Hi all, it has been a while since I've blogged, so I'll just start from the beginning.  It has been proposed that Fallen Leaf Lake (2km south of Lake Tahoe), due to its smaller size and much larger watershed-to-surface area ratio as compared to Lake Tahoe, provides a unique microcosm of the Tahoe Basin for paleoclimate studies.  In particular, several pine trees have been found still standing and rooted at approximately 36m deep.  Although drownings due to earthquakes have been posited as a likely reason for the existence of these trees, paleoseismology has indicated that the last earthquake on the Lake Tahoe-Dollar Point Fault (which runs through the southern quarter of FLL) last ruptured ~4,300 BP. Previous dendrochronology of these trees (difficult due to the short time that divers can be at depth and the practicality of finding waterlogged trees which are invisible to standard sonar systems) has indicated that the ~200 year old pines were killed by rising water levels at approximately 1250 AD.  This suggests that the coincident Medieval Warm Period/Medieval Climatic Anomaly may have initiated a period of intense drought in the Sierras which brought the water level of Fallen Leaf lake down below the level of the trees, for upwards of 200 years, before drowning them when precipitation returned to more 'normal' levels.  These trees provide hugely important climate information, but the cost in time and manpower necessary to find them is prohibitive.  Our research posited that the higher-frequencies of the sidescan sonar system would be able to image both these trees and the paleoshorelines which would corroborate the existence of still-rooted trees.  By creating a map of downed trees, standing trees, and stumps we will be able to focus any future explorations of these climate proxies towards the most likely candidates, drastically cutting the time and expense of searching for these proxies.  In addition, this system could be used to search for downed trees in the numerous other alpine lakes where submerged trees have been found.  

On to the nitty-gritty:  After a week and change in the Tahoe basin I came back to San Diego to buckle down and process our sidescan and Lidar data.  The sidescan itself has taken me over two weeks to process, showing me in the process an interesting part of the human mind: after looking at the same image for a protracted period of time, it is difficult for your brain to acknowledge minor tweaks and changes, it all looks the same.  This is, of course, where Andy's exhortation for us to take breaks comes in.  Using a program called xsonar, the basic process was to choose which of the two dual frequencies' data to use, 100 or 400kHz (low freq. shows the topography much clearer, but the high gives us the resolution to image small reflectors such as trees, rocks, and a boat-wreck), demultiplex the raw sonar data, merge that with the gps data, pick the seabed arrivals, and then correct for range, swath width, etc.  After finding the right values of every option, it is a relatively simple(r) matter to convert the raster files to .raw, then mosaic them together in photoshop.

After finalizing the 500mb tiff (compressed!), I have also begun to process a visualization of the Lake Tahoe-Dollar Point fault scarp situated on the southern end of the lateral moraine that makes up the eastern wall of our study area, Fallen Leaf Lake.  Unfortunately, the scarp is in the middle of a forest, and the problem with a high-res 3d 'camera' is that it will pick out every part of every tree.  Therefore, in addition to poor gps data due to said trees, you have to hand-filter all of the vegetation out.  (the provided filters with the beta of the program can pick some of the topography for you, but they also cut out 80% of the ground, as well.)  It will be interesting to see how much we will be able to get out of it.  This particular lidar system is usually used to measure coastal erosion: the open sky allows gps precision, and the only thing you need to filter out are the occasional people coming up to ask, "is that the beacon for a ufo?'

In addition I have learned how to process CHIRP data in SIOSEIS, and will be adding a few lines of this data to the sidescan for both the AGU poster and for a presentation which I will be giving at SIO in 2 weeks.  

Wow, I can't believe that the end of the summer is nearly upon us.  I'm leaving on the 20th of August, and though it seemed at first like I had so much time, I am already beginning to feel the crunch.  Then again, as I have been told by people who have been doing this for a few years, "relax, at least you have data at this point."

Fallen Leaf Lake.  High frequency passes around the circumference, with low frequency lines in the middle.  The reason for this is that the HF cannot distinguish the seafloor in the 120m deep silty lakefloor, but the LF can, in addition to prominently showing the coarser-grained sediments of the moraine edges.   

 Western shore, looking south towards the glacial valley

Cheers!