# In search of source levels

I love talking about my research with people who are both interested, and way smarter than me (most people are smarter than me; only a tiny subset of those are interested in my research).  It’s really a great combination, because I get to throw all of my harebrained ideas out there, and have them honestly (but not cruelly) critiqued.  I met with Kate S. today, who I’m lucky enough to have on my committee.  She gave great feedback, and also some great ideas.  One of the things she stressed to me was the importance of estimating source levels.  If we know the source levels, we can make an estimate of the detection range for a given bathymetry, sediment type, sound speed profile, surface roughness, etc.  And then in our case, there is the additional problem of understanding the transfer function relating the accelerations measured by the seismometers, and the sound level of the acoustic signal (ie. dB relative to 1uPa @ 1m, say).

My simplistic understanding is that the seismometer is measuring accelerations in three perpendicular directions, two horizontal and one vertical.  Those measured accelerations are converted to displacements using some separate transfer function (measured during instrument calibration, perhaps?).  Acoustic waves from fin whale calls hit the seafloor near the seismometer.  The seafloor is displaced, and that displacement is measured.  So how does it relate back to sound level?

Since I have call location information from Dax’s work, I’ve got a really good place to start looking.  The slant range from the call to each receiver can be measured, and if we restrict ourselves to only the direct path arrivals, then the problem is simplified significantly (no need to worry about losses and phase shifts/flips at the surface or bottom boundaries).  Basically, if you measure the sound level at a certain location, and you know how far you are from the source, you can calculate how loud the source must have been (how loud the fin whale is honking).

So I should be ready to go, right?  … Well, not really.  I actually have a lot of details and coding to work through before this will even begin to take shape.  I will keep you updated!  (By the way, I congratulate you if you’ve managed to read to the end of this post without skipping on to the daily sketches).