Just wondering if anyone can offer some thoughts: what geophysics option(s) would be best in trying to determine timber structure (ie. a ship's hull) buried beneath sand, say, to a depth of 5 metres? (Allowing also for the presence of a water table.) What option would be useful to determine or narrowing down the location of such a structure on a given stretch of coast?
Probably ground mapping radar would be the best bet as it works well in sandy conditions and will penetrate to 10m or so. Waterlogged conditions can cause a few issues from what a geophysics mate tells me. Conventional seismic charges will also work well in such conditions as sand is an excellent medium for accoustic propogation.
Praestat tacere et stultus haberi quam edicere et omne dubium removere.
Hi, Really is a hard question this one. I know that on sites that I have worked on many years on peat and waterlogged conditions we use most of them but that is from a pro unit. Personally sometimes results do not show up at all no matter you do. It down to how low down in the ground the feature is. As for radar mentioned above. Has a huge disadvantage as it best on very dry deposit so I would say use this. The Mary Rose team use this type of equipment.
It down to trail and error. What are you going to do with your results? As any such results must be reported to your County Archaeologist, and NMR for future reference. Also, are you going to interpret them yourself? What is your plan when or if you find the location of the timber will be your next step?
Sorry for all the questions but from a pro point to further help you on your way.
Thanks for your consideration Sie. No, I'm not planning on doing any of this myself and I appreciate the requirements of local Heritage regulations. My understanding is as follows.
On the Southwest coast of Victoria, Australia, there has been an ongoing search for the wreckage of a ship (the "Mahogany Ship") that was sighted on the shore back in 1843.
Owing to the constantly shifting sands, and perhaps other human influences over time, the ship has disappeared from view and its remains are believed to be buried beneath the sand on this particular stretch of coast.
Over recent years there have been a number of geophysical surveys conducted to locate the remains, but none have been successful to date. Although there was very interesting ferro-magnetic data collected back in 1981.
It has been asserted by historians that the ship could have been a Portuguese Caravel, predating Cook's voyage to Australia, I understand, by a century or more.
It is certainly a fascinating piece of local lore which continues to attract different teams searching for the physical evidence.
My question actually seeks to understand the processes better and the difficulties faced by those who have been searching. Why is it that our technology been unable to pinpoint any remains? What have been the shortcomings of different methods and how has the local environment interfered with data collection?
John Gater's "slick" geophysics makes it all look so easy (and I'm sure it isn't!); the lynchpin that gives the "green light" for TT's further excavation of a site. But our local mystery continues to elude us. I hope this clarifies my question for you. If you've got any thoughts I'd be very interested.
Thanks very much to Guttus too, for your reply above.
Not a particularly easy target you've picked yourself there!
In basic terms it's all down to contrast. So first you need to ask yourself is 'is there a sufficent contrast between the target and whatever it may be buried in'? And secondly, up to what depth will that contrast be detectable? Generally, with pretty much every technique, the deeper the feature, the bigger it'll need to be in order to detect it.
So, standard magnetic surveys (of the type most commonly used in archaeology - i.e. the kind of instruments you'll see on T.T.) are unlikely to differentiate between a relatively non-magnetic sand and a relatively non-magnetic ships hull. Of course, the hull may well have ferrous rivets/bolts as part of its construction and, if it were shallow (top metre or so), you may detect an increased area of magnetic noise. But if it were this shallow, there'd probably be bloody great lumps of it sticking out. This makes narrowing down positions somewhat easier. Anything over 2 metres and you're gonna be hard pushed to notice the effect of small ferrous objects.
As for resistance, there is again a question of contrast. For the wood to be in a sufficiently preserved state to detect with anything, it'll probably be waterlogged. Waterlogged wood and waterlogged sand aren't going to provide much of a contrast, ignoring the fact that a res survey in waterlogged conditions is darn near impossible. All the elastictrickery tends to just run away.... Also there's the depth issue again. At 5m, resolution is not going to be great, you're verging on geological techniques to detect to that depth and then the kit is designed to be looking for prett big targets, with sizeable contrasts, not the subtle features associated with archaeology of this nature.
Ground Penetrating Radar. Great in sand. Generally. But wet sand? Wet salty sand? That's not good. Salt water is a great conductor and will attenuate (dissipate) the signal very quickly. Even if you were surveying over sand that is relatively dry near-surface, it won't be long in a fore-shore scenario before you're down to wetness. And inevitably it's gonna be rich in salts. You can minimise the attenuation by using lower frequencies but the resolution drops dramatically with it. So it's a bit of a long shot. Also a bit slow if you've got a big stretch of land to try and cover.
