Tag Archives: Magnetometer

“Hey Venus, Oh Venus”

Many of the younger readers may not be familiar with Franky Avalon and his 1959 hit song ‘Venus’, which contains a plea from a young man to the Roman Goddess of Love to help him find that elusive special someone (if you’re feeling nostalgic you can find him performing the song at: http://www.youtube.com/watch?v=fakpqLDEQAo). Much like the young soul yearning to discover love, we also went in search of Venus as part of the final leg of our magnetometer trials. The small snow-brig Venus made an unscheduled stop in Moreton Bay for supplies in 1855 while transporting sugar from Manila to Melbourne. The Venus attempted to enter the Bay without a pilot and took the wrong channel, foundering off the northern end of Moreton Island on a sand bank system that was subsequently named after the wreck.

A key issue for magnetometer survey was narrowing the search area. Historical research was conducted but sources were few, given the vessel was lost when Queensland was still part of the colony of New South Wales. Newspaper reports from the time did, however, provide quite detailed information on the events leading up to the wrecking, with some vocal critics stating that a missing navigation marker played a key role. In order to help refine the search area, archaeologists Amelia Lacey and Toni Massey liaised with Mr Ian Jempson, Chief Executive Officer of the Queensland Maritime Museum and a former naval navigator. The Queensland Maritime Museum holds copies of historic admiralty charts for Moreton Bay, which were digitised and then georeferenced by Amelia. Ian was able to cross reference the historic sailing instructions with the published account of the wreck’s position and develop a theory of where it is located now. The varying positions of the banks over time were also tracked on the charts.

Image 1: A georeferenced copy of the 1846 navigation chart for Moreton Bay

Image 1: A georeferenced copy of the 1846 navigation chart for Moreton Bay

Not unexpectedly, there was a significant margin of error in the georeferenced charts and this resulted in a larger search area than hoped. To supplement the mapping team’s work I sought information about potential sightings of the wreck from public informants and marine parks staff.

Image 2: The search area (marked by the larger box with yellow contours) for the wreck of the Venus superimposed on the historic chart. Note the name Venus Banks immediately above the search area.

Image 2: The search area (marked by the larger box with yellow contours) for the wreck of the Venus superimposed on the historic chart. Note the name Venus Banks immediately above the search area.

The team set out under clear skies but with a freshening breeze to begin the process of surveying the search grid. Alas, however, as tends to happen in affairs of the heart, there were soon problems when the magnetometer failed after the first transect. After some unsuccessful attempts to redeploy the fish, we realised that there was a connection problem and returned to base to switch to the shorter umbilical cable, which is normally used to link the mag with a towed side scan sonar – fortunately this was a very shallow site. Unfortunately, we had lost a lot of time and could not complete the planned search grid that day; we therefore decided that the best option was to run the mag past a target further to the east north east that was reported to us by two separate sources. The estimated position revealed an almost immediate positive return with the mag, but little was visible on the side scan. This site definitely needs to be investigated further, but to date we have been unable to return to the area as repairs to the mag and vessel availability have caused delays. Love remains elusive but we hope to resume our quest in the autumn of 2015 when the conditions are expected to be more favourable.

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Nice Day for a Fish

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.

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.

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.

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.

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.

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Moreton Bay Magnetometer Survey – Making it Work

By Paddy Waterson

It’s always exciting, and a bit nerve racking, when you get a new piece of ‘kit’.  Will it be easy to put together? Will it work as well as you hoped?  Will it enable you to achieve the results you have promised?  You have probably seen the same piece of equipment at work and know the basics, but the onus is on you now and there are always tricks to be learnt.

