How do find cenotes? (part 4)

'X-ray' Vision to find cenotes!

Over the last few years I have been using some interesting technology to help locate cenotes.  Imagine that you could have 'x-ray vision' to see exactly where a cenote is, when everything indicates that there isn't one there.  

Topo maps show very little information about cenote locations, aerial photographs and hi-resolution digital photography show lots of detail but also contain a lot of conflicting information.  It is through the interpretation of satellite imagery, also known as remote sensing, that we can see things we are unable to see with the naked eye.

 

What is Remote Sensing?

I first heard about remote sensing from the man who would later become my advisor for my masters degree at the University of New Hampshire, Dr. Barry Rock.  Remote sensing in its most simplistic definition is the study of something without actually coming into contact with it.  Our own senses of sight, sound, and smell are amazing remote sensors.  Think of how much information we take in from them. 

Landsat 5 Thematic Mapper. Launched in 1984 it was decommissioned in 2012.

On a much larger scale, remote sensing can be defined as the study of planet earth from outer space.  Images taken from 500 miles above our planet allow us to see broad views of landscapes and change over time.  Scientists like Dr. Rock have dedicated their careers to teasing information out of satellite images.  Satellite data from platforms like the Landsat 5 Thematic Mapper are available for free online

Some important concepts to understand when dealing with satellite data are: spatial resolution, extent, spectral resolution and temporal resolution

 

  • Spatial resolution is the size of the smallest pixel on an image.  In the example of Landsat 5, each pixel measures 30 meters on a side.  This is very coarse when compared to imagery and aerial photos you may be used to seeing on Google Earth. The advantage is that images from Landsat cover very large areas.
  • Extent is the size of an area that a single scene covers. In the case of Landsat 5 the extent of each image is 115 km on each side. 
  • Spectral resolution is where the real power of satellites like Landsat 5 comes into play.  Sensors are designed to record and detect electromagnetic (EM) radiation along different portions of the electromagnetic spectrum and at differing resolutions or bandwidths. Each area along the EM spectrum is able to impart information about conditions on Earth’s surface.  Images from Landsat 5 allow us to see into the infrared which we cannot see with the naked eye.   To find cenotes I am using a combination of spectral bands that show moisture content in vegetation. 
  • Finally, temporal resolution is how often the sensor passes over the same point on planet earth. It is the temporal resolution of a sensor that allows the powerful application of change detection to be used allowing impacts on the earth’s surface to be quantified.

 

Using remote Sensing to find cenotes...

The Yucatan Peninsula has very pronounced wet and dry seasons.  Based on this, my hypothesis is very simple.  If I could find a good satellite image of this region from the dry season then we could hypothesize that any vegetation that surrounds cenotes would have higher moisture content than the surrounding drier jungle vegetation and therefore show up very differently on the image.  After switching on the correct band combination bright green areas appear on the image indicating potential cenotes.

Landsat Image highlighting moisture content in vegetation.  Brighter green areas are potential cenotes.

Proving our theory...

There is only one way we can prove that our hypothesis is valid, and that is to hike out to the spot on the image and see what is there.  For me this is one of the coolest parts of exploration here.  After all, there is nothing like being right especially after the rest of the dive team thinks you have lost your marbles and are leading them on a wild goose chase! 

I am happy to say that we have identified 3 cenotes in the Ox Bel Ha cave system that we otherwise may never have found that have contributed significantly to the exploration of the cave.  We still have many more potential cenotes to hike out and find.  Stay tuned.   I f*&%ing love science!

 Finding Cenote Marina with Pepe Sanchez 2013

Finding Cenote Marina with Pepe Sanchez 2013