geophysical void detection demonstrations

1

6th Biennial ITGAUM Workshop, June 4-16, 2006, Rochester, NY

GEOPHYSICAL VOID DETECTIONGEOPHYSICAL VOID DETECTIONDEMONSTRATIONSDEMONSTRATIONS

By:Kanaan Hanna, Bart Hoekstra,

Jim Pfeiffer, & Brandy Uphouse

MSHA:Steve Vamosy

George Gardner

2

Presentation FocusPresentation Focus

Engineering ProblemProgram Goals and ObjectivesGeophysical Methodology – Field InvestigationsVoid ConfirmationGeophysical Technologies Performance EvaluationsConclusions and Recommendations

3

Engineering ProblemEngineering Problem

• Unknown mines,• Inaccurate historical mine maps, or• Incomplete historical mine maps

MSHA Old mine works present major H&S hazards to our nation’s miners…

DOT’s Abandoned mines beneath roadways impact the performance of transportation infrastructure in terms of…• Cost,• Public safety

4

Program Goals and ObjectivesProgram Goals and Objectives

MSHA’s Overall GoalsTo…

Advance the current state-of-the-practice geophysical technologies for detecting mine voids

ToTo……Accurately & economically

detect underground mine voids

Overall Program Objectives

5

Objectives ContinuedObjectives Continued……

Definition: Old mine works vs.

Mine voids

Old mine works: Identify the general location/boundary of the UG workings

Mine voids: Identify the location of mine opening/entries, and pillars

6

Presentation FocusPresentation Focus

Engineering ProblemProgram Goals and ObjectivesGeophysical Methodology – Field InvestigationsVoid ConfirmationGeophysical Technologies Performance EvaluationsConclusions and Recommendations

7

• Surface and borehole geophysical methods:– 3-D high resolution seismic (HRS) using P-waves (HRPW)

and S-waves (HRSW)– Crosshole tomography (XHT)– Crosshole guided waves– Reverse vertical seismic profiling (RVSP)– Borehole sonar mapping

Geophysical MethodologyGeophysical Methodology

• Demonstration site:– Known abandoned coal mine adjacent to an active UG

room-and-pillar mine, Georgetown, IL

8

Geophysical Methods Geophysical Methods –– An OverviewAn Overview

Basic Concept of the RVSP and HRS Techniques

9

Site DescriptionSite Description

Geologic Setting Thickness of Material

in FtUnconsolidated Clay with Gravel Tan or BuffUnconsolidated Silt with Gravel Red or BrownUnconsolidated Silt with Gravel Med GrayUnconsolidated Sand with Gravel - LargeUnconsolidated Silt with Gravel Med GraySandstone - Med. Gray Sandy Shale - Med Gray Shale - Dark Gray

Danville Coal Seam # 7 (No Mining ~ 140 Ft Deep) 3.70Sandy Claystone - Med Gray Sandstone- Med Gray LimestoneSandstone- Med Gray Sandstone, Limey Med Gray with Nodules Sandy Shale - Med Gray Shale - Med Gray with Coal Streaks

Herrin Coal Seam # 6 (Target Zone ~ 233-235 Ft Deep) 6.80Limey Sandy ClaystoneSandy Claystone - Med Gray Limey Claystone - Med Gray - NodulesBlack Shale with Coal Streaks

Springfield Coal Seam # 5 ( No Mining ~ 253 Ft Deep) 0.80Limey Claystone - Med Gray - NodulesSandy Shale - Med Gray Shale - Dark Gray Black Limey ShaleSandy Shale - Med Gray

Total Drill Depth 320.00

89.30

13.20

66.20

30.00

110.00

Subsurface Strata Above and Below the Herrin Coal Seam #6

Gla

cial

M

ater

ial

Ove

rbur

den

Mat

eria

l 23

3 ft

Und

erly

ing

Mat

eria

l 80.

2 ft

Upper Coal Seam

Target Coal Seam

Lower Coal Seam

10

Grid Location of HRSW and HRPW 3-D Reflection Surveys

HRS Data AcquisitionHRS Data Acquisition

11

HRS Data AcquisitionHRS Data Acquisition

Georgetown Rd

12

Data Analysis and InterpretationAnomalous Coal Response from P-wave

Geophysical Methodology Geophysical Methodology -- HRSHRS

13

• Provided consistent data quality throughout the survey

• Obtained clear reflections from the Herrin #6 coal seam

• Imaged the general location of the old mine works

• Inadequate to identify mine voids (rooms/entries, pillar)

Geophysical Methodology Geophysical Methodology –– HRSHRS

HR primary P-wave 3-D (HRPW):