In 2013, the Queensland Department of Environment and Heritage Protection invested in a new Geometrics G882 Marine Magnetometer to assist with the Queensland Historic Shipwreck Survey (QHSS). The QHSS is a five year initiative to update official records on the state’s estimated 1400 historic shipwrecks. The size of the state, and the number of historic shipwrecks, means that the fieldwork component of the survey is aimed at locating, identifying and documenting wrecks in key strategic areas, such as Moreton Bay. The initial phases of fieldwork in the QHSS used an existing side scan sonar system and had been quite successful in locating a number of wrecks. However, it soon became apparent that we need something more. The dynamic nature of the Queensland coast made locating many timber wrecks problematic, largely because they are constructed from materials that are extremely vulnerable to deterioration in the marine environment and so tend to have a lower physical profile. This is compounded by Queensland’s offshore environment that is a mixture of dense corals, thick muds and highly mobile sand, all of which can significantly inhibit the effectiveness of visual and side scan sonar searches for low profile historic shipwrecks. A business case for a magnetometer was subsequently developed and the G882 was purchased using funds from the Commonwealth Historic Shipwrecks Program—now I just have to make it work!

A project was developed to configure and test the magnetometer in local conditions to ensure we achieved the best potential outcomes when it was deployed across the state. This project has two phases:

  1. The initial testing of the magnetometer on five known shipwrecks to determine its operational limits and develop a signature profile guide for different wreck types.
  2. Conducting preliminary research into two previously un-located wrecks in the Moreton Bay Region.

The initial testing phase will use five known wrecks within the greater Moreton region. These wrecks were chosen for their comparative signature profile testing, as they are a good representative sample of the different wreck types commonly encountered along the Queensland coast. The test wrecks range in type from a small wooden schooner and a large iron hulled barque, through to steel hulled trawler. By comparing the different magnetic signatures of the wrecks, and their relative detection ranges, we will be able to refine future survey methods and better interpret results when searching for previously un-located historic shipwrecks.

Table 1. Details of the five wrecks used to test the magnetometer, build a signature profile and refine search methods. These wrecks were chosen due to their variation in size, physical profile and construction materials.

Table 1. Details of the five wrecks used to test the magnetometer, build a signature profile and refine search methods. These wrecks were chosen due to their variation in size, physical profile and construction materials.

The initial configuration and preliminary tests were conducted in November 2013. The hardware configuration for the magnetometer was relatively simple, as it came correctly calibrated for the region. Some minor assembly was required, but this was quickly achieved with the support of staff from Marine Sciences and the Queensland Parks and Wildlife Service.

The Geometrics G882 Marine Magnetometer

The Geometrics G882 Marine Magnetometer

The magnetometer being deployed from the Queensland Marine Parks vessel Caretta.  Assisting are Ranger Rohan Couch (left) and Technical Officer James Fels (right).

The magnetometer being deployed from the Queensland Marine Parks vessel Caretta.  Assisting are Ranger Rohan Couch (left) and Technical Officer James Fels (right).

The initial software configuration proved more challenging, as the magnetometer software was configured to integrate the GPS data via a ‘pin-port’ rather than the more common USB connection—this was resolved through the acquisition of an additional ‘pin-port’ aerial output cable.  The use of a specialised laptop that could cope with the movement of the vessel was also essential—many laptops simply lock up the hard drive when vibration is detected.

The laptop, data junction box and GPS configured and ready for deployment.

The laptop, data junction box and GPS configured and ready for deployment.

With the initial set-up and preliminary systems testing complete the surveys of the known wrecks could commence—and a new range of challenges could begin. More on that in my next blog.

