Geophysics as a Mapping Tool

Transcription

1 SOCIETY OF ECONOMIC GEOLOGY EXPLORATION IN 2025 TOOLS AND TECHNIQUES TO EXPLORE UNDER COVER Using Geophysical Data for Geological and Structural Mapping Jon Woodhead, Ph.D. Principal Geologist Condor Consulting Inc. October Colorado School of Mines, Golden, Colorado Geophysics as a Mapping Tool Quick, Cheap and Effective Why wouldn t you? 10 km Magnetics (1VD) with radiometric ternary Interpretation Framework 1

2 Geophysics as a Mapping Tool Mapping geology..why/how is it possible? Airborne surveys allow rapid and inexpensive coverage Coverage is continuous Measurements are a direct representation of the physical property of underlying rock units What attributes can be resolved? Stratigraphy, Structure, etc. Burial depth and 3D orientation of structures Metamorphism, Alteration, Mineralization etc. When/where is it required Wherever the geology is regarded as a key driver of the exploration process (most cases?) Geophysical Data What geophysical data can be used? 2

3 Magnetic Expression of Geology Direct verses Indirect Targeting Indirect targeting (most common) Geological interpretation and structural analysis of magnetic data assists us by: Identifying favorable hosts (and source) rocks for mineral deposits Highlighting faults/folds that provide fluid conduits and traps for mineralization Direct targeting Kimberlites; BIF s; Some porphyry, epithermal and skarn deposits; Some IOCG s Ernest Henry IOCG Cu-Au 122Mt 1%Cu 0.5g/t Au 3

4 Indirect Targeting Yilgarn Craton Au and Ni (Western Australia) Kambalda Ni deposits St. Ives Au deposit Junction Au deposit Indirect Targeting Unconformity U (Athabasca Basin, Canada) Sub-Athabasca basement structure showing structural controls on U deposits 4

5 Direct Targeting Unconformity U (Athabasca Basin, Canada) Radiometric anomalies over the Patterson Lake South U deposit Direct Targeting Porgera Au mine Epithermal Au / Oil & Gas: PNG Highlands Oil & Gas RTP eu Porgera K% Hides & Karius anticlines Radiometric ternary image (KThU-RGB) draped over topography 10 km 5

6 Regional Geological Studies New interpretation map series: PNG Highlands (CGG, 2010) WABAG 1:250,000 map sheet (1 of 6 sheets) Solid Geology Interpretation What can we get out of the data? Distribution of structures (folds / faults etc.), stratigraphy, intrusions and alteration Form surface mapping which can be used to extend our understanding from the 2D plan view to 3D kinematics Relative fault character, timing and displacements Thin, straight (brittle, shallow, low T); Broad, discrete breaks (brittle ductile: mod P & T); or Broad, no breaks (ductile deep, high T). Development of tectonic models (local & regional); Predictive targeting: Direct anomaly detection; Direct structural analogy; and Indirect structural targeting (new models?); Extend our models outside immediate area, to other terranes 6

7 Structural Analysis Key observations Associated features: Folds (parallel / oblique); 2 nd order sediment depo centers / basins; Intrusives / extrusives; Veining / alteration; and Recognition of fault hierarchy (1st, 2nd order etc.) Strain variations inferred from block geometries: Structures at block margins; and Strain within blocks (possible modified stress fields) Regional context: Relationship to regional structures; Setting (e.g. basin, mobile belt, arc setting etc.); Orogenic events at inferred time of faulting? Reactivation of older structures, etc. Interpretation workflow Interpretation workflows typically consists of 3 stages: Observation / Planning Decide on scope (time needed/available), scale, and expected outcome Assess the availability and quality/resolution of the geophysical data Obtain complimentary geological data (maps, sections, reports/papers, etc.) and geophysical/satellite data (SRTM, Aster, regional magnetics etc.) Data Processing and Compilation Processing and imaging of all digital data (incl. 3D inversions) GIS compilation of data (using common projections and formats) *May include printing of base maps for overlay interpretation Interpretation structural framework (i.e. trends and discontinuities) domain delineation (i.e. areas with common character) integration of secondary data (e.g. published geology) map compilation 7

8 Data resolution and quality Critical to interpretation outcomes! Dictates the scale at which an interpretation can be undertaken and the achievable mapping resolution 100 m line-spacing, 50 m elevation Demagnetization (Cu-enrichment) conventional gridding gradient enhanced 1 km 400 m line spacing, 150 m elevation 300 m line-spacing Magnetic data processing A variety of derivative products are used to enhance magnetic boundaries (edge filters) or textural domains (block filters) Key products include: Reduction to Pole (or Equator) Vertical Derivatives (1VD, 2VD) Total Gradient (ASM) High/Low Pass filters, AGC etc. Tilt Angle Pseudo depth slicing frequency filters Edge and block filters 3D Inversion unconstrained Depth or elevation slicing Isosurfaces (3D DXF s) Unconstrained 3 D inversion 8

