Page 44 - The ROV Manual - A User Guide for Remotely Operated Vehicles 2nd edition

32 CHAPTER 2 The Ocean Environment
the surface layer. As stated previously, the deep ocean is uniformly cold due to the higher density cold water sinking to the bottom of the world’s oceans. The temperature change from the warm sur- face at the tropics to the lower cold water can be extreme, causing a rapid temperature swing within a few meters of the surface. This surface layer remains near the surface, causing a small tight “surface duct” in the lower latitudes. In the higher latitudes, however, the difference between ambient surface temperature and the temperature of the cold depth is less pronounced. The thermal mixing layer at these latitudes, as a result, is much larger (over a broader range of depth between the surface and the isothermal lower depths) and less pronounced (Figure 2.5). In Figure 2.5(a), density profiles by lati- tude and depth are examined to display the varying effects of deepwater temperature/density profiles versus ambient surface temperatures. Figure 2.5(b) and (c) looks at the same profiles only focusing on temperature and salinity. Figure 2.5(d) demonstrates a general profile for density at low to midlati- tudes (the mixed layer is water of constant temperature due to the effects of wave mechanics/mixing).
A good example of the effect of density on ROV operations comes from a scientific mission conducted in 2003 in conjunction with National Geographic magazine. The mission was to the ce- notes (sinkholes) of the Northern Yucatan in Mexico. Cenotes are a series of pressure holes in a cir- cular arrangement, centered around Chicxulub (the theoretical meteor impact point), purportedly left over from the K
T event from 65 million years ago that killed the dinosaurs.
The top water in the cenote is fresh water from rain runoff, with the bottom of the cenote becoming salt water due to communication (via an underground cave network) with the open ocean. This column of still water is a near perfect unmixed column of fresh water on top with salt water below. A micro-ROV was being used to examine the bottom of the cenote as well as to sam- ple the salt water/fresh water (halocline) layer. The submersible was ballasted to the fresh water on the top layer. When the vehicle came to the salt water layer, the submersible’s vertical thruster had insufficient downward thrust to penetrate into the salt water below and kept “bouncing” off the hal- ocline. The submersible had to be re-ballasted for salt water in order to get into that layer and take the measurements, but the vehicle was useless on the way down due to its being too heavily bal- lasted to operate in fresh water.
2.2.7 Depth
Depth sensors, discussed below, measure the distance from the surface of a body of water to a given point beneath the surface, either the bottom or any intermediate level. Depth readings are used by researchers and engineers in coastal and ocean profiling, dredging, erosion and flood moni- toring, and construction applications.
Bathymetry is the measurement of depth in bodies of water. Further, it is the underwater version of topography in geography. Bottom contour mapping details the shape of the seafloor, showing the features, characteristics, and general outlay. Tools for bathymetry and sea bottom characteriza- tion are as follows.
2.2.7.1 Echo sounder
An echo sounder measures the round trip time it takes for a pulse of sound to travel from the source at the measuring platform (surface vessel or on the bottom of the submersible) to the sea bottom and return. When mounted on a vessel, this device is generally termed a “fathometer” and when mounted on a submersible it is termed an “altimeter.”