Achieving good image quality and complete coverage
The DVL enabled the research team to efficiently map a section of a subsurface wall with still images, while ensuring good image quality and complete coverage. The test was carried out in Trondheimsfjorden, a fjord in Norway.
“Getting the velocity and altitude measurements from the forward-facing DVL enabled us to run the ROV in an automatic pattern on a vertical rock wall while maintaining a constant distance to the wall,” says Stein Nornes.
The ROV was equipped with a combined stereo camera and DVL rig. The DVL was a standard 1 MHz DVL from Nortek, with no hardware modifications. The chosen survey area was a rock wall with an approximate inclination of 70 degrees located at 175–200 m depth. The main guidance module of the ROV was programmed to move along a pattern of five parallel vertical 5-meter-long lines with 1 meter linespacing to cover the face of the rock wall with sufficient level of overlap.
“The DVL was mounted in between the two still image cameras on the front of the ROV, facing forwards. The input from the DVL is transmitted via serial communication to the dynamic positioning system we are continually developing at our university,” adds Nornes.
This helped the NTNU team get data on velocity relative to the wall, which was used to track the ROV’s movement, as well as the four distance measurements, which were used to keep the ROV at a constant distance from the wall.
“The velocity is essential in order to maintain a smooth positioning of the ROV. Keeping a constant distance to the wall helps counteract difficulties with image quality related to light attenuation in water,” Nornes explains