9 Magnetic data fundamentals The magnetic anomaly is complex and needs to be simplified for interpretation Magnetic data processing Example: Reduction to Pole (RTP) Reduced to pole (LL 20) Note striping Used to simplify the data so that the anomaly center is positioned directly over its causative body Problems occur at low magnetic latitudes (+ 20 ) where the field is near horizontal (e.g. Chile, W. Africa) This often requires a trial and error approach to get the best result. Reduced to pole (LL 50) N S artifacts are attenuated by using a limiting angle in the RTP (e.g. 20, 30, 40 and 50) Usually done by applying an amplitude correction angle to minimize the directional amplification or bias (but can amplify noise and artifacts ) but at the risk of attenuating real N S signatures 40 provides best outcome 9

10 Magnetic data processing or Reduction to Equator (RTE) At very low latitudes, a simple numerical inversion of the RTE (or the TMI) grids can be used to approximate the RTP Ambiguity Many possible models can interpret the same observed anomaly! (ambiguity) Gravity and magnetic data contains no inherent depth information 10

11 Forward and Inverse Modeling The problem of non uniqueness Forward modeling: Structure and physical properties are known Compute the geophysical response The process is unique Inverse modeling: We have the geophysical response but want to know the the structure and/or physical property of the model e.g. a + b = 7 The process is non-unique Radiometric data processing Radiometric data closely reflects bedrock in residual terranes (vs. transported regolith) Ratio and classified products may show areas of alteration (e.g. anomalous K) K Th U classed as high, med or low (i.e. 3 3 = 27 classes) Classified K Th U K / Th ratio Ternary K Th U (RGB) U 2 / Th ratio 11

12 Interpretation Workflow *Methodology by SRK (Australia) Extract line data from 1VD image Determine movement direction and dips on faults and dip direction of stratigraphy. Add data from other sources (radiometrics and gravity) annotate on a separate layer Extract Lithology data from RTP image. Divide area into major lithological packages. Interpret faults breaks, truncations etc. Annotate accordingly. Add exploration model parameters Add geological model parameters Add geological data from surface geology maps, drilling results identify formations and marker units Improve on unit boundaries, annotate and connect marker units from magnetics Finalize map presentation, prepare cross-sections and block model, check geology for internal consistency SRK Consulting Interpretation Workflow 1VD data SRK Consulting Stratigraphic Form Line mapping Fault/deformation zone mapping (including timing) Fold mapping (including timing) Total Field RTP data Lithology mapping Integrated interpretation 12

13 me # me me me me me # me me Data Integration Regional geophysics Published literature Local geophysics mn E E mn mn E E EE Integrated interpretation E E mn Drillhole data Regional mapping Final Products and Outcomes SRK Consulting 13

14 Semi automated interpretation A semi automated process can be employed to rapidly derive the position and accurate extent of magnetic sources by utilizing the Tilt Angle grid The Tilt Angle is the ratio (restricted to ±90 degrees) of the vertical and horizontal derivatives and is used to define the edges of magnetic sources The zero angle provides a good approximation of source edges, while the horizontal distance between the +45 and 45 contours gives an approximation of depth. 4 km deep Actual edge of magnetic body 8 km deep Forward model Tilt depth output Salem et al. (2010) Magnetic anomaly map Individual magnetic sources are extracted as discrete tilt angle values and colored by relative magnetic susceptibility (here dikes are colored black) Provides improved spatial resolution 14

15 Interpretation workflow Solid geology interpretation is built on geophysically constrained contacts (magnetic domains), thus limiting interpretation bias Each domain can be directly characterized by its geophysical attributes Classification Domains Fabric Sources 1 km *Classification of geophysical domains based on average magnetic intensity values Magnetic Domain Classification Magnetic sub domains can be assigned value statistics according to the original (RTP) magnetic data The output is a geophysical domain classification and structural interpretation map (not necessarily lithologically assigned) 1 km 15

16 Interpretation outcomes Geophysical domain classification provides a proxy for geology in covered areas or in the absence of outcrop (or no detailed mapping) Provides a detailed representation of the magnetic data in a geological context that can be used to as a base for field mapping The patterned overlay shows the magnetic texture based on the total gradient Color domains (9 classes) represent the main variations in magnetic amplitude, i.e. reflect major rock units 1 km Interpretation vs field mapping Geophysical Domain Classification Published geology (1:100k) marble, carbonate schist quartzite ironstone schist alluvium 1 km Magnetics Radiometrics 16

