Moultrie Group Subsurface Geophysical Surveys

Transcription

1 Moultrie Group Subsurface Geophysical Surveys Martin Brook Moultrie Geology, Banyo, Brisbane, QLD

2 Subsurface Geophysical Surveys Martin Brook / Senior Project Geologist / Moultrie Geology & Adjunct Senior Research Fellow, The University of Queensland

3 Outline Overview Near Surface Geophysics Gravity Magnetometry Ground Radar Resistivity Summary

4 Overview of the Moultrie Group

5 Locations Key Locations Growth Plans Satellite Operations

6 Moultrie Group Integrated Mining Solutions Geological Services Exploration Geology Mine Geology Map, Model, Interpret Technical studies & Reports + Earthworks, Hire, Safety Services, Mine Services, Project Management

7 Near Surface Geophysics

8 Near Surface Geophysics <100 m depth

9 Contrasts in Electrical Properties Between Target and Host

10 Sinkholes

11 Sinkholes

12 Underground storage tanks

13 Brownfield Sites Near Surface Geophysics, 2011, 9,

14 Abandoned Mine Workings

15 Buried Utilities Water Power Comms Sewage

16 Exploration Geology

17 Gravity Surveying Subsurface density / gravitational acceleration Surface measurements of the Earth s gravitational field Microgravity surveys seek to detect areas of contrasting density Relative High: dense material, acceleration (g) is + gravity anomaly Relative Low: Low density feature (e.g. an air-filled cavity) gravity anomaly

18 Gravity Surveying Units Acceleration: 1 Gal (Galileo) 1 cm s 2 milligal (mgal) 1/1000 th Gal for regional geological mapping microgal (µgal) 1/1000,000 Gal 50 µgal void mapping Microgravity= very subtle variations in density! Issues Scintrex CG-5 Time consuming Gravimeter Corrections/Modeling Terrain, latitude, tides, surface structures

19 Gravity Surveying Northwich, UK Former salt mines Post-Mining 2005, November 16-17, Nancy, France, p.1

20 Gravity Surveying Northwich, UK Former salt mines Peter Street subsided 23 cm Low density body (D) increased in size, top propogated 4m toward surface (A) decreased in amplitude due to collapse and infilling

21 Ground Magnetometry Variations in Magnetic field Geology Man-made features Surface-processes Magnetic field strength (sometimes direction) Teslas (T) strength 200 nteslas 1000 nteslas

22 Magnetometry 1. Magnetic Surveys for Geology Base station magnetometer (proton precession) Survey Magnetometer (Cesium vapour magnetometer) Rapid data collection 20 km/day line surveys

23 Magnetometry 2. Magnetic Gradiometer Surveys Buried objects Instrument has 2 magnetometers, measures mag gradient between the 2 Very good for near surface buried objects Archaeology etc Journal of Applied Geophysics 68 (2009)

24 Ground Penetrating Radar (GPR) Electromagnetic wave propagation and scattering due to contrasts Highest resolution subsurface imaging of any geophysical method (cm) Depth of Investigation <1m to >5km over ice sheets Depends upon properties: electrical conductivity dielectric permittivity

25 Ground Penetrating Radar (GPR) Trade-off between Penetration Depth and resolution Depth (m) Centre frequency (MHz)

26 Ground Penetrating Radar (GPR) Reflection profiling Two-way travel time Convert to depth based on velocity of subsurface Costs: $20k for basic Plumber s unit More for sophisticated units Borehole GPR Issues: 1. Signal attenuation - high ground conductivity 2. Unshielded antenna 3. Clay 4. Water 5. Simplicity of use

27 Cart GPR Shielded antenna system MHz Multitude of targets Integrated GPS

28 Sled GPR 100 MHz

29 Rough Terrain Antenna 50 MHz Sled GPR 100 MHz

30 Getting the most out of GPR data: Post-processing (ReflexW software) 1. Migration and use of gain experimentation (SEC, AGC etc) removes background noise 2. Allows easy identification of point targets, which were covertly buried waste-oil drums

31 Seam horizon tracking identify seams depths below mine floors or in opencast pit

32 Structural geology mapping of structures in quarries

33 Abandoned Bord-and-Pillar workings Depth slice maps (modelled in GPR-Slice)

34 Abandoned Bord-and-Pillar workings

35 Utility Mapping isosurface in Voxler

36 Concrete Scanning 1.6 GHz antenna

37 Electrical Resistivity Imaging Determine subsurface resistivity distribution from surface measurements True resistivity of the subsurface is then estimated Resistivity related to mineral and fluid content, porosity, groundwater etc Used extensively in hydrogeological, mining & geotechnical investigations Issues: Time consuming Electrode contacts Data Processing RES2DINV Utilities Length of line proportional to depth

38 Electrical Resistivity Imaging Electric current is injected into the ground through 2 Current electrodes Resulting potential is measured between 2 other Potential electrodes Injected current and resulting voltage (potential) is measured and apparent resistivity is then calculated Lots of different electrode arrays Usually 2 Current, 2 Potential electrodes

39 Electrical Resistivity Imaging Wenner Array length of cable 300m = 51 m depth

40 Summary: Near Surface Geophysics Many near surface geophysical methods useful for mining & exploration 1. Target/host contrasts 2. Costs 3. Depths of investigation 4. Simplicity 5. Resolution 6. Other methods: seismic; EM conductivity etc

41 FOOTER DOCUMENT DETAILS MOULTRIE GROUP 2012