Detection and Characterization of Cavities, Tunnels, and Abandoned Mines

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Detection and Characterization of Cavities, Tunnels, and Abandoned Mines

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Detection and Characterization of Cavities, Tunnels, and Abandoned Mines
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Presentation at 2008 International Conference on Environmental and Engineering Geoscience (ICEEG 2008): Detection and Characterization of Cavities, Tunnels, and Abandoned Mines by Dwain K. Butler, PhD, PG US Army Engineer Research and Development Center Vicksburg, Mississippi
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(2008)
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PAGE 1

Dwain K. Butler, PhD, PG US Army Engineer Research and Development Center Vicksburg, Mississippi Detection and Characterization of Cavities, Tunnels, and Abandoned Mines ICEEG, June 2008 Wuhan, China

PAGE 2

DANGER! Abandoned Mines ???

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One Way is to Look e.g., Visual Inspection and Exploratory Drilling

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Lessons Learned (?) Geophysical investigations should be integrated. Selected boreholes to guide geophysical program planning. Follow on borings to validate geophysical survey results. Problem: Locate abandoned mine openings / portals in the abutment of a proposed lock and dam (openings indicated in mine maps) Location: West Virginia, USA Exploration Program: 1.) Fifty (50) exploratory boreholes -No evidence of mine openings 2.) Small, follow on, geophysical survey program -Geophysical evidence of two potential mine openings One opening found just upstream of last borehole One opening found between two borings

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DANGER! Abandoned Mines requirements and concepts the history of geophysical cavity detection. the need to integrate geophysics into the overall characterization program. noninvasively detect and characterize with geophysical methods. Another way Recognize

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Cavity and Tunnel Detection Requirements and Drivers Development of Geophysical Technology for Cavity and Tunnel Detection Both Military and Civilian Requirements have been Drivers for Tunneling / Utilization of Existing Cavities and Tunnels have Played Important Roles in Conflicts Throughout History Military Tunneling is a Method of Choice for Activities Such as Smuggling, Prison Escape, Robbery, and Caches Criminal Activities Natural Cavities and Abandoned Mines Pose Safety Threats to Man and His Engineered Structures Safety and Engineering

PAGE 7

Man Made Mines -Shafts, Tunnels, and Chambers Known: Active Mines; Abandoned (w/records) Unknown: Abandoned (Records Lost; Forgotten); Ancient Tunnels -Known: Railway, Highway, Sewer, etc. Unknown: Covert, Intrusion, Abandoned CAVITIES AND TUNNELS

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Man Made (Cont.) May be Lined, Unlined or Braced -Dependent on Soil or Rock Type and Condition Complete or Partial Wood, Concrete, Steel, PVC May be All or Partially Water Filled, Clay Filled, Debris Filled May Contain -Rails Power Lines Ventilation Pipes and Shafts Pumps and Other Motors Other Metallic Objects CAVITIES AND TUNNELS

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Natural Underground Caves, Tubes, Chambers, Pipes, etc. Formed by Natural Processes Frequently Formed in Limestones Lineaments and Intersections Fractures and Bedding Planes Soil Bridges Pinnacles and Grikes May be -Air filled Water filled Soft sediment filled Partially filled CAVITIES AND TUNNELS

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Lineaments and Intersections Medford Cave Site Fracture and Bedding Plane Intersections Soil Bridge Pinnacles and Grikes Karst Site Features Examples: Medford Cave Site Florida

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DANGER! Abandoned Mines Understand the applications and the physics and fundamental concepts of the geophysical methods.

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Anomalies Caused by Cavities and Tunnels Density anomaly Seismic velocity anomaly Electromagnetic velocity anomaly Electrical resistivity anomaly Anomalies caused by secondary effects around the cavity Stress redistribution Cracking and fracturing Subsidence Induced ground water flow; Increased / Decreased water content Anomalies caused by materials within the cavity Water Tunnel lining: Steel, reinforced concrete Metal: machinery, rails, etc. Power lines Clay; Rockfall Debris

PAGE 13

Survey Types: Anomaly Detection; Imaging Profile (1 D) Area/Grid (2 D) Single Borehole (Anomaly Detection; Reflection Mode) Multiple Boreholes (Reflection; Transmission; Diffraction; Tomography ) Definitions: Anomaly, Background, Halo Halo Amplitude Distance Background Anomaly Borehole Cavity/Void

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Anomalies Caused by Cavities and Tunnels WHAT DETERMINES IF A CAVITY OR TUNNEL CAN BE DETECTED ON THE SURFACE ? Magnitude of the Anomaly Size; Geometry Depth Physical Property Contrasts Noise Manmade or Cultural Natural Sensitivity and Accuracy of Measurement System to the physical expression of the anomaly Signal / Noise Caution: often theoretical anomalies are compared to system sensitivity to predict detectability,

PAGE 15

Concepts of the Geophysical Methods Seismic Methods Passive Applications Tunnel location -"Triangulation" Activity monitoring Sources -Within tunnel Vehicles Walking Motors Active tunneling Requires only Sensors At surface or within boreholes Lost Miner Location (MSHA) Sensors Geophones Impacts, tapping, hammering, etc. Sensors / Geophones Extensive archival documentation of in tunnel sound/seismic sources exists, associated with construction activities and general use activities.

