I'm back from the darn cottage weekend. Oh well, I can always say it was a good work weekend, provided I could do things between the rain showers! Thank god for fast-drying outdoor latex paint!
I'm recalling another major project which, although it wasn't an absolute total disaster, it wasn't quite what I expected. We still had money for geophysics, and we were flush from the success of our marine geophysics find. There were some areas on land that the marine surveys didn't cover, so we decided to go for a land seismic (vibroseis) survey. This was in cooperation with several government agencies, so our good 'industrial' money was well amplified.
The details might be buried somewhere in Lithoprobe, but I can't find anything in my papers. We got a survey crew from Alberta with the giant vibroseis trucks (Dancing Elephants). These things are absolutely incredible, and when they start up sending a signal, you feel like you are in an earthquake!
Now, for my favourite blogger, I will explain things in very simple terms. The big trucks have huge hydraulic pads which they place on the ground. They are all linked together, and then they vibrate the pads. Not just up and down, but with a very special little 'song'. Elephants do the same thing with their infrasound.
This song penetrates down into the crust, perhaps up to 30 km deep, down to the Moho (where all crustal structure dissolves). On the way down, the seismic waves start reflecting from layers and bounce back to the top, where they are picked up by a very long string of microphones (geophones). The computers automagically correlate the frequencies of the song, and come up with the depth of the reflector, which is plotted as those ugly reflection plots I showed once.
The nice thing about vibroseis is that we could run the surveys along roads without any trouble (or so we thought....). Turns out that Alberta roads were made for this in the first place, and they have huge compacted shoulders. Our country roads are crap! Also people are a big tetchy about having earthquakes generated near their houses! We were just helping them get ready for the big one!
We had to pay a lot of damages, and I wasn't too happy about the results. Because the roads had a nasty habit of not going exactly straight over the important zones, we had to do a lot of dog-legs which messed up the results. As well, we got perfect reflections off the good rock, but nothing back from the bad rock (where the earthquakes were!). In the end, the results just confirmed what we saw in the marine section, and it wasn't until recently that I figured it all out.
Still, it was a magnificent experience!