Well, I’ve had a successful initial go at using GMT to create a map of where my research will take place. It took a couple of days but I finally got the map projections right and the positions for all the labels just right. The cool thing is I’ve already got something that will go on my poster (after I put the maps together in another program)!
My data for the summer will come from 3 sources: a permanent installation of seismometers, an array of geophones and a sharp, pointed stick (I know, high tech, right?).
Why a stick?
The use of the stick is simple: poke the ground and see how far down it goes. When the pole gets stuck, you’ve hit permafrost.
About the permanent stations: H/V Experimental technique
There are currently 7 stations installed with Nanometrics Trillium Posthole Seismometers and Reftek-130 digitizers. These stations are set up in a triangular array with one station in the center (see one of the fancy maps I created in GMT). They’ve been taking data from the ambient noise since October.
I’ll be using a technique with this data called H/V Spectral Ratio Analysis to track changes in the depth of the active layer (see my profile). While H/V has been used to categorize the vulnerability of an area to shaking caused by an earthquake, using H/V for the depth of permafrost is a new technique.
Geophones: The ReMi Method
In addition to the data from the permanent stations, I’ll be collecting new data with the geophones. This will be an active survey that will, in addition to the pole measurements, act as ground truth for the experimental H/V technique.
A FEW TECHNICAL DETAILS
Normally geophones are used in refraction surveys but I’ll be using the ReMi (Refraction Microtremor) method. The data will be recorded in the time domain and then transferred to the p-tau domain. The p-tau domain is slowness (inverse of velocity) vs what is essentially time. In this domain noise as well as the reflected and refracted waves get stacked out so that just the direct waves are left. This data is then transferred to the p-frequency domain via Fast Fourier Transform (FFT) and then inverted until a velocity model is obtained that matches the data.
Tools of the Trade
I’ve already mentioned that I’m using GMT to create nice maps for visuals for my poster. I’ll also be using GEOPSY for the H/V analysis along with Matlab. I don’t know all the details just yet.
So the point off all this is try to see if this new technique will work for measuring depth to permafrost. If it works it will be a cost efficient way to map and track the permafrost layer through time. There are other geophysical techniques to map it, but these require multiple site visits if you want to track it through time. H/V could work on a daily basis and especially could be used as the US Array is installed in Alaska beginning this year.
You must be logged in to post a comment.