Let’s get Geophysical! Non-invasive Underwater Archaeological Survey Methods

Trends are not a new concept to archaeology. The patterns found in the archaeological record are what lead to the wider inferences made about past cultures or behaviours. However, the latest trend in archaeology isn’t about similarities in information sets or assemblages,but rather the movement towards in situ (in place) preservation of archaeological sites, especially in underwater archaeology. I use the term ‘trend’ loosely, as it implies that in situ preservation is a ‘fad’ that will become obsolete given enough time or with the arrival of a newer, en vogue concept. I actually believe the opposite is true, that in situ preservation is here to stay and that it is the future of archaeology, above or below the water. This is not so much my opinion, but more of an observation. Looking at the international legislation that surrounds underwater cultural heritage (UCH), one cannot help but see that in situ preservation is pressed as the primary approach (UNESCO, 2001: Article 2,5; UNESCO Annex, 2001:Rule 1) and in many introductory texts, non-invasive survey methods are considered the future (Bowens, 2009:5). We need to know what is under the seabed in order to know if archaeological sites lie beneath, but we are trending away from invasive methods of surveying like subsurface testing. This leaves non-invasive approaches like geophysical surveys and remote sensing.

Geophysics in underwater archaeology is the scientific study of features below underwater and under the seabed using a range of specialized instruments while remote sensing is obtaining images of a phenomena from a distance (Bowens 2009: 217). It is common for these two methods to be grouped together, as they both deal with the ability to collect large amounts of data quickly and understand the scale of the surveyed site without having to be directly on or necessarily near it. In the past, geophysics was used primarily for site prospection but has been applied more recently to research and site management (Bowens 2009: 103). Geophysical and remote sensing surveys allow for the coverage of large areas relatively quickly and economically. They are not meant to replace divers on a site, but aid in timely identification of site locations, site distribution, site boundaries, and sub-seabed phenomena and are particularly useful in environments with poor underwater visibility, strong currents, or any other environmental hazards. Geophysical and remote sensing surveying methods will be discussed and can be grouped into three types: acoustic systems, magnetometers, and submersibles. These methods are used over a large area to ensure complete coverage of the site and its environmental context and are very accurate when used with global positioning system (GPS) satellites and differential global positioning system (DGPS) land-based reference stations. Using both will increase site position fixing as DGPS makes range corrections for GPS satellites; the addition of an on-boat GPS antenna increases accuracy (Bowens 2009:94).

Acoustic Systems:

These systems are the most commonly used geophysical method for underwater archaeological surveying. Sonar,or sound waves, are used in order to obtain the desired information. Some forms of acoustic surveying systems are: echo-sounders, multibeam sonars, side scan sonars, and sub-bottom profilers (Bowens 2009:104). The general idea behind these types of non-invasive systems is to use reflected sound waves (echoes) to construct a picture of what the underwater site and bathymetry, or depth over seabed, looks like.  Figure 1 shows the different components and general setup for using side scan sonar. Side scan sonar uses a wide-angle pulse of sound (emitted from the towfish) and the strength of the reflected scattered sound to display an image (Figure 2). The coverage of the side scan sonar can reach over 100m on either side of the track line. The track line is a gap in between the two sides; its size varies by size of coverage and depth. It is a ‘dead space’ of sorts where there is too much interference between the two sides to get an accurate image. This problem can be countered by overlapping boat runs to ensure full coverage. Acoustic shadows are also important as they can give a general description of objects that sit proud (vertical) to the seabed (Bowens 2009:108), see Figure 2.

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Figure 1.  The components and set up of a side scan sonar (image created by author)

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Figure 2. The results of a side scan sonar survey (after Kainic 2012)

Echo-sounders and multibeam sonars are generally used to gauge vertical measurements or depth. Echo-sounders were first to be applied to maritime archaeology and used a single transceiver to send an acoustic pulse straight down to the seabed and read the reflection or echo on a single prescribed spot. Multibeam sonar (also known as swath bathymetry) records a continuous thin strip of depth directly below and to the side of the boat (Figure 3), effectively scans the surface of the seabed, and creates a 3D image via colour gradations to highlight depressions and outcrops, as represented in Figure 4 (Bowens 2009:106). Sub-bottom profiling is the only means to locate buried wooden material culture underwater; metal material culture will be discussed in the next section. Strong short pulses of sound are shot into the seabed sediment and ‘reflect’ anything that sends the echo back earlier than the rest. The two forms of sub-bottom profiler are single-frequency pulse (also known as ‘pingers’ and ‘boomers’) and swept-frequency pulse (‘chirp’) (Bowens 2009: 109). Using both devices ensures the best coverage and penetration of the seabed.

