Inspired by caving friend Nathan Williams photos of this technique I decided to try to duplicate his results and then write some great software. The idea is to make profiles of cave tunnels known as cross sections very easily and accurately. Cross sections are commonly sketched by a cave mapper by eye with a very rough scale. Sometimes the passage is measured in height and width with a tape.
Here we use a motorized laser level and a DSLR camera to try to construct profiles. After seeing Nathan's photos I got the laser level from Harbor Freight Tools (~$60) and used my Nikon D40X in a local Arkansas cave.
Today I just did a quick test about 100 ft. into the passage. Below is a picture looking toward the level with flash so the tunnel profile can be seen. Then I did a 20 second exposure with the level running and all lights off. There was a small amount of light from the entrance, but negligible.
I then read the image into python, remove tripod reflections by subtracting the average of the blue and green channels from the red and then inverting the resulting monochrome image. The result is seen below:
The big thing I need is the software to then produce a set of points that describe the profile so I can implement routines to compute area and make a pseudo 3-D model of the cave by stacking many closely spaced profiles. I also tested the scale of the image by counting how many pixels wide the level appears and then determined the pixels/cm count to get the size of the tunnel. This process will be improved and automated as the software develops.
I'm open to suggestions from cavers and numerical methods folks. I have a contouring algorithm (Moore-Neighbor Tracing) coded, but it doesn't handle the breaks in the profile. Any ideas on making it continuous and possibly minor smoothing? I plan to build a "T" shape device with 4 dim LEDs to provide a larger scale target.
Those of you who live in the northwest Arkansas area were recently 'rocked' by two earthquakes. These were both small being under 3.0 in magnitude. I hesitate to give out exact magnitudes because these are determined by looking at at data from several seismic stations (as is the location and depth). You can imagine that there are relatively few earthquakes in this part of the state, so the seismic network is lacking.
So, why did these earthquakes occur? Have they happened before? Well, in short we don't know and not in recorded history. Yes there have been movements on faults that have been felt in Benton county before, but these two events are the only earthquakes originating in the county we know of. The first map shows recorded earthquakes since 1974 to the present (not including these two). You will observe almost none within 200km of the county.
The initial hypothesis by a University of Arkansas professor, given the shallow depths initially calculated, was that these were the results of cave collapses. Karst topography (what we have in NWA) consists of layers of caves carved into the subsurface. It is not uncommon for these to collapse and the UofA professor cited the loud noises heard as support for the collapse theory. Though the theory is nice, but my initial thoughts were 'where are the sinkholes'? None have been discovered, not that they would be like the recent sinkhole in Guatemala (picture from Guatemalan Govt.). I had settled on the personal hypothesis of these being slips of old faults. There are many of these faults throughout the area, but they have been inactive for a very long time (even in geologic time). This slipping can be stress built up over long periods of time, introduction of fluids, or loading of the land. I even thought about a fault running under the railroad bridge in Decatur which does not break, but creeps slowly. That bridge has been rebuilt several times due to fault movement.
I heard that people were rushing to buy earthquake insurance and couldn't help but to (almost literally) fall out of my chair laughing. These are not indications of further earthquakes. We don't know what exactly is happening, but that is mostly because the area is poorly mapped and we have little geophysical data available about it. Could more earthquakes happen? Absolutely. Will they? We can't say.
Next we must discuss how earthquakes are rated. We use the Richter scale, which is a logarithmic scale. This means that each step is an order of magnitude more energy. A 3.0 is ten times larger than a 2.0, so a 4.0 is 100 times larger than a 2.0. News media commonly misses this and says it was 'twice' as large which is simple ignorance. We also know it takes a 4.0+ to start breaking glass and doing serious damage. I have only head reports of a few cracked driveways and see no reason to expect anymore. Finally there is no 'trend' with only two data points to support earthquakes getting stronger.