Laser Scanning for Cave Surveying and Visualisation Kevin

Laser Scanning for Cave Surveying and Visualisation Kevin Dixon CPC Geospatial 3 D

Overview u u u u Why ? Technology Issues Hardware Software Cave Surveying Summary

Why Laser Scan ? u Expensive £ 15 k-90 k Big, heavy (>10 kgs) sensitive equipment Lots of batteries required Time consuming 1 -4 hr per detailed scan u So why do it ? u 3 D realisation Record the geological structure Visualise black holes Virtual cave exploration for non-cavers u u u

Laser Principles u u u Light Amplification by Stimulated Emission of Radiation Gain medium is energy pumped to excite electrons to emit light which is amplified in optical cavity to produce a coherent light beam often at specific frequency Most common uses – DVD/CD Players and Burners, Fibre-optic Communication – Bar-code Readers, Laser Printers, Laser Pointers, Mouse u Other uses – – Medical, dentistry, lightshow industrial cutting and marking Military target painting, ranging Directed energy weapon

Semiconductor Lasers 375 to 1800 nm u Most common wavelengths for ranging: – – – 532 nm visible green, can measure through water 650/690 nm visible red 785/870/905 nm near infra-red 1064 nm near infra-red, typically used for airborne LIDAR 1535 nm inherently eye-safe, not absorbed by water

Laser Safety u u u Class I/1 is safe Class II/2 is safe during normal use < 1 m. W Class IIIa/3 R pose small risk of eye damage < 5 m. W Class IIIb/3 B severe eye damage < 500 m. W Class IV/4 can burn skin > 500 m. W Protection – Wavelength specific glasses – Security Key Control – Labels

Range Performance Range requires some laser energy to be reflected back to the laser detector Reflectivity is a function of: Surface Shape Colour Prism Target Size Flat Texture Atmosphere Convex Incidence Angle

3 D Lasers Station Range u Laser plus – Angle Encoders – Motors u Vertical Angle Optional – – – u Coordinates Viewfinder Camera Accelerometers Gyro Compass Horizontal Angle Reference Azimuth Polar Coordinate System (Range, HA, VA) – Add Station Coordinates – Add Reference Azimuth u Get Cartesian Coords (Easting, Northing, Height)

Different Scanner Hardware $ $$ $$$$$$

Scanner Gear for Caves Normal Transit Case Large Tackle Bag plus Scanner 10 kg, Case 4 kg Internal Protection 2 kg Lightweight Case 2 kg (Floats) Plastic Bags for Protection Small Battery 3 kg / 3 hrs Big Battery 5 kg / 8 hrs Mini 3 kg or Normal 6 kg Tripod

Laser Scan Integration u u Two Options Coordinates – – u Traverse Series of control points Requires more time Best for complex spaces Overlap – Adjacent scans have sufficient common data – Software registration – Quick – Suitable for simple spaces

Shadow u u Line of Sight System Creates ‘Shadow’

Shadow u Line of Sight System u Scan From Another Direction Reduces Shadow u

Shadow u Line of Sight System u u Scan From Other Direction Fills Shadow u Shadow Under Scanner ?

Scanning Issues u u u High humidity limits range Water buildup on scanner lenses Reflectivity ≡ Range – Best: white flat perpendicular limestone – Poor: Wet, Muddy, Mossy, Dark u u u Short ranges <0. 5 m can be difficult Difficult Setups for Vertical Shafts Curious cavers and tourists – ‘Tripod kick’ and ‘Laser Block’ u u Equipment Weight Very Little Data from Narrow Rifts and Flat-out Passage ‘Cloud’ Waterfall

Stay Out of the Scan !

Software u u Quarryman. Pro Scanner logs data on internal CF card Void. Scanner requires ruggedised laptop MDL Model Software Creates Cartesian Space MDL Void. Works - Dedicated for Underground Mining – Data Edit and Registration – Void Modelling And Volumetrics u u Data colour coded by area, height or reflectivity Output options OBJ, DXF, CSV, VRML, Vulcan (Mine data format)

Cave Survey Grades u u u u 1 Low accuracy sketch with no measurements made 2 Intermediate survey between Grades 1 & 3 3 Magnetic survey. Angles ± 2. 5º; distances ± 50 cm; station position error < 50 cm. 4 Intermediate survey between Grades 3 & 5 5 Magnetic survey. Angles ± 1º; distances ± 1 cm; station position < 10 cm. 6 Magnetic survey that is more accurate than grade 5. X Survey based on theodolite or total station.