17 Final Products Interpretation Synthesis Integration of all geological and exploration datasets to derive structural histories and prioritized exploration targets. 17

18 Exploration Targeting Orogenic gold (West Africa) Large scale fold closure on regional shear zone, with proximal intrusions Local concentration of dikes suggest deep seated cross structure Association with anomalous magnetic units and elevated K (? felsic intrusions)? high K intrusion?? Interpretation Guidelines Guiding Principals Airborne geophysical data shows the continuous distribution of magnetic radioactive, conductive/resistive or dense minerals, from which the geometry of the rock units can be resolved Interpretation is designed to translate the geophysical data into a form suitable to a broad exploration user group. The aim is to improve our knowledge in such a way as to significantly improve exploration efficiency and success Structural geology is a natural partner since it also relies on geometrical patterns Models (structural/stratigraphic) are needed to guide the process, but Interpretation needs to be objective (i.e. the data needs to tell its own story and not be used to confirm/reject pre existing geological mapping). Not an end in itself, but is a first step towards a coherent 3D geological model 18

19 Interpretation Guidelines Guiding Principals (cont.) The objective is to produce a map which depicts all relevant and appropriate geophysical features in the context of the known geology. This product is designed to be used in conjunction with the outcrop geological map to develop a 3D solid geology interpretation. This integrated product is a record of the combined geological and geophysical observations, from which a range of interpretations and analysis can be derived The map should provide the user with a relatively non interpretive set of observations from which the user can also develop their own ideas The knowledgeable user will use the this map in conjunction with the original raw data to test hypotheses and progressively build and refine the geology of a project area. Interpretation Guidelines Additional 19

20 Interpretation Guidelines Mechanics of Interpretation Flexibility is required to take into account a wide range of geophysical responses that can be observed across different geological environments Data resolution and spatial limitations should guide outcomes may require a trade off between aesthetics and geological data since there is generally more information in the data than can be presented in a single map product. Multiple scales are needed to integrating the regional with the detail Workflows are required that involve three or more distinct stages (a crack map is not an end in itself ) Recycling the best work usually comes from the second (or third!) pass. Thinking time and revision is essential! Geophysics does not lie. There are very few non geological contributions to airborne geophysical data don t dismiss unexpected features as spurious Interpretation Guidelines Don t ignore the spurious data 20

21 Interpretation Guidelines When relating lithology or stratigraphy to magnetics, think: Which rocks contain the magnetic minerals (field evidence, susceptibility measurements, petrology)? Do these rocks always contain magnetic minerals in this area? How and when did the magnetic minerals form? Be wary of making the following generalizations: The XYZ Formation is highly magnetic Is it?? Always?? Everywhere?? Uniformly?? The igneous and metamorphic rocks will be more magnetic than the sedimentary rocks They are frequently not!! The mafic rocks will be more magnetic than the felsic rocks They are frequently not!! Think in terms of the magnetic rock body in the ground. not the magnetic field it causes. (after SRK Consulting) Interpretation Guidelines Key Structural Questions: What structures occur? What is their extent? Relative geometry? What strain was produced? What P/T conditions did they form at? What is the 3D geometry? What was the tectonic driving force & history? What is the relationship of all this to mineralization? Ontario Superior/Grenville Compilation, 400m line spacing, RTP1VD. Wabigoon subprovince. (after SRK Consulting) 21

22 Geological Models for Mapping Geological models are a primary component of the interpretation process for producing maps; In order to present an interpretation of the geology of the region, there needs to be a coherent framework of stratigraphic and structural principles that form the basis of the interpretation; A coherent regional geological pattern can be followed through the series of maps; Individual structures can be interpreted in both a time and space context. We can interpret when and how faults moved; The spatial distribution of mineral deposits in relation to structures becomes evident, and can be related to the geological evolution of the region, not just the geometry; Areas and structures with potential for reactivation at later times become apparent; and Cross sections can be developed which provide a realistic 3D form consistent with the geological models. (after SRK Consulting) Interpretation Defining magnetic domains Source: CGG (prev. Fugro) 22

23 What is an Airborne Geophysical Survey? GPS Satellite Navigation Specially Modified Survey Aircraft Aircraft Height Determined by Radar or Laser Altimeter Fixed-Wing m Heli < 80m Flight Lines 100m - 1 km Apart (Black Lines) Tie Lines 5 10 x Flight Line Spacing (Red Lines) 10 km to 500 km 45 Typically Fly 400 to 1500 km per day 23