Tag: multibeam sonar

NOAA Teacher at Sea
Christy Garvin
Onboard NOAA Ship Rainier
June 1 – 8, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: June 4, 2005

Weather from the Bridge

Latitude: 56 deg 59 min N
Longitude: 135 deg 17 min W
Visibility: 5 nautical miles
Wind Direction: 300 deg
Wind Speed: 10 kts
Sea Wave Height: 0-1 ft
Swell Wave Height: 0 ft (we are in a protected bay)
Sea Water Temperature: 53deg F
Sea Level Pressure: 1009.8 mb

Science and Technology Log 

On the RAINIER, the crew works right through the weekend, so the workday began at 0800; again, four launches were deployed to run survey lines and take bottom samples.  I was assigned to launch RA-3, and we worked an area on survey sheet Y.

Launch RA-3 ran approximately 40 miles of hydrography using the Sea Bat 8101 Multibeam Echosounder.  The Sea Bat is a 240kHz echosounder that measures the relative water depths across a wide swath that is perpendicular to the launch’s track.  The system is comprised of 5 main parts: the sonar processor, the sonar head, the sonar processor to sonar head signal and control cable, a color monitor, and a computer mouse.  The transmit array, which is a projector section of the sonar head, transmits a pulse of sound energy that travels through the water and is reflected by the sea floor or any object in its path. The reflected signal is received by the hydrophone section of the sonar head, digitized, and then sent to the sonar processor for beamforming.  The processor then generates a video display of the ocean floor that can be viewed on the color monitor.  The Sea Bat can “see” approximately 300 meters, but it is more accurate in depths of 150 meters or less.

The physical process of running survey lines with the Sea Bat is nicknamed “mowing the grass;” this is because the launch actually follows the parallel lines drawn by the survey techs and the launch’s path resembles an individual mowing a lawn.  The survey lines are displayed on a computer screen so the survey tech can highlight a given line for the coxswain to follow. As the launch approaches the line, the survey tech logs the computer data for each line.  Lines vary in length from a few meters to several kilometers.

Personal log 

I learned to drive the launch today, and it was a lot of fun.  I was able to “mow the grass” for about an hour, and I also drove during a man overboard drill.

Previous question of the day:  What is refraction? 

Answer: Refraction is the bending of a sound wave.  In the case of sound traveling through water, different temperatures and pressures cause sound to travel at different speeds; this in turn causes the waves to bend.

NOAA Teacher at Sea
Christy Garvin
Onboard NOAA Ship Rainier
June 1 – 8, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: June 2, 2005

Weather from the Bridge

Latitude: 56 deg 59 min N
Longitude: 135 deg 17 min W
Visibility:12 nautical miles
Wind Direction: 275 deg
Wind Speed: 10 kts
Sea Wave Height: 1-2 ft
Swell Wave Height: 0 ft (we are in a protected bay)
Sea Water Temperature: 54deg F
Sea Level Pressure: 1016 mb

Science and Technology Log 

Each evening aboard the RAINIER a POD, or plan of the day, is posted to inform all hands of the work scheduled for the following day. The POD for today, Thursday, June 2, showed that work would officially begin at 0800.  Two launches were to be deployed in order to run survey lines; I was assigned to launch RA4 working near Sitka and off Baranof Island. Running survey lines is one of the most important tasks accomplished by the RAINIER.  Since the ship’s scientific mission is hydrography, or charting of the ocean floor, the running of these lines is a major part of accomplishing that task.

Before the launches can actually run survey lines, there is a lot of preparation that takes place behind the scenes in the plotting room.  Here, each area to be surveyed is subdivided into smaller plots and placed on survey sheets to make large areas more manageable.  Then, a survey technician pulls existing nautical charts of the area and uses previously obtained depth information to determine where and how far apart to space the survey lines. In order to ensure 100% coverage of the ocean floor to the 8-meter mark, survey techs multiply the depth in fathoms (a fathom is 6 ft) by 5 to calculate the meters of line spacing.  For example, if a launch is working in an area that is 7 fathoms deep, then the amount of space between the survey lines in that area would be 35 meters apart. The closest any lines are drawn is 25 meters apart.

