NOAA Teacher at Sea
Onboard NOAA Ship Rainier
June 1-12, 2009
Mission: Hydrographic survey
Geographical area of cruise: Trocadero Bay, Alaska; 55°20.990’ N, 33°00.677’ W
Date: June 7, 2009
Weather Data from the Bridge
Temperature: Dry Bulb 12.8° C (55°F)
Wet Bulb 11.7°C (53°F)
Cloudcover: Overcast 8/8
Visibility: 4 nautical miles
Wind: VRB, light speed
Sea Wave Height: 0-1
Sea water temperature: 9.4°C (49°F)
Science and Technology Log
Today we left Craig to finish our grids in Trocadero Bay, Alaska. It was a time to clean up or capture data from isolated locations which had either been missed or not completely surveyed. For the first few hours we spent our time surveying areas very close to the shoreline and areas very difficult in which to maneuver.
We did our first cast with the CTD (Conductivity, Temperature, Depth) equipment and I finally asked if I could run it. Ian, the survey technician, happily obliged. The CTD calculates speed of sound through water. I have finally gotten the gist of sonar settings. The following information will help you understand why it is all necessary for getting accurate data to the surveyor and coxswain.
Range- How long it takes for the sonar beam to go to the bottom and return, or in layman’s terms, tells the sonar when to ping and listen.
Pulse length– Pulse length sets how long the sonar transmits, thus allowing more power to be put out bythe sonar, but it results in decreased resolution. The longer the length of the pulse the lower the resolution, so shorter is optimal. For instance, when going through kelp it should be set at low so the kelp isn’t all being picked up by the sonar beam.
Sonar Beams- There are 512 beams at high frequency (400khz). Low frequency (200 khz) equals 256 beams. There are two yellow gates on the screen which surveyors utilize. One is positioned above the shallow water, one is positioned beneath the deepest water measurement. When in shallow water most surveyors disable them. When in deep water, if the top gate is positioned too low, you lose valuable data on the outer limits. If the lower gate is positioned too low it records too much noise. However, if it is set too high the outer beams are missing and no data is recorded. Surveyors must constantly watch this screen when these gates are active to ensure all data they want is being captured.
The surveyor must ensure the data is placed in appropriate folders, enter data in spreadsheets, and basically keep things running smoothly for the entire time data is being logged. So, in essence the surveyor must watch the sonar screen, set the polygons on the screen for him/herself and the coxswain, continually check the settings, remember to log on for data retrieval and log off when the swath is completed, set the CTD for casts every four hours, and monitor as many as ten folders at one time.
The coxswain’s job is to drive the launch into areas to be charted, based on the POD, the Plan of the Day, grids. When data is being recorded he/she drives approximately four to eight knots, depending on the wave action. High swells require slower forward progress. The coxswain has two computer screens-one showing the grid being logged or charted, and another displays depth of water in feet, meters, and fathoms and several other pertinent pieces of data. He or she is ultimately responsible for making decisions about when to enter dicey locations and determining when to stay out of a risky situation.
When traveling in either extremely shallow water or water full of kelp and known rocky locations, a bow watch will stand on the bow and give visuals for the coxswain to avoid. Obviously, this person must wear a safety jacket and hold a rope around an arm or wrist, due to the precarious position he or she is in. High swells could cause serious accidents in a second.
Did you know when backing up a launch, sonar cannot penetrate the bubbles formed when the water is getting stirred? The readings inside the launch show the color red, or dangerous zones, because the sonar thinks the boat is at the bottom. As the surveyors and coxswains say, “No worries! We know where we are.”
Question of the day: What is a patch test and why is it run?
Now that I felt much more comfortable with understanding the sonar I was able to relax more on the launch today. Perfect timing, as this was such a great day for biological observations. Five different humpback whales were sighted in the bay with us; in one location two were in position as forward observers on either side of our launch. The last whale we spotted surfaced fairly close to our launch so we had to stop, mainly because the regulations state you must stay 100 yards from humpback whales. This whale went under the launch and surfaced about 50 meters from us. Off and on during the day they would surface in the areas we were surveying so we had to just wait until they moved along.
I also observed at least eight bald eagles either sitting in trees, flying over the water, or harassing the whales. One eagle flew down close to the water and looked as though it was taunting the whale! Then it quickly flew back up to a tree top and perched on a branch. Several eagles would fly off together, separate, then come back together before landing on a tree. Early in the morning we ran into a group of seals swimming around in kelp. They poked their heads out and just stared at us as we drove by. Luckily we saw them in time to slow down, so as to not disturb them anymore than necessary.