Reventador, located in northern Ecuador, is an active volcano. In 2002 it erupted with such massive force that it blanketed Quito, Ecuador's capital city 60 miles to the west, with a layer of ash so thick the airport had to shut down.
Today it blows its stack with lesser force but with great frequency. And just as storm chasers want to be where the cows are flying around, volcanologists want to be as close as they can get to 11,500-foot peaks that spew grapefruit-sized molten rocks several times a day.
What scientists want even more than taking in the sights and sounds, however, is gathering precise records of seismic activity and studying the data, which is where Matt Welsh, assistant professor of computer science at Harvard University, and his wireless sensor mesh network come in.
Previously, seismologists from the University of New Hampshire (UNH) and University of North Carolina (UNC) collected data by lugging bulky, heavy sensor stations up the mountain - each one containing a car battery for power plus sensors, cables, and data logging and data storage equipment.
Every few days someone had to trek several hours back up the mountain to collect the microdrives. This put severe limits on the number of sensor stations deployed and consequently on the amount of data collected. The setup also had other limitations - for example, there was no way to tell if one of the stations had stopped functioning.
Welsh had a better idea. He had a 190-node wireless sensor mesh running at Harvard that was designed to detect changes in a building's temperature and humidity. Now it was time to bring his mesh network out of the lab and into the line of fire.
It's a jungle out there
For the Reventador expedition, Welsh created a tiny, low-power sensor station that runs on ordinary D-cell batteries and fits into a plastic case the size of a lunchbox. It sends seismic and sonic data around the clock to a base station that collects data in real time. Total cost: US$400 per node.
In August 2005, the seismologists from UNH and UNC and the computer scientists from Harvard flew to Quito, drove three hours, then bushwhacked through the jungle for several hours to the base of the mountain, where they camped. At 5 a.m., Reventador issued a loud wake-up call.
"The top blew,'' Welsh says, "and I freaked out.''
Welsh was far enough away from the top of the volcano so that he wasn't in danger, but just to be on the safe side he sent his grad students up the mountain while he manned the base station. His team deployed a total of 16 nodes 650 to 1,300 feet apart, in an ad hoc mesh network that spanned almost 2 miles up one side of the volcano.
Each node, which consisted of a Tmote Sky sensor from Moteiv, an interface board and a battery holder, was covered with rocks to anchor it and protect it from the elements. (The area had been defoliated in the 2002 eruption.)
The seismic sensors were buried nearby, connected to the nodes by USB, and the sensors that measure infrasonic waves were mounted on PVC tubing Welsh used to elevate the high-gain antennas off the ground.
The network used IEEE 802.15.4 low data-rate radios and connected to a FreeWave radio modem at the base of the mountain that backhauled the data to a laptop about 2.5 miles away in the tiny hotel where the team was staying. (Welsh says he decided against using 802.11 because it would have required too much power.)