Electrical Imaging of Mine Electrical Imaging of Mine Voids: Voids:

Lessons LearnedLessons Learned

Peter J. Hutchinson, PhD, PGThe Hutchinson Group, Ltd., USA

CONTINUOUS VERTICAL ELECTRICAL SOUNDING

• VES profiles using multicore cable and switching box

• Microprocessor produces geo-electric model• Rapid and inexpensive• Image depth of 40 m• Function:

– Lithology– Porosity– Permeability– Fluids

“NON-UNIQUENESS”

• Term that plagues the application geophysicist

• Lack of recognition not knowledge• Interpretation

COMMON ARRAYSCOMMON ARRAYS• Wenner – shallow, coarse data

5ba ]R;a4

b-1[ba =

2

22

a ≥πρ 5ba ]R;

a4b-1[

ba =

2

22

a ≥πρ 5ba ]R;

a4b-1[

ba =

2

22

a ≥πρ

aRnnna )2)(1( ++= πρ aRnnna )2)(1( ++= πρ aRnnna )2)(1( ++= πρ•Dipole-dipole – deepest, densest data

•Schlumberger – deeper, denser data

aRa πρ 2= aRa πρ 2= aRa πρ 2=

WENNER ARRAY

0 30015010050 200 250

Depth(meters)

15

0

7.5

11.5

3.5

SCHLUMBERGER ARRAY

Depth(meters)

18

0

9

4.5

13.5

0 30050 100 150 200 250

DIPOLE-DIPOLE ARRAY

Depth(meters)

18.5

0

9.25

4.5

13.5

0 30050 100 150 200 250

GEOLOGIC MATERIALGEOLOGIC MATERIALAPPARENT RESISTIVITIESAPPARENT RESISTIVITIES• Clay = 1 Wm• Soil = 10 Wm• Coal/Lignite = 50 Wm• Sand (wet) = 100 Wm• Conglomerate = 20,000 Wm• Sandstone = 100,000 Wm• Quartz = 1,000,000 Wm• Granite = 3,000,000 Wm• Basalt = 13,000,000 Wm• Limestone = 50,000,000 Wm

SITE LOCATION MAP

SITE 2

SITE 1

SITE 4

SITE 5

SITE 3

SITE 1 SITE 1 -- OHIOOHIO

•• 100100--Home subdivisionHome subdivision•• 77--m thick Massillon Sandstone m thick Massillon Sandstone •• 22--m thick Pennsylvanian coalm thick Pennsylvanian coal•• Shallow late 1800s coal mineShallow late 1800s coal mine•• Recent subsidenceRecent subsidence

SITE 1 SITE 1 -- OHIOOHIO

~100 Homes overlie mine~100 Homes overlie mine•• 47 Borings47 Borings

•• 25 Voids25 Voids•• 12 coal12 coal•• 10 non10 non--coalcoal

•• ~56 Homes impacted~56 Homes impacted

RESISTIVITY SURVEY LINESRESISTIVITY SURVEY LINES

SURVEY LINESSURVEY LINES

BORINGBORING LOCATIONSLOCATIONS

BARBERTON COAL MINE BARBERTON COAL MINE CONCLUSIONSCONCLUSIONS

•• 69 homes within footprint69 homes within footprint•• 46 homes appear safe46 homes appear safe•• 25 homes at risk25 homes at risk

•• 9 low risk9 low risk•• 10 medium risk10 medium risk•• 6 high risk6 high risk

HOUSES NEAR ANOMALIES

BARTGES MINEOUT OVERLAYBARTGES MINEOUT OVERLAY

SITE 2 - PENNSYLVANIA

• Recent road cut• Pennsylvania Warrior Fm• Steeply dipping• Clay-filled void in limestone• 100-meter dipole-dipole array

SITE 2 - PENNSYLVANIA

SITE 3 - INDIANA

• Pennsylvanian-aged 1.2-m coal seam• Coal at 15 – 20 m below grade• Horizontal bedding• Room-and-pillar and retreat mining• 1,500-m Schlumberger array

ROOM-AND-PILLAR MINING

SITE 3 - INDIANA

DEEP COAL MINE

SITE 3 - INDIANA

SITE 3 SITE 3 -- PENNSYLVANIA•• PennsylvanianPennsylvanian--aged coalaged coal•• 22--m thick coal/clay unitm thick coal/clay unit•• Split seam (0.4 meters each)Split seam (0.4 meters each)•• 1.21.2--m thick Mercer Coal underclaym thick Mercer Coal underclay•• 2525--m of shale/m of shale/ssss overburden overburden •• Shallow 1870Shallow 1870--1920 clay mine1920 clay mine•• 100m Schlumberger array100m Schlumberger array

SITE MAP

CROSS SECTION

POTTSVILLEGROUP

SITE 4 - PENNSYLVANIA

CLAY MINE OVERLAY

SITE 5 - PENNSYLVANIA

• 2.5-m thick Pennsylvanian-aged coal• Surface-mined• Deep-mine• Abandoned highwall• Hill-topping and cut burial• 150-m Schlumberger array

SITE 5 - PENNSYLVANIA

CONCLUSIONSCONCLUSIONS

www.geo-image.com

•• Subsurface voids can be resolved Subsurface voids can be resolved through CVES (>40 m depth) through CVES (>40 m depth)

•• AirAir-- and clayand clay--filled voids w/ contrastfilled voids w/ contrast•• NonNon--uniqueness uniqueness –– resolved throughresolved through

•• Site knowledgeSite knowledge•• Experience and trainingExperience and training•• Deductive and inductive reasoningDeductive and inductive reasoning•• InterpretationInterpretation

•• CVES is effective screening toolCVES is effective screening tool