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Concepts of the Geophysical Methods Seismic Methods (Cont.) Active Requires both Source and Sensors Surface and Borehole Application Cavity and Tunnel Detection and Characterization Source Sensors Surface Methods Reflection; Refraction; Surface Waves Source Borehole Sensor Borehole Crosshole and Tomographic Methods S and P Wave Applications * * * Fundamental Properties: Density; p and s wave velocities Sensors

PAGE 17

Surface Seismic Methods Surface Seismic Methods and Shallow Cavity Systems Seismic method and geologic setting constraints exist for detection and tracking Seismic surface wave methods: SASW, MASW Seismic reflection: mining with marker bed/layer disruption 0,0 0,280 300,0 (50,10) Medford Cave Site, Florida, USA 10 m Seismic Fan Shooting Results Travel Times, ms Geophone Number Time, ms Known Cavity System Cavities discovered by drilling based on microgravity anomalies Reference 0 10 20 30

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Conceptual variation of Rayleigh wave velocity with depth for cavity in multi layered geology. Distance Depth 7620 5145 5145 5145 4650 3595 3595 5115 5430 5430 V 1 = V 2 = V 3 V 1 < V 2 < V 3 V 1 > V 2 > V 3 Distance, m 95 90 85 80 75 70 65 60 55 50 Distance, m 55 50 45 40 35 30 25 20 15 10 Depth, m 0 5 10 15 300 200 100 300 200 100 m/s Crosshole S wave and P wave surveys (simple and tomographic) can detect cavities and halo Current research in Surface Wave Methods for cavity detection and imaging Often used with GPR or ERT {

PAGE 19

Concepts of the Geophysical Methods Electrical and Electromagnetic Methods Passive Application -Detection of and Monitoring for Electromagnetic Signals from Cavities/Tunnels Requires -Surface Electrodes Magnetometers Loop (Dipole) Receivers Receiver Electronics Active I V C C p p 1 2 2 1 1 2 Electrical Resistivity Methods Application -Detection of Cavities and Tunnels Requires -Transmitters and power supplies Types Electrical Resistivity Electromagnetic Induction Ground Penetrating Radar 1 2 Tx Rx Electromagnetic Induction Methods Hp Hs Same type receivers as for passive applications Fundamental Properties: electrical conductivity; dielectric permittivity; magnetic susceptibility

PAGE 20

Collage of Resistivity, EM Induction and Ground Penetrating Radar Systems Tx Tx Tx Rx Rx Rx

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Residual Real, ppm Residual Imaginary, ppm Residual, ppm 100 50 0 50 100 1 0 1 2 3 Tx 6 m Rx Range, m Anomaly calculation for 6 m Tx Rx spacing, at 1 m height; FDEM System, 10 kHz; Tunnel Depth 10 m; 25 Ohm m background Purely air filled tunnels/cavities generally difficult to detect (S/N) with EMI systems unless very shallow and/or large Theoretical FDEM Anomaly for Air Filled Tunnel Otay Mesa, California, USA

PAGE 22

GPR Example Survey maps large diameter oil / gas pipeline (2007) ( metal reinforced cavity) Position/Depth Determination Prevents Pipe Rupture GPR Example Mill Creek Dam, WA 1989 Filled Sinkhole and Piping Time / Depth ~ 1 m 15 ns Time, ns 0 0 6 m 6 m 9 m 9 m

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10 m << Detect and Estimate Depth Conductivity, mS/m 20 m Tx Rx Spacing, Array Parallel to Tunnel Axis 20 m Tx Rx Spacing, Array Perpendicular to Tunnel Axis Station, ft Otway Mesa Tunnel Test Site EM 34 Data Reinforced, Concrete Lined Section 15 m Depth Angle Between EM Array Axis and Tunnel Axis Tunnel Axis Tunnel Axis Conductivity, mS/m Determine Orientation >> > > 0 Degrees 90 180 270 0 360 Conductivity, mS/m Conductivity

PAGE 24

Geologic Section 3 m Spaced Boreholes N S Distance, ft Resistivity Profiles a = 10 ft (3 m) a = 40 ft (12 m) 10 m Elevation, ft msl N S Distance, ft 10 m 2.5 m Apparent Resistivity, m Known Cavities Limestone Clay Top of Rock 4 to 1 Vertical Exaggeration A A Medford Cave, Florida, USA Apparent Resistivity Map A A 10 m Known Cavity System Wenner Array a = 40 ft (12 m) C. I. = 200 ft (60 m) 100,160 0,0 260,160