Cave Survey Detail Grades u u A All passage details based on memory. B Passage details estimated and recorded in the cave. C Measurements of detail made at survey stations only. D As per C and at significant changes in passage u Is another classification required ? ? E Measurements of detail made for majority of passage u XE Laser Scan Survey ? ? u

Yordas Cave, Yorkshire u u u First trial laser scan for caves Quarryman. Pro Laser Scanner used on Tripod 3 Setups, 5 hours scanning

Reads Cavern, Mendips u u u Cave Electronics and Radio Group 19 -20 April 2008 Demonstration of Quarryman. Pro Laser Scanner 2 Setups, 2 hours scanning

OFD 2, Wales u u u Main Chamber and Top Entrance Series Quarryman. Pro Laser Scanner on Tripod 11 Setups, 8 hours scanning

OFD 2 TBCNTE

St Michaels Cave, Gibraltar u u u Quarryman. Pro Laser Scanner 45 stations 5 days scanning Determine cave volume for study Laser scans identify geological features running through rift

Gaping Gill, Yorkshire u u u Quarryman. Pro and Void. Scanner Main Chamber, Entrance Shaft, Shakehole and River Bed Also Mud Hall 20 setups including half way down winch 4 weekends scanning

GG Mud Hall

GG From West to East

GG Revolution

GG Heights for 2008

GG Jumbo x 2

Cave Scanning in the News Yorkshire Post 27 Aug 2008 Yorkshire Dalesman Oct 2008

In-Work u Titan, Derbyshire – 8 scans in 1 weekend so far – Humidity Issues Below Event Horizon – Yorks v. Derbys (GG v Titan) u York Minster – Is GG as big as Minster ? u UK Big Cave/Pothole List – Dimensions, Volume – GG, GB, Titan, Time Machine, Smoo, Mud Hall. . . . u Big Chambers Worldwide Scanning at Titan Breakthrough Pitch Head Picture Rob Eavis

Titan Side Elevation Scans so far

Titan Revolution

Summary u Current Laser Scanners for Cave Surveying are – Expensive, Hard Work – Not Suitable for Very Small Passages – Need to be Controlled by Traverse and/or Scan Overlap u 3 D Realisations – Aid Cave Studies – Add Detail to Cave Mapping - Class XE Cave Survey ? ? – Provide a new means of Promoting Caves to non-Cavers u A smaller, low power, faster scanner with photogrammetry and ‘dead-reckoning’ would be useful – Unlikely to be within typical Caver budget !!

Thank You u u u Yordas Cave - Roo Walters Reads Cavern - BCRA CREG OFD 2 - Allan Richardson, SWCC, Meg Stark St Michaels Cave - Dave Mattey, Gibraltar Caving Group Gaping Gill - Meg Stark, Bradford PC, Craven PC Titan - Dave Nixon, Rob Eavis, Katie Dent, Meg Stark

Future u u Set up new Company Geospatial 3 D Riegl Laser Scanner – 6 mm Accuracy – Camera Photo Overlay – 100 m Range u 3 D Laser Scanning – Heritage – Underground – Quarries and Mines u OFD 2 Columns ? ?

Laser Range Techniques Pulse (Time of Flight) u u High Measurement Rate Long Range Better for Passive Targets Good Outdoor Phase (Carrier Wave) u u High Accuracy Lower Cost Can be Visible Low Power

Phase (Carrier Wave) Turn on the Laser Beam and modulate the intensity LASER Monitor the reflected signal LASER Compare the two signals Time Delay = Phase Angle 2 x π x Modulation Speed Range = Time Delay x Speed of Light 2 Range = Speed of Light 2 x Modulation Speed

Pulse (Time of Flight) Send a short pulse of light out from the laser LASER REFLECTOR Monitor the reflected signal LASER REFLECTOR Compare the two signals Range = Time Delay x Speed of Light 2

Handheld Lasers u. None u. Only Are Waterproof 2 have Tilt and Compass