I've seen surveys over ancient trackways preserved in peat. Standard archaeological res (twin-probe) and mag (fluxgate gradiometer) weren't overly successful despite the shallow nature of the archaeology. Pretty much all down to the lack of contrast and abundance of wetness. The GPR has been relatively successful in some examples, but as I say the archaeology is generally very shallow.
I heard seismics mentioned. That's a good plan. There's a lot of Wreck work going on at the National Oceanography Centre / Southampton University here in the UK, mainly under Dr Justin Dix. They've been developing and using 3D-CHIRP systems which are very high resolution seismic systems that allow GPR-esque resolution levels. They're towed behind a boat, can cover a few hundred square metres in a go and penetrate up to 15 metres of seabed in the right conditions. If I remember rightly. But... Still only used on a handful of sites and not really a commercially available kinda system. But do look it up, I'm sure a quick google search (try "chirp archaeology Dix") will turn up some articles - it's really cool stuff.
Hope this helps. No need for hot milk before bed tonight...I feel sleep coming on already!
Man...that is just great. Thanks so much for that detailed explanation. I understand a lot better now.
You're dead right...it does seem to be a difficult target, both in locating the site of the presumed remains and the physical evidence. Particularly in light of the many searches that have taken place over recent years. The proverbial needle in a haystack.
I'm going to chase this up some more find out the current state of play with the local teams. I'll get back here if I find anything to add to your information above.
Thanks again for your time and explanation. Hope you slept well too!
A couple of other thoughts...the area is volcanic. I understand that the geology underlying the sand has made it difficult also.
The methods relying on acoustic propagation rely on reflected signals, correct? Which attenuate on the return journey, as well as on the way out, depending on the density of the material through which it is travelling?
In simple terms, igneous geology is very magnetic - so always going to be a bit of nightmare if you're attempting a magnetic survey above it as the natural response from the rock is likely to swamp the effects from archaeological-scale features. Not an impossible situation, just depends on what it is you're looking for - research in Iceland (a very large lump of magnetic rock) has suggested that you can pick out variations within the strong field that are due to anthropogenic intervention - i.e. from using the rock for building purposes.
And yes, signals will attenuate the entire time that they are propogating through a medium, so that is both on the way in, and the way out. Strong reflectors (i.e. horizons where there is a large material property contrast) will also reflect some energy back down as it passes up through it. This energy can bounce around for a while before it gets back to the receiver which produces 'multiples' or 'ringing' - basically repeated patterns of reflections that appear lower in your section because the energy has taken something of meandering route home. A similar principle is often applied to my arrival time home from the pub due to bouncing off numerous high contrast surfaces rather than taking the direct route...
I can relate to that. OK, you can feel free to call me a "propeller-head" for the next question.
Is there any technique that uses GPR, for example, in a cross-strata fashion? The configuration I've in mind is a bore sunk to a set depth with instrumentation lowered into it that sends signals horizontally to a "reader", at the same depth, at a set distance away, in a "cathode" to "anode" analogy.
If an object was buried between them, I imagine that you'd register its "shadow" - areas of exclusion - on the "reader", rather than relying on contrasts and reflected signals to the sending unit.
A grid of this configuration could build up a pretty good picture over a large distance, in theory, could it not?
This question probably also indicates that I, too, should follow an inbound path to my local pub, and enjoy the reflected journey. But in the meantime, I'll appreciate your thoughts...
Borehole geophysics is used a lot in geological / oily surveys with all manner of sensors measuring different properties. Use in archaeology is limited. I'd imagine this is largely because sinking bloody great holes around and potentially through archaeology is generally frowned upon somewhat.
It is possible to use GPR as you would from the surface, with just a single borehole and measuring the reflections. Much work has been done by a firm in Holland on single bore hole GPR where the antenna rotates as it passes through the passage and produces a 3-D volume.
Using a receiver and transmitter in seperate holes (cross-borehole or transillumination surveys) again are very rare in archaeology. It's a relatively slow methodology and quite complex in terms of interpretation, as far as I'm aware. Also, to do it effectively you'd need a knowledge of where the archaeology is to be able to sink the boreholes either side of it....
Thanks again for this...I'm glad that the ideas that run around in my rat's maze actually have names.
Given that the search, from what I understand, has not turned anything physical up as yet, it's good to know that other techniques exist, even though they may not be archaeology-friendly. From memory, I think bore holes may have been sunk in one attempt, but not as a means to conduct further surveying. So the notion may not be as unpopular as it would be in a site with higher density finds likely.
The area under consideration is a few kilometres long, along with coastal dunes, so the search may require techniques out of the ordinary (along with budgets out of the ordinary no doubt!)
Time for more research at this point and my sincere thanks for your help.