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Elevation, ft msl Sand Clay Limestone Known Cavity Lows Highs 170 150 130 110 N S Distance, ft 60 100 140 160 220 260 170 150 130 110 Pole Dipole Resistivity Anomaly and Geologic Cross Sections Medford Cave, Florida, USA 3 m C 1 P 1 P 2 24 m Manual Data Acquisition; Graphical / Qualitative Interpretation ( ca 1980 ) Advances in Resistivity Data Acquisition and Interpretation for Cavity/Tunnel Investigations 10 20 30 50 40 60 70 Automated Data Acquisition; Interpretation by Iterative Forward Modeling ( ca 1994 ) Dipole Dipole Resistivity Pseudo Section Otay Mesa Tunnel, California, USA

PAGE 26

Elevation, ft msl Possible Clay Filled Cavity Resistivity ( m) Dipole Dipole Resistivity Cross Section Automated Data Acquisition (w/ 28 electrodes, 2 m spacing) 2 D Inverse Imaging of Shallow Cave and Associated Karst Features ( ca 2005 ; courtesy Schnabel Engineering and Advanced Geosciences, Inc.) 3 m 15 m 2 D / 3 D Resistivity Inversions are Now Routinely Available

PAGE 27

Gravimetry and Gravity Gradiometry g Z g g Z or g Surface Cavity Gravimetry Cavities Detectable to Depths to ~ 6 10 x Effective Diameter Ex. -2 meter diameter tunnel can be detected to depth of ~ 20 m (Assuming 5 microgal gravity measurement accuracy) Gravity Gradiometry Higher Resolution than Gravity Ex. -2 meter diameter detectable to depths ~< 25 m; limited by S/N (Assuming 1 2 Eotvos gravity gradient measurement accuracy) Anomaly Signature Relatively Simple and Reliable Technology; Slow Complex Technology Concepts of the Geophysical Methods Fundamental Property: density Requires: Gravity Meter

PAGE 28

Microgravity Profile N S Distance, ft 10 m Residual Gravity Anomaly, Gal 30 20 10 0 10 20 30 0 40 80 120 160 200 240 Geologic Section 3 m Spaced Boreholes N S Distance, ft Elevation, ft msl 10 m 2.5 m Known Cavities Limestone Clay Top of Rock 4 to 1 Vertical Exaggeration Test Borehole Exploratory Borehole Confirmatory Borehole Positive Gravity Anomaly 20 Gal contours Base Station Microgravity Survey Medford Cave, FL, USA

PAGE 29

Room and Pillar Mine ~ 5 m Depth, Northern France Detected by Reconnaissance Grid 20 m Delineated by Follor on High Resolution Grid 5 m Bouguer Anomaly (10 gal contours) Residual Anomaly (5 gal contours) Regional Anomaly Abandoned Mine Characterization Microgravity Example (Robert Neumann, ca 1977)

PAGE 30

150 ft 360 ft 1 2 3 Repeat Microgravity Survey Example Tunnel Detection in an Urban Environment Sewer Tunnel Construction Atlanta, Georgia, USA Tunnel Diameter 8 m; Tunnel Depth 49 m I II Approx. TBM Location I & II Microgravity Survey Lines Approx. Tunnel Centerline

PAGE 31

2D Analytical Model for Gravity Anomaly over a Tunnel Segment 2D Analytical Model for Gravity Anomaly over a Tunnel Segment Conceptual Models for Repeat Gravity Survey Differences

PAGE 35

Comparison of Gravity Repeat Values for an Urban Area Survey 1 Survey 2 Survey 3

PAGE 36

Recommended Geophysical Methods for Subsurface Void/Cavity Detection NRC CR 2062 1981) Candidate Geophysical Methods for Cavity Reconnaissance Surveys Most Promising Surface electrical resistivity profiling Microgravimetry Constant Borderline Standard seismic refraction Pole dipole electrical resistivity Surface ground penetrating radar Candidates for High Resolution, Delineation Survey Most Promising Crosshole radar; Crosshole seismic methods Pole dipole resistivity Microgravimetry Acoustic Resonance (Subsurface source) Borderline Seismic refraction Surface ground penetrating radar

PAGE 37

Seeing into the Earth (NRC 2002) 2 1 2 2 2 3 1 3 3 Cavity detection


Description
Presentation at 2008 International Conference on
Environmental and Engineering Geoscience (ICEEG 2008):
Detection and Characterization of Cavities,
Tunnels, and Abandoned Mines
by Dwain K. Butler, PhD, PG
US Army Engineer Research and Development Center
Vicksburg, Mississippi


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