How to Improve Sound Velocity Accuracy and Lower its Affect to Water Depth Survey?

Why we need sounde velocity productd during underwater application?

We all know sound can transmit much longer than light or radio signal, which will attenuate seriously in water, especially in turbid water. So most underwater exploration equipments' working principle is base on acoustic trasmition.

When we use single beam or multibeam echo sounder to survey the bottom depth, the sound velocity accuracy is also the key factor. Look at each beam, it is projected from transducer to the bottom downwards. When the sound wave hits the waterbottom, it is reflected. Then the transmitted echo signal returns to transducer and is received by its receiving arrays.

From above picture, we can see, the water depth is determined by the round-trip time of sound signal multiple by sound speed in water. h = sv * t. So how to get accurate sound velocity become the key point to get precise water depth.

Sound Veloicy is more important for multibeam echo sonar survey

Multibeam sounding system is to use transducer array project more sound beams covering a wide swathe sector to the seabed, and the receiving array receives the sound waves in a narrow beam. Because of the orthogonality of the direction of transmitting and receiving sectors, the irradiated footprints of the seabed terrain can be formed.

After these footprints areproperly processed, we can obtain over one hundred and even more water depth of surveyed points which are in the vertical plane perpendicular to the coursethrough one single survey. We can then accurately and quickly measurethe size, shape and height changes of underwater targetswhich locate within a certain width along the course, and describe the three-dimensional characteristics of the seabed topography more reliably. Combined with the navigation positioning and attitude data collected on site, we can get digital outcome drawings with high precision and high resolution.

Difficulty to get accurate the sound velocity

There are mainly three factors that affect the sound velocity in water.   

1) Temperature:

The sound speed will increase along with the temperatrue getting higher.

2) Salinty:

The Sound velocity will increase when the salinity increases. So sound transmit quicker in sea than in lake or river.

3) Pressure:

The sound speed will increase along with the pressure getting higher.

In conclusion, the speed of sound in water increases along with increasing water temperature, increasing salinity and increasing pressure. So we need device to measure accurate sound veloicity in different environments.

Type of Sound Velocity Products:

1) Sound Velocity Sensor

It is used to measure the sound velocity in water at the location of the sound velocity sensor and usually integrate together with transducer. In fresh water river, where current and flow is big and not deep, the sound veloicy does not vary that much. Then one sound velocity sensor is enough. We can regard it as average sound velocity.

SVS1500 is very sensitive and can know any minimum change of sound velocity caused by environment. The accurcy can reach 0.05m/s. Its operation is quite simple and can send the real-time data to multibeam or single beam echo sounder and enhance the depth accuracy.

2) Sound Velocity Profiler

When the fresh water is peaceful, such as reservoirs or lakes, the surface sound velocity value cannot represent the average sound velocity.   It is recommended to use both sound velocity sensor and profiler, which can measure the sound velocity at different depths from surface to bottom of the water area. The same request to sea application.

SVP1500 is   a self-contained device that can automatically store sound velocity data in internal memory. The internal battery enable It built in a high precision pressure sensor, which can mesure depth corresponding to each sound velocity value. Its standard software named HydroSonar is user-friendly and easy operation, which can intellegent calculate the average sound velocity values in every depths.