The problem with perfect conditions is that everyone has the same idea – let’s get out there! Of course the popular idea of fishing was not the same as ours—i.e. towing a magnetometer fish as part of the Department of Environment and Heritage’s magnetometer trials. The initial focus of the trials was to perfect the hardware and software ‘set-up’ and investigate the detection range and signature profiles for different known wreck types. The four wrecks selected for initial testing were:
1. Tiwi Pearl: a modern fishing trawler sunk as an artificial reef.
2. S.S. Dover: a former ferry converted to a machine gun platform
3. The wheel house of the Captain Nielsen, a suction dredge that capsized in 1964.
4. Grace Darling: a wooden schooner that ran aground in 1894.
Figure 1: Map of Moreton Bay showing the location of the four wrecks used for initial magnetometer testing.
It took a little while to get the magnetometer and computer configuration working the way we wanted, but we eventually set out only an hour later than expected on a glorious, sunny, calm day. Upon arrival at the starting point of our search grid for the S.S. Dover it was immediately obvious where the wreck was located, as there were three recreational fishing vessels anchored in the middle of the grid. We commenced the survey and as we moved closer to the group, they became increasingly curious about what we were doing. By the time we came in close proximity they were actively enquiring why a Marine Parks vessel was trawling back-and-forth around them. When advised of our intention they freely offered to provide the marks for the wreck if we would immediately leave. Interestingly, the aluminium hulled fishing vessels caused no significant magnetic interference. Fortunately there were no hook-ups or problems on our part, but the final nail in their fishing efforts came with a large pod of dolphins, which also forced us to slow down and recover the tow-fish. Seems like everyone was taking advantage of the great conditions.
Figure 2: A large pod of dolphins passing the vessel during the magnetometer survey.
The search for the wheel-house of the Captain Neilsen was based on marks taken during a recreational dive inspection. The site was meant to be located within the centre of the grid to test detection range. However, when we finished the grid we had only a marginal reading in the top N/E corner of the grid. We immediately extended the search in that area and found a strong signature that was confirmed via side-scan sonar to be the wheel house.
Figure 3: The side scan image (left) and vessel track (right) showing the location of Captain Neilsen’s wheelhouse.
The Grace Darling site proved quite problematic due to its shallow depth and proximity to shore. Some of the planned transects were in too shallow water and the alignment of some transects had to be adjusted to suit the topography of the seabed.
Figure 4: A print out of magnetometer readings for the Grace Darling. Note the gaps between positive returns (in red) indicating the transects were too far apart to detect the shallow and highly degraded timber wreck.
The last wreck, the Tiwi Pearl, was again immediately obvious, as there were nearly a dozen recreational fishing vessels in the immediate vicinity. Similar to the S.S. Dover, the fishing fraternity kept a close eye on proceedings as we endeavoured to run transects around them. Again, we found the recreational vessels caused minimal instrument interference. Importantly, we also found the magnetometer worked more effectively when side-on, or adjacent to the wreck, than immediately above it.
The key outcome of the initial testing was greater understanding of the detection capacities of the system and how to configure the surveys. While it was no surprise that the detection range was directly proportional to the depth of target and its ferrous concentration, it was surprising that, for the timber wreck of the Grace Darling, transects needed to be considerably reduced, as there were gaps in the positive signal. While this enabled the results to be pieced together to get an overall picture, it was reliant upon our understanding of the nature of the site and indicated that buried wooden wrecks would require tighter transects to achieve complete coverage.
During post-processing it also became apparent that the way we had configured our searches in the software prevented us from separating out certain finds during post-processing. This meant we needed to repeat some of the searches, which was serendipitous, as the revised search grid for the Captain Nielsen led to the discovery of more large pieces of wreckage nearby. The new information about transect width was also important as it forced us to revise our strategy for the planned search of the Venus. For now, though, the next step is to test the mag on a large historic iron barque and conduct a preliminary search for another of the same configuration lost nearby.