Friday, 14 November 2008

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.


pacalaga said...

Not to be gross, but how long does the body stay intact enough to trap the gas? I'd guess that has as much to do with what else is in the water as it does with regular decomp rates. I'd guess the volume scattering would be less in areas where the little fishes are snacking on your subjects.

Mark said...

True. That is a major reason why we're planning on doing what we're doing! :)

Very little is known about the way bodies decompose underwater, and the effect crawly nibbly things have on this. It's something I'll talk about in a later post, but one of the major goals is to quantify when and for how long the cadaver can be imaged using acoustics.