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Figure 3. The setup of a multibeam sonar survey (image created by author)

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Figure 4. The results from a multibeam sonar survey, red are closer to the surface while blue is deeper (after Cox 2012)

Magnetometers:

Magnetometers measure the strength of the earth’s magnetic field and are used to detect the presence of ferrous material (iron) by the variations they cause in said field (Bowens 2009:111). This may include both man-made objects, like the cannon in Figure 5, or geological formations. They are usually deployed in a towing array to inhibit interference from the tow boat and the data they collect are plotted (or ‘contoured’) according to varying magnetic intensities (Figure 6).

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Figure 5. The setup of an underwater magnetometer survey (image created by author)

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Figure 6. The results of a magnetometer survey (Spirek 2001: Figure 2)

Submersibles:

Submersibles for archaeological surveying come in three forms: remotely operated vehicles (ROVs), autonomous underwater vehicles (AUVs), and manned submersibles. They can perform many tasks including visual assessments or searches, and photography, thereby negating the need for divers in the water (Bowens 2009:112). ROVs are piloted from the boat and can be outfitted with an array of data-collection devices like acoustic systems or video recorders (Figure 6). AUVs can be outfitted with these devices as well, but are not piloted nor are they attached to a vessel. Manned submersibles can complete the same aforementioned tasks but with an on-board pilot for more control and precision. manned submersibles fall into three categories; commercial, tourism, and research (Kohnen 2005:121).  (Figure 7).

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Figure 6. The setup of a remotely operated vehicle (ROV) (image created by author)

 Carolyn sub

Figure 7. The Institute of Nautical Archaeology’s (INA) manned submersible Carolyn in operation in the Aegean Sea (Kohnen 2011)

We know that the goal is to try to leave the archaeological site in its original context, as well as the best non-invasive ways to survey it, but why go through all the trouble? As archaeology is an ever changing field that progresses in parallel with new technology, it is undeniable that the information we gather ten years from now will be of a higher quality and degree of accuracy than what we collect today. This means that whatever we choose not to disturb today may never need to be disturbed in the future. Yet we must still yield a high degree of archaeological data,and therefore non-invasive survey methods, like those mentioned above, are an investment for our future AND our past.

References

Bowens, Amanda (editor)

2009 Underwater Archaeology: The NAS Guide to Principles and Practice. 2nd ed. Blackwell Publishing, West Sussex.

Cox, Marijke

2012 Building an Estuary Airport Close to Sunken Warship Branded ‘Bonkers’. Electronic document, http://www.kentnews.co.uk/news/building_an_estuary_airport_close_to_sunken_warship_branded_bonkers_1_1416102, accessed 30/08/2013.

Kainic, Pascal

2012 Search and Recovery Side Scan Sonar. Electronic document, http://www.yousaytoo.com/search-and-recovery-side-scan-sonar/1924787#:image:2729577, accessed 30/08/2013.

Kohnen, William

2005 Manned research submersibles: State of technology 2004/2005. Marine Technology Society Journal, 39(3): 121-126.

Kohnen, William

2011 Carolyn‘s 10-year Aegean voyage for INA. Electronic document, http://nauticalarch.org/news_events/news_events_archives/prior_to_2011/carolyn_takes_a_break/, accessed 01/09/2013.

Schott, Becky K.

2013 The Wrecks of Thunder Bay: A Photo Essay. Electronic document, http://www.alertdiver.com/m/?a=art&id=780, accessed 30/08/2013.

Spirek, James

2001 Port Royal Sound Survey: Search Begins for Le Prince. Legacy, 6(2):28-30.

UNESCO 2001 Convention for protection of underwater cultural heritage.