Conclusions…

14

• Obtained some reflections from Herrin #6 coal seam

• The deployment of two vibrator sources were still insufficient to compensate for energy loss

• Provided limited information on the old mine works

Geophysical Methodology Geophysical Methodology –– HRSHRS

HR shear S-wave 3-D (HRSW):

Conclusions

15

Geophysical Methodology Geophysical Methodology –– XHT/GWXHT/GW and and RVSPRVSP

Borehole Location Overlaid HRS Image

16

Geophysical Methodology Geophysical Methodology –– Boreholes DrillingBoreholes Drilling

17

Geophysical Methodology Geophysical Methodology -- RVSPRVSP

RVSP Data Analysis and InterpretationSeismic Amplitudes from RVSP Profiles along the Target Zone

18

Peak Seismic Amplitudes at Target Zone Overlain on Historical Mine Map

Geophysical Methodology Geophysical Methodology -- RVSPRVSP

19

Geophysical Methodology Geophysical Methodology -- RVSPRVSP

Peak Seismic Amplitudesat Target Zone fromNS #6 to NS #4

20

Presentation FocusPresentation Focus

Engineering ProblemProgram Goals and ObjectivesGeophysical Methodology – Field InvestigationsVoid ConfirmationGeophysical Technologies Performance EvaluationsConclusions and Recommendations

21

Boreholes Layout Basedon RVSP Results andHistorical Mine Map

Void ConfirmationVoid Confirmation

22

Drilling along NS Mains and Sonar Mapping

Void ConfirmationVoid Confirmation

23

Void Confirmation Void Confirmation –– Sonar Mapping Field ResultsSonar Mapping Field Results

Conf NS#4

24

Void Confirmation Void Confirmation –– Sonar InterpretationsSonar Interpretations

Sonar Void Models Overlay Historical Mine Map

25

Presentation FocusPresentation Focus

Engineering ProblemProgram Goals and ObjectivesGeophysical Methodology – Field InvestigationsVoid ConfirmationGeophysical Technologies Performance EvaluationsConclusions and Recommendations

26

Criterion HRPW HRSW XHT GuidedWaves RVSP Sonar

Mapping*

Ability to LocateMine Works/Voids Fair Fair Poor Poor Good n/a

Resolution Poor Poor Poor Very Poor Very Good Excellent

Depth ofInvestigation Good Poor Good Good Good Very Good

AnticipatedRepeatability Good Fair Good Fair Good Very Good

Robustness underVariousGeologic/SurfaceConditions

Fair Poor Fair Fair Good Very Good

Cost High Very High Medium Low Medium Medium

Void Contents Poor Good Poor Poor Good Very Good

*Sonar mapping can only be used in a borehole that has intersected a water-filled void in the mine.

Performance EvaluationPerformance Evaluation

Performance Evaluation of Geophysical Methods

27

Have these geophysical technologies met the overall project objective...

Performance EvaluationPerformance Evaluation

28

NO

Partially

YESYES

Performance EvaluationPerformance Evaluation

…..……. XHT and guided waves

…………HRPW/HRSW

…….…..RVSP

Objective Select the most accurate and cost-effective method…!

29

Presentation FocusPresentation Focus

Engineering ProblemProgram Goals and ObjectivesGeophysical Methodology – Field InvestigationsVoid ConfirmationGeophysical Technologies Performance EvaluationsConclusions and Recommendations

30

The geophysical technologies demonstration at the The geophysical technologies demonstration at the site has verified the effectiveness of the.site has verified the effectiveness of the.……

RVSPRVSP

WhenComplemented by Sonar or Laser Mapping

Combined with Engineering Practices – the most viable geophysical method to…

Accurately and Economically Detect Mine Voids

Conclusions and RecommendationsConclusions and Recommendations

31

Proposed RVSP Geophysical/Geotechnical Investigations for Commercial and Residential

Development

Where to go from here

32

RVSP Proposed Methodology RVSP Proposed Methodology

A systematic approach – integrating 2-D and/or 3-D RVSP geophysical investigations with subsidence engineering evaluation for optimum foundation design…

Geophysics RVSP Subsurface Investigation:• Identify old mine works• Identify mine voids

Engineering GeophysicsSubsurface Characterization in terms of:• Subsidence evaluation• Void detection evaluation

Engineering Evaluation

Engineering Analysis:• Geotechnical evaluation• Foundation system evaluation and risk assessment• Engineering decision

33

Schematic of Recommended RVSP Survey for Mine Void Detection

RVSP Proposed Methodology RVSP Proposed Methodology

34

RVSP Investigation

35

Questions