MT Prospecting. Map Resistivity. Determine Formations. Determine Structure. Targeted Drilling

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2 MT Prospecting Map Resistivity Determine Formations Determine Structure Targeted Drilling Cross-sectional interpretation before and after an MT survey of a mineral exploration prospect containing volcanic rocks.

3 MT Overview Magnetotellurics (MT) is a geophysical method for imaging the electrical resistivity and structure of the earth, from the near-surface down to hundreds of kilometers. MT is a passive geophysical method which uses natural electromagnetic field variations (caused by solar radiation/particles or atmospheric sources, e.g., lightning discharges), which induce electric currents in the subsurface. Magneto inferring magnetic fields and telluric inferring electric fields in the ground (tellus, Latin for earth ).

4 Resistivity Ranges Electrical resistivity of rocks and minerals is an important physical property to measure as part of attempts to understand geological structures and processes. It varies by many orders of magnitude, from very resistive crystalline igneous rocks, to very conductive saline-filled sedimentary rocks.

5 Technology Comparison Depth information is obtained by measuring the time variations over a range of frequencies. High frequencies penetrate into the earth to shallow depths only, while low frequencies penetrate deeper. Information is obtained from a few tens of metres depth to hundreds of kilometres depth.

6 MT Signal Sources Worldwide lightning activity at frequencies of 10,000 Hz to 1 Hz and geomagnetic micropulsations at frequencies of 1 Hz to Hz provide the majority of natural signal used by the MT method. Varying on hourly, daily, yearly cycles Ground Wave Lightning Solar Wind 10,000 1, Frequency in Hz

7 Applications and Benefits Exploration Base metals (nickel and precious metal exploration, as well as for kimberlite) mapping High-resistivity surface (volcanics, carbonates, igneous) Overthrust, fold belts, volcanics Can be combined with seismic or other methods to enhance understanding Explores to depths in excess of 500m, typically kilometers Geothermal Energy Mapping of very deep seated structures bearing high temperature fluids. Reconnaissance or Detail Detail: prospect definition - spacing = 250m - 500m on profiles or grids Recon: areal coverage - spacing = 1-5 km on profiles or grids Adaptable In any topography stations can be put out, in even the roughest conditions Multiple profile directions can be provided from a single survey

8 Advantages and disadvantages of MT Data Pros Great depth of penetration (10's of kms) No transmitter or source required Light-weight equipment --very portable Good production rate (2-5 km/day) Better resolution than gravity/mag Well-developed and fast interpretation procedure Little impact on environment Can access almost anywhere Cons Static Shift - Near-surface distortions to electric field created by resistivity variation at surface additional TDEM data required for correction Static shift of apparent resistivity curves sometimes significant Resolution less than seismic Data processing and modeling are complex Inversion techniques rely on smooth models, tougher to interpret in complex areas

9 Survey Planning Customized survey layouts are constructed to cover areas of interest. Denser and coarser stations layout according to size and target depth. In the field stations are placed where the best contacts are available with both safe access and within reasonable specifications. Profiles are created as per geological strike and client requirements. Data are analyzed to client specifications, either single profile directions, or multiple as shown.

10 Data Acquisition & Field Setup Five channels at each station Flexible site locations Two to five stations simultaneously No Line restrictions GPS synchronization between stations 24-hour recording/layout/pickup cycle In-field processing and editing

11 Site Deployment

12 Remote MT - Data Record The time series record, showing (from top) electric and magnetic data varying with time for five channels plus remote reference (i.e. for correction) Electric X Electric Y Magnetic X Magnetic Y Magnetic Z Magnetic X R Magnetic X R 1000 sec

13 How Resistivity is Computed Impedance tensor is measured at surface Computing apparent resistivity (and phase) as a function of frequency a 1 Ex / Hy 2 r a 5f Two values computed, r xy and r yx, for the two orthogonal pairs of Electric and Magnetic sensors in horizontal directions, Thus can interpret for strike and dip directions

14 Depth of Investigation The depth of investigation is a result of the frequency and resistivity of the subsurface (Skin depth) Lower frequency = deeper penetration Higher resistivity = deeper penetration Skin depth is an approximate estimate of depth of penetration at particular frequency and resistivity Skin depth (in meters) = 500 r / r f where = resistivity and f = frequency

15 Processing and Interpretation Process Raw time series data to create a database Multiple Tensors (components) Review all Tensors (components) for QA/QC Review the initial data to ensure it makes geological/geophysical sense Production of 1D ( sounding) models ( Depth Profiles) Perform any valid edits / filtering for noise or signal strength Produce 2D inversion models using the latest software Combine and produce 3D representations of data and inversions Create Coloured Cross-sections and Maps Integration with Geology, Drill Holes and Legacy Data

16 Many Data Components Apparent Resistivity with Tensor Data Phase Showing Tensor Data Tipper Magnetics and Tensor Data Dimensionality Typical Exploration Range

17 Data Analysis Shallow Resistive Conductive Deep Apparent Resistivity Apparent resistivity changes in 2 directions (x and y depending on layout) Deeper Conductor Phase Phase changes in 2 directions Resistive Conductive Deeper Conductor Tipper (Vertical) Magnetics Vertical Magnetics with depth Complex Complicated Induction Vectors Useful in detecting conductors Dimensionality (Polar Diagrams) Defines Layers, 1D to 3D, i.e. complexity

18 1D - Depth Profiles Defines Layering under a single station (sounding) Apparent resistivity changes in multiple layers under each station Layers are determined from near surface to great depths Models help to QA/QC all data Data collection enables kilometers of depth information This sounding shows good data to over 10km Multiple sites are combined to produce a 2D image of the property Depending on the survey design, many profiles (lines) can be extracted, giving both strike and cross strike profile information

19 2D - Sections Multiple sites are combined and inverted Mapping of conductive and resistive areas Structural Interpretation Resistive Zone Future Drill Planning

20 MT Sections 3D - Presentation Presentation of Multiple Cross-sections to Analyse Changes Property Wide Understanding Near Surface and Deep Geological Understanding Integration and Compilation of Different Data Sets in One Database Good Drill Hole Results! 3D Magnetics

21 The Exploration Process Survey Design Data QA/QC 1D Target Generation 3D 2D