Large Bottom Tripod

Measurements of marine parameters near the sea bed are often made with instrumentation that is attached to a fixed bottom platform. A bottom tripod serves as a framework on which to fix instrumentation in exact positions in the water column, as a protective frame against natural and human intervention. It is often the case that oceanographic studies are performed in heavily fished areas. The tripod frame is designed to sit on the ocean bottom and tolerate large storm generated currents and hits from nets, grappling hooks and other gear.

We have been using this particular tripod design (and some of the same tripods) for more than 20 years. Its height allows a clear, unobstructed view of the area 1 m above the sea bed for taking bottom photographs and measuring current velocity and direction at multiple heights. It is bolted together from several pieces, the top pyramidal part, the legs and lower crossbars. All the pieces will fit on a small stakebody truck and are assmbled at the pier or aboard ship.

Figure 1 below shows a tripod which is being used at the USGS Long Term Monitoring program off Boston, MA. It carries sensors and systems for sediment sampling, bottom photographs, recovery, and measurement of salinity, temperature, current, turbidity and pressure. It is deployed at a depth of about 32 meters.

Figure 1

Legend

1 - Recovery float
2 - Acoustic release
3 - Rope cannister
4 - Battery pack
5 - Current sensors
6 - Main controller, data logging system
7 - Underwater camera
8 - Compass

This tripod is constructed from 4" dia. stainless steel pipe with 300 lb lead weights on each foot. It is recovered by sending an acoustic command through the water to the release, which lets go of the recovery float. The float pulls a lifting line out of the rope cannister to the surface, where
it is snagged and used to lift the tripod up to the recovery ship.

Figure 2

The tripod in Figure 2 is being deployed from the USCG White Heath. All fitted out, it weighs about a ton.

Figure 3

A photograph of the sea floor taken by the tripod's underwater camera system. On the right, the compass shows the direction of the prevailing current. On the left, the bottom of one of the current sensors is visible. A starfish can be seen bottom right. The rocks have been scrubbed clean of sediment by a recent storm.

Figure 4

Newly recovered from the sea floor at the end of an experiment, this tripod shows one of the greatest challenges to oceanographic instrumentation engineers: biofouling.