Subsurface Mooring

The figure below shows a mooring which is being used at the USGS Long Term Monitoring program off Boston, MA. It carries sensors and systems for sediment sampling, recovery, and measurement of salinity, temperature, current, and turbidity. Scientists analyzing the data get a better picture of how storms such as nor'easters transport sediments (and hence pollutants) in the water column. It is deployed at a depth of about 32 meters, and stays in place for 4 months. Scroll down to see the entire figure and an explanation of each part of the mooring.

Legend

Subsurface moorings are just that: moorings which do not break the sea surface. They are designed to position each sensor at a specific depth. Scroll down to see the top of this mooring... it starts at 20 meters above bottom.

A steel float keeps the mooring upright.

Stainless steel chain is used as a damper below the float.

A small tube trap catches sediments which fall through the water column. These are often used as controls for other traps on the same mooring.

An Anderson Trap catches greater amounts of sediment with a larger opening. The sediment accumulates in layers in a tube below the funnel.

Plastic jacketed wire rope offers a lightweight, low drag alternative to chain for connecting mooring components together.

A Vector Measuring Current Meter is set to record currents at 10 meters above the bottom. Attached to it is a Sea Cat data logger which measures temperature and salinity, and records turbidity levels sensed by a Sea Tech transmissometer.

Another tube trap is attached closer to the sea bed to compare the sediments in this part of the water column with those caught by the traps above.

A nylon bridle is used instead of wire at this point to minimize the introduction of foreign matter into the trap below.

A Honjo time series trap works just like the Anderson 10 meters above, except sediment is collected in bottles, rather than a tube. A computer and motor rotate the sample bottles so that scientists know exactly how much sediment was collected in a given time period. Attached to the Honjo trap's titanium frame are two acoustic releases (one is a backup). When given an acoustic command from the surface, the release lets go of the chain that attaches the mooring to the anchor below, and the entire mooring rises to the surface to be recovered by a ship.

This mooring's anchor weighs 2200 pounds.