Measuring source levels of marine mammal vocalizations is complicated, and even more so when using ocean bottom seismometers. I’m trying to look at some of these complications and sources of error and uncertainty.

One of these is the interference between the direct path arrival and the surface bounce. Because they take different paths to reach the receiver, they arrive at slightly different times. These offsets result in interference patterns – sometimes constructive and sometimes destructive.

R_1 and R_2 are the direct path and the surface bounce. D_s and D_r are the source depth and receiver depth. H is the horizontal distance between the source and the receiver.

I wrote some code to model these effects, and I’ll just start out with a little video clip that shows what I mean by constructive and destructive interference. I’m plotting the RMS (root-mean-square) amplitude of the received signal. You can see that in addition to the interference pattern, the amplitude of the input signals is decreasing – this is to account for transmission losses along the travel paths, modeled simply using a spherical spreading assumption (scaled by range).

Next, I wanted to see how this effect might manifest itself in our particular setup.  I ran the code using an approximate receiver depth of 2200 meters, a source depth varying between 0 and 100 meters, and a horizontal range of 0 to 2200 meters.  I chose the source depths based on what I think are likely depths from which a fin whale might call.  The horizontal ranges are restricted such that the incidence angles will be small(ish) – a constraint that is imposed in part to reduce ambiguity with later multipath arrivals, and in part because of the physics of converting ground motion back to an acoustic pressure level (details I won’t go into here).  The surface bounce is given a 180 phase flip (and no loss of amplitude) since the surface is treated as a perfect pressure release boundary.

Here are the results of that model:

Interference patterns for a series of horizontal ranges and source depths.

Wow, that is a lot of possible variability!  This has been just a quick little experiment, and there’s a significant possibility that I’m still doing something wonky in my code, but based on looking at examples from the literature (for example, Charif et al., 2002), it seems to be in the right ballpark.  Very interesting – this will definitely affect how I interpret my source level estimates.

There are a few options for doing this. From what I can gather, it’s not something that is easily done within Python (someone correct me if that’s wrong!). So instead, I’m creating a series of image files (.png’s), and converting to video outside Python. I looked at a couple of options and settled on ffmpeg, which seemed really simple.

I grabbed it from fink:

fink install ffmpeg

All of my image files were saved in a single folder, with names like 001.png, 002.png, … 198.png. Converting them to video format:

ffmpeg -i %03d.png -vb 1024K video.mpg

The -i flag is for input file(s) and -vb is video bitrate. Here’s the output:

The whales probably aren’t really trying to talk to us.  But there is a good chance that they use those low frequency sounds to talk to each other.  And being able to get their messages to each other is probably very important for their livelihood – they might be telling each other about a delicious krill buffet.  Or maybe they need to attract mates with a whale-themed pick-up line.  Either way, it’s kind of bad if the noise in the ocean is so loud that they can’t hear each other.

found at http://abbotlab.wordpress.com/2011/04/12/the-new-depths-of-noise-pollution/

HAPPY BIRTHDAY, DAX!

This is how I’d like to celebrate.  Poutine!!

from http://www.heartsf.com/2011/07/27/poutine-poutine/

So whales probably do have conversations with each other, explaining life’s mysteries. Right? No? Okay, fine. But while we’re on the topic of unlikely depictions, check out this artistic rendering of a whale and her calf by Conrad Gesner:

by Conrad Gesner, published in 1560 in "Icones Animalium" and more recently in "Monsters of the Sea" by Richard Ellis. Found on the internets at http://www.strangescience.net/stsea2.htm

Okay, just so you know – this is just a silly drawing, and doesn’t actually represent anything that happens in reality.  OBSs don’t look like that.  And fin whales probably wouldn’t care if they saw one anyway.

This is what a real OBS looks like:

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Or, something that kind of resembles a fin whale.  Why are they so hard to draw?

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This drawing (and whatever follow-on drawings that may go with it) is for Vera, who loves all animals, large and small.  Even big, scary spiders!

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To use line wrapping where lines are split on the spaces between words (instead of splitting at the screen width regardless of where you are in a word) is much better on the eyes.  To do that, it’s:

cmd-M Visual-Line-Mode

Hey, presto!

And if you don’t want to have to type that in every time, just put the following line in your .emacs file:

(global-visual-line-mode 1) ; 1 for on, 0 for off.

AND… because I love to see what Google Images will kick back, the exact search terms “line wrapping in emacs 23″ gave me this:

To a certain friend of mine (you know who you are):  I tried using Bing, but there was nothing nearly this exciting or totally unrelated as this.  Sorry.  Next time!

Dax was looking for a cartoon about geophysicists.  And I was looking for a way to avoid doing actual work this evening.

This one was made using the Intuos tablet and Sketchbook Pro (All Autodesk products are FREE for students!  How cool is that??)  In case you’re curious, this took me about 2 hours.