The Seismic Survey of Lake Rotomahana is proceeding well this week. Whilst it is being led by GNS Science, the University of Waikato and NIWA are providing technical assistance with some of the equipment being used.
The first photo shows the survey boat being loaded with the the cable that contains the hydrophones. These pick up the reflected sound waves that are sent down below the surface by the ‘boomer’, the white object in the background, at the end of the pier.
In the graphic you can see how the set up works. The boat tows the seismic source (either the low frequency ‘boomer’ or the higher frequency ‘CHIRP’). This sends sound waves down through the water and into the rocks below. These signals get reflected back up from the different rock layers and are received by the hydrophones in the cable floating behind the boat. Lower frequency sound waves can penetrate deeper into the rocks, whilst higher frequencies give shallower penetration, but provide more detail.
During our survey we are using the boomer to give an overall view of the lake floor first. We are then using CHIRP to go over specific locations that we want to observe in more detail, such as the sites of any terraces and particular volcanic structures.
On this map of the lake floor, you can see how the seismic lines criss cross the lake back and forth to give overall coverage. This is the planning map, but sometimes the scientists change their plans during the survey, depending on the time they have available, and how well things are progressing.
Chris Leblanc is set up with all the computer hardware and software to process all the data produced by the survey. He creates graphic cross sections of the lake floor that reveal the sub surface geological features. You can see one of these sections on his computer screen.
There has been a great deal of media interest in our investigation of Lake Rotomahana. In the last photo Cornel de Ronde is being interviewed by John Hudson with cameraman Clint Bruce for TV1’s Sunday programme.