This week was seismic week for us here in Canon City. We carefully selected a site that crosses from sediment into basement, but the nature of the contact is unknown. It ran across the property of a nice couple who moved here from Iowa three years ago. They are interested in the geology and were more than happy to have us tramp all over with magnetic, gravity, and finally seismic gear including a larger thumper mounted on an ATV.
The first part of the week involved Cullen and I working on a mounting system to attach the thumper for the four-wheeler. Seismic surveying works on the premise that different rocks have different wave velocities as a function of the type of rock, fluid content, etc. We set out a long line of geophones (basically a vertical seismometer) and then hit the ground very hard to induce a signal. From the return of the signal we can learn a lot about what the subsurface looks like.
For some surveys we hit the ground with a sledgehammer, shoot it with a gun, or even use dynamite! In this case we tried a new device that pulls a 40kg weight up with an electric motor and then drops it. There is also a giant rubberband that accelerates the weight towards the ground. There is currently a battle going on between naming the machine the seismic thumper device or the seismic thumper and utility device. We added weights, battery mounts, and even a flashing safety light to the four-wheeler.
Before we could even use the thumper the control box failed due to a cable issue, so I had to rewire the control system (actually just a solenoid control) and mount the switch in a box on the ATV. It was a midnight patch up, but it worked well all day!
The survey was laid out on Wednesday. The line was almost 750m long, then we even rolled it forward! Geophones were placed every 10m and the thumper was shot at each geophone three times to 'stack' the data (this helps us reduce random noise). We don't have any images yet, but tomorrow we begin processing. The line took a day to layout, a day to shoot, and tomorrow morning to roll up. Several long days for us here. I also put together a quick video of the thumper shooting.
On a side note, we also took a great hike and field trip in the past week, so I've added a few photos of the Collegiate Peaks, and Tunnel Drive Trail.