter otherwise. TRMs must be lowered to the seafloor off the back of the ship via winch, which allows us to know their seafloor locations exactly.

The second type of OBS is a deep-water (up to 5000m) variant, which is recognizable thanks to the yellow floats that will eventually bring it back to the surface. They are about half the weight of a TRM and are easier to deploy since they can just be dropped over the side of the ship. These instruments may not fall directly to the bottom, so its important track their descent and record their resting location as accurately as possible. When its time to recover these instruments, a signal is communicated to them to return to the surface.

Technically, each of the two types of OBS we just discussed should really be called an ‘instrument package’. Each package contains a seismometer (the device that actually responds to seismic ground motion), a datalogger (to digitize the signal from the seismometer), pressure sensor, hydrophone and battery pack capable of powering these sensors for about a year.

 

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The guts of a datalogger. Each instrument is tested before being connected to a seismometer (not shown) and bolted to an OBS instrument package.

 

Since data recorded by the seismometer is usually of most interest to geophysicists, lets explore this recording process in more detail. Each of the OBS instrument packages carries what’s known as a broadband seismometer. This is a sensitive instrument that can measure ground motions (also known as seismic waves) over a very wide range of frequencies and amplitudes. Inside each seismometer is a suspended mass. When the instrument is vibrated, it applies a force to the mass to prevent it from moving. This force is proportional to the amplitude of the vibration, and the size of the electrical current used to produce the force is recorded by the digitizer as a measure of ground motion. Technically, the size of this current is directly proportional to the velocity of the ground.

This is called an electronic force-feedback system, and it is the basic principle used by all modern seismometers. Ground motion is recorded in three components: Vertical (i.e. motion up and down, perpendicular to the Earth’s surface) East-West and North-South. With all three components we can understand the three dimensional shapes and sizes of seismic waves.

Everything from the sub-mm amplitude, high-frequency seismic vibrations from distant earthquakes to the low-frequency expansion and contraction of the soil due to temperature variations can be detected, so long as the seismometer is properly protected from sources of noise. This ability makes these instruments incredibly useful when it comes to studying the sources of seismic waves and the materials through which they travel.

Noise is just any part of the seismic record that is undesired (although for some purposes, one researcher’s signal can be another’s noise!). For a seismometer on the sea floor there can be a variety of noise sources, some of which are unavoidable, others of which can be reduced. Internal electronic noise from the instrument is present but usually small compared to the signals we might want to study. External noise from the ocean is more problematic. It’s caused by seismic signals generated by waves as they break against the shore or as they pass over the seismometer. This explains why shallow water stations tend to be noisier than those in deeper water.

Data from offshore seismometers is often noisier and more challenging to work with than that from their onshore cousins, so why go to all this effort to deploy them? The answer is simple: 70% of the Earth’s surface is ocean, and the only way to do high resolution imaging of the crust and mantle beneath this 70% is to deploy seismometers there. On top of that, our deployment region is special because it has to potential to generate – and has generated in the past – megathrust earthquakes (more about these in a later blog post), whose epicenters are all offshore. The only way to fully understand the zone in which these earthquakes are generated is to place seismometers directly above it.

Robert Martin-Short, UC Berkeley Seismic Lab

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