Once the distance between lines has been established, the survey tech draws the lines on a map of the area.  Lines are drawn parallel near the shoreline since close lines are necessary to achieve 100% coverage. Further from the shore, lines can be drawn in any direction, but they are usually drawn parallel to each other for ease in running the lines.  These maps are taken out on the launches and entered into a specialized computer program that helps the launch find and follow the survey lines that have been drawn.

Personal Log 

One of my favorite activities today was having the chance to drive the launch during a man overboard drill.  I’m sure the life preserver in the water (our man overboard) breathed a huge sigh of relief that it wasn’t run over by the crazy coxswain (driver) at the helm.

Question of the day: What is a CTD? 

Previous question of the day: What is hydrography and why is it necessary? Answer: Hydrography is the charting of the ocean floor. It is necessary because many ships use the information on the charts to ensure safe passage through narrow or shallow channels. Many of the old charts contain data that is very old or that was obtained by unreliable methods.  It is important that charts are updated so that ships are able to pass through these areas safely.

NOAA Teacher at Sea
Christy Garvin
Onboard NOAA Ship Rainier
June 1 – 8, 2005

Mission: Hydrographic Survey
Geographical Area: Aleutian Islands, AK
Date: June 1, 2005

Weather from the Bridge

Latitude:  56 deg 59 min N
Longitude: 135 deg 17 min W
Visibility:  12 nautical miles
Wind Direction: 285 deg
Wind Speed:  15 kts
Sea Wave Height:  1-2 ft
Swell Wave Height:  0 ft (we are in a protected bay)
Sea Water Temperature:  51deg F
Sea Level Pressure: 1022.5 mb

Science and Technology Log 

Due to the Memorial Day weekend, today was the RAINIER’s first operational day since I arrived in Sitka, Alaska. Sitka is located off the western coast of Baranof Island and is surrounded by snow-covered mountains.  The weather has been sunny with temperatures around 55 deg F. Currently, the RAINIER is anchored in the Aleutkina Bay, which is just south of Sitka.

Because several interns and new hands joined the crew for this leg of the journey, the CO, or Commanding Officer, declared today a stand down day.  Basically, this meant that normal work operations were suspended so that the entire day could be devoted to training. A large part of our training was concerned with the deployment and recovery of small boats called launches.

The launches are stored on a system called the gravity falls davit.  The system was originally designed for emergency boat deployment and is capable of working without any electricity; by lifting a lever and removing a small pin, the weight of the boat will cause the davit to slide down the track and drop the boat in the water.  Although this works well in emergency situations, it causes a great amount of stress and strain on the equipment.  Therefore, this method is not used for the daily deployment of the launches.  Instead, a somewhat more complicated process requiring a team of ten or more people is used to safely put the boats in and out of the water.

The first step in deploying the boats is to unplug the boats from the main ship and loosen the gripes that secure the boat.  Once the boat is no longer constrained, a team of four people standing on the deck below grabs ropes called frapping lines and helps stabilize the boat as it is lowered down to the deck level by a controller allowing the davit to slowly descend. Once the launch has reached the main deck level, a survey team boards the launch with all of their equipment.

At that point, one member of the survey team sits on the bow of the boat while another stands on the stern. Their job is to help detach the chain falls (a hook with an attached shackle) from the padeyes (a horseshoe shaped attachment bolted to the deck).  This detachment occurs after the boat is slowly lowered into the water by a controller using a winch drum.  Throughout the entire process, the boat is continually stabilized by the four linesmen who provide or remove slack from the frapping lines.  The deck crew and survey teams have practiced this process many times, and the process is quick and efficient when run by an experienced crew.

However, it was much less graceful as all of the new hands took their turns on deck; ropes ended up in the water, the launches bumped the ship a few times, and hooks didn’t quite catch the padeyes; but by the end of the day all of the new hands were feeling much more confident in deployment and recovery of launches.  It was an excellent opportunity to learn and practice new skills that will be used each day as survey teams are sent forth to accomplish the hydrography mission of the RAINIER.

Personal Log 

I really enjoyed learning how to tie different types of knots and handle line today.  Working as a linesman is very intense because someone could be seriously injured if you fail to do your job properly. The teamwork aspect is so important to safety as the launches are deployed, and I am having a great time actually being involved in the procedures that take place on the ship.

Question of the day: What is hydrography and why is it necessary?