How proxy is a proxy? - Part II

So, I believe that last time I threatened to, for once, have a series of evolving posts that gradually elucidate on a single topic. Henceforth, here is round two.

As a reminder, last time I posed the question:

How accurate are experiments where non-human, proxy cadavers are used?

In particular, I'm considering experiments where we want to use existing acoustic technology to image submerged cadavers.

To start with, lets think about precisely what we're going to be imaging with the acoustic returns. Sounds simple, doesn't it?! Sadly, as with almost everything to do with acoustics, simple questions tend to result in complex answers.

In order to image something using sound, it needs to present a measurable change in acoustic impedance (i.e., basically a measure of the strength to which the material resists the passage the sound wave) to it's immediate surroundings. The human body is generally considered to consist of 60 -70 % water, suggesting that a cadaver sitting on the seabed, lakebed, or riverbed will tend not to offer as strong an acoustic target as, say, the sediments on which it is resting because it closer resembles the water around it than the sediments do.

However, the acoustic backscatter from a target (the sound which travels from the source to the object and is reflected back towards the source again) is the combined response of two processes:

1. Surface scattering: the energy reflected back by the water/cadaver interface.

2. Volume scattering: the energy reflected back from within the target.

Of these, the volume scattering is the one we're particularly interested in. As I said earlier, the surface scattering will not be very strong for a cadaver. The volume scattering, on the other hand, will be. This is because, during decomposition, gas builds up within the tissue and internal cavities of the cadaver. This gas will present a very strong change in acoustic impedance.

So, when we want to acoustically image a cadaver, we would expect the dominant acoustic signature to be from the build up interstitial gas as a result of decomposition.

How proxy is a proxy? - Part I

As I've mentioned in a blog a couple of months ago, Piggy hide and seek, I've recently been looking into the idea of using shallow water marine geophysical techniques in helping Law Enforcement Agencies conduct underwater body searches. Going through the frantic research process involved with writing any kind of research proposal, a particularly interesting thought occurred to me, namely:

How accurate are taphonomic experiments where non-human, proxy cadavers are used?

In certain states of the USA it is possible to use human cadavers when people have donated their body to science, or their body remains unclaimed. This has enabled researchers at the University of Tennessee's Anthropological Research Centre, in Knoxville, to setup their 'Body Farm', where human cadavers are placed in a variety of environmental conditions and their decay monitored over a period of days, weeks, months, and sometimes years. As a result of the ground-breaking research conducted at Tennessee, other body farms have now sprung up in Western Carolina University and Texas State University, although on considerably smaller scales.

The work undertaken by these institutions has been truly astonishing, advancing forensic entomology immensely. Without this work there must be thousands, perhaps hundreds of thousands, of homicides the world over that would never have been solved. However, outside of these specific locations, the vast majority of people trying to undertake research in this, or related, fields have to rely of using proxy cadavers, normally domestic pigs. This leads to the question of how accurately a pig cadaver can imitate a human one? Taphonomically they have been shown to be very, very similar; their skin is close enough to ours for use in skin grafts for burns victims, whilst, also being omnivores, they have much the same gut bacteria, leading to a decomposition progression that very closely mimics our own.

This is all well and good for taphonomic and forensic entomology studies of beetle or fly larvae colonization, etc, but for our purposes, where we want to image the acoustic properties of the cadavers, can we truly say the same?

This is something I'm, hopefully, going to explore in the next few blogs by discussing the physics behind the variety of acoustic profilers that can be used. In this way, it should be possible to see where the potential differences between the different cadavers could result in different observations.

The Obama-wagon...quick, jump!

Firstly, and probably most importantly, congratulations to Senator Obama on his resounding victory Tuesday evening. Secondly, congratulations America!

It was quite a surreal evening for me. I attended my first ever 'Election Party' (I'm not sure what it says about me that it was for an American Election rather than a British one, but...). Throughout the evening there was a lot of talk about how, if Obama got in, people would feel 'proud to be American again', which says a lot for the damage our friend George W. has done to international relations on all levels. However, the best comment of the evening, I think, came when someone said:

'We've really dodged a bullet by avoiding electing Palin as Vice President.'

I couldn't agree more, it's hard to imagine what could be worse that having the gun-toting creationist loitering in the wings, just in case something happened to McCain. I have to admit, though, there is a part of me that feels sorry for Senator McCain. The guy is not a bad politician, and probably wouldn't have made a particularly bad President, but suffered because somehow his party decided Sarah Palin would make a great running-mate!

However, what has grabbed my attention most about the immediate outfall from the election result, is the level of childishly cynical hero worship that has swept around the political world. Nothing describes this better than the pitiful exchange between Gordon Brown and David Cameron in the House of Commons yesterday.

What is more pathetic than two grown men fighting it out to be associated with Senator Obama's victory? Why is it necessary - is the result of the US election really going to influence British voters in their choice of who they will vote for in our forthcoming election? Had the elections happened the other way around (which they might well have), do they really think that Obama and McCain would have reciprocated and been fighting tooth and nail as to whose campaign closest resembled that of the British PM? I think not!

After all the comments on the blog-o-sphere and articles in newspapers and news websites in the UK criticizing American politics over last few years, on the evidence of this we should perhaps be looking a little closer to home!

E-Day is nigh

Sadly, there's been little activity on this blog in recent weeks. I assure you, it's not that I don't have anything to say, simply too much to do. As anyone who's ever done a PhD will tell you, as the deadline looms everything else in life gets pushed aside as the behemoth of a thesis grows (you should see the amount of facial hair I now have)!

Today is an interesting day. As I'm sure everyone is aware, it's US Election Day! Over recent weeks the newspaper, together with quite a few of my favorite blogs, have been discussing this topic ad infinitum:

Cosmic Variance
Lab Lemming
Shores of the Dirac Sea
Michael Berube

Plus, probably a million more that I don't read! So, I'm not going to say anything else on the subject, other that try reading some of the above if you're interested.

Oh, and, "Come on Obama!"