Cave mapping with Raspberry Shake

Accurate maps of intricate cave systems help improve the safety of intrepid divers. In issue 150 of The MagPi, Rosie Hattersley hears about Raspberry Shake’s contribution.

Richard Wylde describes himself as “a sort of physicist and engineer living between the business and academic worlds” whose passion for cave diving is “closer to an obsession than a hobby”. He is co-founder of Terahertz, an advanced engineering company which, among other impressive achievements, developed remote sensing instruments for the European Space Agency’s EarthCARE mission. Richard is also one of several experienced cave explorers involved in mapping the subterranean network of cenotes [sinkholes] in Yucatan, Mexico. “The caves are stunningly beautiful and [mapping them] is technically difficult,” he says. “A lot of effort goes into staying alive.” Acoustic and magnetic mapping can help plot the location and direction of these unexplored passageways, improving safety for all who visit them — an endeavour made more robust using Raspberry Shake, a Raspberry Pi-based device more commonly used to detect earthquakes. 

Cave measurements are made manually underwater using a compass and tape measure

The maps are also useful for dive guides keen to show off the speleothems (mineral deposits such as stalactites and stalagmites), and for developers to know whether building on a particular area is possible and permissible. Their dives also reveal the effects of developments such as golf courses, which are built by clearing jungles, use nitrates to maintain their greens, and may also be drawing water from the aquifers. 

Distinguished company 

Richard often dives with renowned cave explorer Fred Devos in Mexico’s Quintana Roo region, which has no overground rivers. Mapping its subterranean cave network is “incredibly dangerous and physically challenging”. 

Team Raspberry Shake’s kit, including a Raspberry Shake acoustic seismograph, oxygen tanks, and maps of remote and barely accessible stretches of cave systems

Exploring the caves involves following taut lines of string with knots every ten feet to mark the way, just like Theseus in the Greek myth. Richard mentions the trust and focus needed to accurately read a compass and count out distances travelled ten feet at a time based on how many knots you’ve passed. Visibility and human physical resilience are all factors too — if you’re exhausted from a lengthy dive, you probably aren’t noticing arrows or counting knots accurately. “It’s milk of magnesia down there when the bubbles hit the ceiling.”  

Richard explains the process: “We mark the depth, the distance and the azimuth, and the angle to the next station” — often simply where the line is wrapped around a rock. Painted arrows help ensure divers don’t get lost, but some caves have more than one entrance, or arrows pointing in more than one direction. 

Richard Wylde, Fred Devos, and colleagues published a booklet mapping the cave system at Actun Koh

The maps are written on specially printed paper and include geographical features such as cave openings, changes in cave and water depth, and height. Relating this information to the outside world requires a way of referring it to the surface and getting a GPS position from it. “The map is linked to an absolute position by taking the line out of the cave entrance and accessing a GPS coordinate in an area with few obstructions to the sky,” says Richard. “In caves which have more than one entrance, it is possible to ascertain and correct for the build-up of errors by taking GPS measurements at the entrances. Programs such as Ariane [a widely used mapping tool] can then be used to distribute the correction through the map.”

Instrumental improvements

The team previously used a fluxgate magnetometer to match above-ground and subterranean locations at the Sagitario cenote. In the summer of 2024, Richard and his cave-mapping colleagues trialled a new means of confirming their findings, using both the Raspberry Shake 1D vertical motion seismograph and a far more sensitive acoustic magnetometer. Getting to the site after hacking through the jungle, the Raspberry Shake and acoustic magnetoscope were placed directly above where divers believed the cave was located. 

Golfing greens are treated with nitrates that poison the cenotes with algae

“Raspberry Shake helped confirm our findings and add a degree of accuracy that was not previously possible,” says Richard. The results were promising enough that the team ordered an RS3D three-axis model for their planned return trip in early 2025. This time, the Raspberry Shake will be placed in an IP67 waterproof box, and the team hopes the additional measurements will allow direction to be determined from the relative amplitudes of the disturbance in the X, Y, Z frames. 

Richard, Sam, and Chris embark on a cave dive at Actun Koh

The post Cave mapping with Raspberry Shake appeared first on Raspberry Pi.

from News - Raspberry Pi https://ift.tt/GCxq63n