NOAA Teacher at Sea
Onboard NOAA Ship Hi’ialakai
April 4 – 25, 2005
Mission: Coral Reef Ecosystem Survey
Geographical Area: Northwest Hawaiian Island
Date: April 5, 2005
Location: Latitude: 28.5 N, Longitude: 49.3 W
Weather Data from the Bridge
Visibility: 10 nautical miles
Wind Direction: 42
Wind Speed: 16 kts
Sea Wave Height: 3 feet
Swell Wave Height: 3-4 feet
Sea Water Temperature: N/A
Sea Level Pressure: 1021 mb
Cloud Cover: 3/8 SC, AS, Ci
Science and Technology Log
Today’s scientific goals involve running survey lines at Nihoa. Survey lines will begin at the 12:00 position and run counterclockwise one and a quarter times at Nihoa. The ship will be using its multibeam sonar equipment to do this and it will in turn fill in missing data to complete benthic habitat maps of this area. A formal in-service was given by senior surveyor, Joyce Miller, on multibeam sonar equipment. Some of the interesting facts from that presentation are provided below. There are 3 multibeam sonar devices available for use on the HI’IALAKAI. Sonar concepts from the in-service: An echo sounder sends sound down to the sea floor and then back up. A single beam echo sounder sends a pulse out that comes back to 1 point on the ship. The center of the beam right under the ship, or swath, is termed nadir.
Nadir is the shortest distance between the sensor and the location of the beam. Ensonification is energy within the main part of the beam pattern which radiates toward the sea floor. Decibel is a unit used to measure the relative strength of a signal. Beam width is an angle that defines the main part of the energy that is radiated within a 3db solid angle. The footprint size beneath the sonar beam changes as the water gets deeper because it comes out of the ship at an angular direction. The deeper the water, the less accurate the information will be from the beam because the footprint pattern below the beam gets larger. A narrow beam echo sounder ensonifies a smaller area, so it gets more accurate information because of its narrower angle. A transducer is a device that converts electrical energy into sound energy and vice versa. The “ping” is the sound going down the beam. The frequency is the number of times per second that the same waves of sound repeat itself (vibrations per second). The pulse length, or duration of outgoing pulses of the sonar equipment, in part determines the system’s resolution.
The shorter the pulse length, the greater the resolution. Other facts: The transducer range, or how far the sound is effectively transmitting, is determined by a number of factors, including; frequency, transmit power, beam width, transmit pulse length, received bandwidth, absorption, ocean floor composition, and noise level (heavy rain). In summary, high frequency sonars with narrow beam widths provide the highest vertical resolution. If you need both range and resolution, pick a medium frequency sonar to do the job. What is being measured then? The 2-way travel time of a sound wave and this information is converted to distance. The speed of sound in water ranges from 1450 meters/sec to 1550 m/sec. CTDs, or Conductivity, Temperature/Depth devices are dropped at intervals off the side of the ship daily because the information they gather are the most accurate way to get sound velocity data and is needed when multibeam error sources are being defined. Multibeam concepts: Side scan sonars are sonars that are towed behind a boat. Backscatter is the term for when the sonar signal provides information about the character of the sea bottom (smoothness, roughness, etc). Multi-beam sonars were first designed to provide information on depths and they just happen to also give information on backscatter.
Benthic habitat maps are maps pf the sea floor, so backscatter information is extremely useful and the goal of this expedition. To get good backscatter data, many factors need to be kept constant. The ship should be driven in straight lines and kept at a constant speed. Some of the area around the Hawaiian Island chain has already been mapped using this technology but there are many gaps to be filled in. The cruise aims to fill in more of that missing information for the benthic habitat maps. Three multibeams are being utilized on this trip. Finally, it is important to understand sources of error in multibeam use. They consist of sound velocity or physical oceanographic parameters that influence the sound velocity structure. These include temperature, salinity, depth, and density, which are all recorded and gathered during CTD drops. Changes in these parameters affect the multibeam because they are used to create a sound velocity profile.
I awoke to the hustle and bustle of the ship, as my stateroom is located a few doors down from the mess. After eating (I eat better here than I do at home) I attended a formal inservice presentation by the senior surveyor, Joyce Miller, on Multibeam Training. I took notes during her PowerPoint slideshow, to try to better understand the type of sonar equipment they are using onboard. The transducer on the sonar equipment turns electrical energy into sound energy and is sent down to the ocean floor. It bounces and scatters and provides data which is used to create a map of the ocean floor (a benthic habitat map). I also learned some new vocabulary words like nadir, ensonification, and beam width. We broke for lunch and after lunch I attended my first CTD cast on the deck and took some pictures. After noon the scientists met back in the forward mess lounge to finish the multibeam training. The rest of the evening was just left to typing logs, watching a movie, and resting. I am not assigned a watch schedule so I have been sleeping normal hours of 10 to 6am.
QUESTION OF THE DAY for my fourth grade students: Locate the ship using the latitude and longitude coordinates above. Remember “latitude” lines are fat (horizontal) and longitude lines are long (vertical). What are the 5 major Islands of Hawaii? What does the word salinity mean? Would the Pacific Ocean or the Potomac River be measured for salinity, and why?