AN INEXPENSIVE TERRESTRIAL LASER SCANNER FOR CAVE AND MINE MAPPING

Airborne lidar remote sensing has been revolutionary in providing high resolution elevation measurements of the earth’s surface that can be applied to several scientific fields including geomorphology, volcanology, forestry and active tectonics. Terrestrial Laser Scanning (TSL) is now extending lidar technology to develop precise ground surface measurements of landforms at the millimeter to centimeter scale. TSL can also be applied to mapping underground features such as caves and mine tunnels. There are some limitations for TSL when working in an underground environment, especially working in a cave environment where conditions include narrow, wet and muddy passageways. TSL equipment can be bulky, require significant battery power and may require the use of a laptop computer. The cost of a commercial TSL unit can also range from the tens to over one hundred thousand dollars. All these factors have discouraged speleologists from using TSL for routine cave mapping.

The device described in this abstract has been designed for cave environments. It is small, light weight and has limited battery requirements. Total cost for the unit is approximately $400. The entire TSL system is stored in a watertight container that is easy to transport in narrow cave passages. The frame of the TSL is produced by 3D printing using PLA filament. The TSL laser rangefinder is a Garmin Lidar Lite V3 operating at 904 nm. The rangefinder measures distance using time of flight, pulsed phase-difference with a maximum range of 40 meters and typical accuracy of ± 10 cm. The divergence of the laser beam is 8 millradians, producing a target cross section of 40 cm at the maximum range of the beam An IMU (inertial measuring unit) attached to the rangefinder uses a magnetometer to obtain initial bearing orientation and an accelerometer to determine vertical (z axis) orientation. Stepper motors are used to rotate the rangefinder in x,y,z orientations. An Atmega 1280 (Arduino Mega) is used to control the TSL and store measurements. Distance measurements are recorded in polar coordinates. Post-processing in R is used to filter the measurements and convert the values to Cartesian coordinates. Programs such as Blender can be used to convert the Cartesian coordinates to a 3-D point cloud with supplementary mesh networking to provide a visually interpretable image.