Moultrie Group Subsurface Geophysical Surveys Martin Brook Moultrie Geology, Banyo, Brisbane, QLD 04 37887 362 mbrook@moultrie.com.au
Subsurface Geophysical Surveys Martin Brook / Senior Project Geologist / Moultrie Geology & Adjunct Senior Research Fellow, The University of Queensland
Outline Overview Near Surface Geophysics Gravity Magnetometry Ground Radar Resistivity Summary
Overview of the Moultrie Group
Locations Key Locations Growth Plans Satellite Operations
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
Near Surface Geophysics
Near Surface Geophysics <100 m depth
Contrasts in Electrical Properties Between Target and Host
Sinkholes
Sinkholes
Underground storage tanks
Brownfield Sites Near Surface Geophysics, 2011, 9, 483-496
Abandoned Mine Workings
Buried Utilities Water Power Comms Sewage
Exploration Geology
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
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
Gravity Surveying Northwich, UK Former salt mines Post-Mining 2005, November 16-17, Nancy, France, p.1
Gravity Surveying Northwich, UK Former salt mines 1998-2001 Peter Street subsided 23 cm 1998-2001 Low density body (D) increased in size, top propogated 4m toward surface (A) decreased in amplitude due to collapse and infilling
Ground Magnetometry Variations in Magnetic field Geology Man-made features Surface-processes Magnetic field strength (sometimes direction) Teslas (T) strength 200 nteslas 1000 nteslas
Magnetometry 1. Magnetic Surveys for Geology Base station magnetometer (proton precession) Survey Magnetometer (Cesium vapour magnetometer) Rapid data collection 20 km/day line surveys
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) 294 300
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
Ground Penetrating Radar (GPR) Trade-off between Penetration Depth and resolution Depth (m) Centre frequency (MHz) 0.5 1000 1.0 500 2.0 200 5.0 100 10.0 50 30.0 25 50.0 10
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
Cart GPR Shielded antenna system 200-800 MHz Multitude of targets Integrated GPS
Sled GPR 100 MHz
Rough Terrain Antenna 50 MHz Sled GPR 100 MHz
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
Seam horizon tracking identify seams depths below mine floors or in opencast pit
Structural geology mapping of structures in quarries
Abandoned Bord-and-Pillar workings Depth slice maps (modelled in GPR-Slice)
Abandoned Bord-and-Pillar workings
Utility Mapping isosurface in Voxler
Concrete Scanning 1.6 GHz antenna
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
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
Electrical Resistivity Imaging Wenner Array length of cable 300m = 51 m depth
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
FOOTER DOCUMENT DETAILS MOULTRIE GROUP 2012