What is sound velocity? It is the speed at which sound travels (through salt water, fresh water, air, etc.). There are many different things that can influence sound velocity, especially in the ocean. The main influences are:
- Conductivity (salinity)
- Temperature
- Depth (pressure)
Just to give you an idea of how these factors impact sound velocity, here are some figures:
1 degree Celsius change in temperature ---> 4.0 m/sec change in velocity
1 ppt (parts per thousand) change in salinity ---> 1.4 m/sec change in velocity
100 m change in depth (or 10 atm's pressure) ---> 1.7 m/sec change in velocity
SALINITY
On average the ocean contains 32-38 ppt (parts per thousand) of salinity. Some of the influences on salinity include:
- Evaporation
- Prescipitation
- Fresh water influx from rivers
- Tidal effects
TEMPERATURE
Temperature is one of the major influences on sound velocity. Some effects on temperature include:
- Solar heating
- Nighttime cooling
- Rain/runoff
- Upwelling
REFRACTION ERRORS
Applying the wrong sound velocity profile to your data can result in refraction errors. Visually, the swath will curl up in a convex manner if the velocity is higher than supposed to be. The swath with curl downward in a concave manner if the velocity is lower.
Here is a picture detailing exactly that...the wrong sound velocity profile was applied and this is what happened to the data:
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| Data with incorrect sound velocity profile (svp) applied results in errors |
TRANSMISSION LOSSES
A ping is the transmission of an acoustic pulse. The ping moves through the water column and returns to the transducer. Certain things can influence the pings and can result in transmission losses. Sound velocity can effect that as well as spreading and absorption which is frequency dependent.
Our launches have Reson 7125 dual-frequency multibeam sonars installed.The frequency output of these sonars is 200 kHz and 400 kHz. (We also have one tilted Reson 8125 multibeam with a frequency of 455kHz. This multibeam allows us to survey closer to shore since it is tilted). The ship uses a Kongsberg EM710 multibeam system which has a frequecy of 70-100 kHz.
Because of absorption we use different frequencies to obtain data. Lower frequencies are used for deeper depths, higher for more shallow areas. Usually, we will use the 400 kHz for areas up to 50-60m depth. Anything deeper than that we will switch to the 200 kHz.
SOUND VELOCITY PROFILES
Our method for collecting sound velocity profiles is by using a CTD on the launch, or an MVP on the ship.
The CTD has a sensor that measures the conductivity, temperature, and depth of the water (thus CTD). We drop the unit to the seafloor, bring it back up, and collect out data from there.
The MVP is the Moving Vessel Profiler. It is basically the same thing as the CTD just on a larger scale. We use a crane to lower a towfish that contains the CTD sensor to the seafloor.
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| CTD |
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| MVP |
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| Sound Velocity Profile |
DATA ACQUISITION
Due to the weather, we haven't been able to take the launches out for surveying too often this past week. So, we have been collecting some data with the ship's Kongsberg multibeam system. It can go much faster to acquire data with the ship because the swath is much wider due to the lower frequency.
Here is the setup we have for our ship data acquisition process.
The large monitor on top shows our coverage areas and the water column. (Sometimes you can see whales and/or large schools of fish swimming through it). The far right monitor is hooked up to the MVP and allows us to remotely control the MVP (that is located on the fantail) to collect sound velocity profiles. (We have been collecting profiles approx. every 15 minutes to ensure accuracy.) The other two monitors are used for navigational purposes, our position, a log to track the data acquisition, and shows our coverage.
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| Hydrography |
We have arrived in Seward, AK where we will be in port for the next 9 days. After that we will head back to Chirikof Island to gather more data for our survey.
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