A Salty Situation for Surveyors Working in Western Australia
The Lake Macleod salt mine, operated by Rio Tinto subsidiary Dampier Salt, is located in Western Australia about 860 kilometers (535 miles) northwest of Perth. The lake was previously an opencast gypsum mine, which has been further developed as a rich source of natural salt.
To recover the brine, a collection ditch is cut into the halite (rock salt) layer. The liquid is pumped at an average rate of 55 cubic meters (72 cubic yards) per minute from the collection ditch into a transport channel that runs 16 kilometers (10 miles) to a common collection point, before being pumped into the crystallizers and recovered.
Local survey firm Hille, Thompson & Delfos was commissioned to prepare a general profile survey of the channel, which presented assorted obstacles:
- Stringent safety concerns working in close proximity to a concentrated chemical solution
- Mechanical limitations using a small craft due to draft, motor cooling problems and metal part corrosion
- Technical hydrographic constraints with extremely high sound velocity settings and shallow depths
Technologies Team Up
The solution for the surveyors was a manually-towed “rubber duck” boat with Trimble RTK GPS providing position fixes every three seconds and an Ohmex SonarMite echo sounder generating two depths per second.
As the GPS antenna was directly over the echo sounder, there were no complications due to heading and offset correction calculations. The system was very light, so the speed had to be fairly slow to prevent the boat lifting and causing cavitation under the echo sounder’s transducer.
“We collected a mass amount of data that has given us the confidence to prune out the outliers in the plots that inevitably appeared due to the shallowness of the survey, thus giving us good smooth results,” said Edrick Delfos.
Following similar work in Israel’s Dead Sea, the SonarMite echo sounder has the ability to set its sound velocity settings up to the unusually high rate of 1,800 meters (1,970 yards) per second to allow for the high density in saturated brine. The only parts in contact with the liquid were the transducer, the casing of which is predominantly plastic sealed in epoxy resin to avoid the aggressive corrosion associated with using metal parts in concentrated brine.