How can we make the most of magnetic data in building regional geological models?

Transcription

1 How can we make the most of magnetic data in building regional geological models? Clive Foss, Tony Meixner (Geoscience Australia) and James Austin MINERALS DOWN UNDER FLAGSHIP

2 Excellent national magnetic field coverage readily available through GADDS Regional magnetic field data (much from State and Territory pre-competitive data initiatives) is the most widely used geophysical data in greenfield exploration, geological mapping and the building of 3-D geological models. The data can be downloaded via the Geoscience Australia GADDS utility. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 2

3 Australian Crustal Elements Map Based on the distribution of geophysical domains This regional magnetic field data is a major source of information in mapping basement geology and structure from tenement to national scale. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 3

4 Australian Crustal Elements Map Based on the distribution of geophysical domains Definitions used in the index highlight how significant the geophysical input to this map is in particular the input from magnetics.. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 4

5 Overview Review the inverse problem of estimating source depth from magnetic field data Suggest a process for optimal recovery of source depth from a regional magnetic field data set, with associated quality factors This talk will firstly consider the challenges of recovering source information from magnetic field data (the inverse problem of working from the field to its source), and then outline an approach that CSIRO and Geoscience Australia are investigating to value-add to the primary field data. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 5

6 Flight-path for a typical regional aeromagnetic survey Flight lines 400m ties 4km 10 km Aeromagnetic surveys are flown on a regimented highly uniform flight plan, with time, effort and resources generally deployed consistently over the survey area. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 6

7 Source information distribution 10 km The distribution of information recovered from the survey is generally non-uniform. In this example (a TMI image from a survey over the Capricorn Area in WA) source information is available at the many flight-line intersections with dykes. That information is specifically about one component of the geology the dykes and by inference the elevation of the basement surface which is interpreted to be the surface at which they are truncated. Very little reliable information can be recovered about other components of the geology. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 7

8 Source information distribution 10 km In this nearby area the distribution of information is highly irregular. Away from the influence of the dykes it is possible to interpret magnetic field variations due to other basement units. The geological source of these magnetisations is uncertain, and there is less justification of assumptions of simple source geometry that are required for their interpretation. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 9

9 Insensitivity to source depth Depth offset identical sources (appropriate to looking for a known source) 20% shallower 20% deeper This figure shows the magnetic anomaly profiles over a tabular source of large strike and depth extent (such as can be used to represent a dyke). The problem of finding the depth to this otherwise fully specified source can be solved with high sensitivity. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 10

10 Insensitivity to source depth Depth offset inversion-matched sources (appropriate to looking for an unknown source) Anomaly changes due to depth variation can be mostly compensated by changes in other parameters especially magnetisation and thickness This figure shows the magnetic anomaly profiles for the case that the shallower and deeper sources have been inverted to have anomalies most like that of the source at its reference depth. The changes are achieved by appropriate adjustment of thickness and magnetisation, and highlight the much lower sensitivity of the common geophysical inverse problem in which depth is only one of several unknowns. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 11

11 Properties of the best-fit models at different depths The best-fitting solutions at different depth show an increase in magnetisation and decrease in thickness with depth. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 111

12 Thickness-magnetization product The best-fit model has an approximately linear increase in the magnetisation-thickness product with depth. These anomalies are highly suitable for depth estimation, but at depths in excess of about 500 metres there are few dykes that generate anomalies of sufficient amplitude to be inverted reliably. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 13

13 Evaluating sensitivity to depth Non-uniqueness prevents derivation of true error statistics in any a priori analysis, but as a proxy we can at least supply model parameter sensitivity estimates. These cannot be used in any exact discrimination to accept or reject a model, but they are better than having no quality measure. uncertainty for known source Level at which the misfit is proposed to be significant (interpretational) uncertainty for unknown source Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 15

14 Almost all the reliable information in a magnetic field image is in the discrete anomalies We can consider the sweet spots in magnetic field data where all of the reliable source information is concentrated as equivalent to the holds on an otherwise smooth climbing wall. They are the only features that are of any use in reaching an interpretation objective. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 19

15 Manual Workflow Select sweet-spot data Adjust the strike length and azimuth of bodies to fit the map image Run individual inversions to derive optimum models We term this process as sharp focus magnetic field interpretation (or more commonly sharp focus magnetic source depth estimation). The sharp focus refers to the isolation of features and the concentration of effort rather than the quality o the output. This method is bound, as are all others, by non-uniqueness, and it would require case by case post-priori ground trothing to establish that it provides superior solutions. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 150

16 Manual Method Case Study the Waukarlycarly Basin, WA The Waukarlycarly Graben in WA is one area where this analysis has been performed. The TMI image above shows many high-amplitude, short-wavelength anomalies over shallow basement, and areas of lower-amplitude, longer wavelength anomalies, interpreted to be due to deeper burial of the basement containing the strong magnetisations. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 161

17 Waukarlycarly Basin, WA Selected profile inversions Individual anomalies were selected from flight lines (for the shallower sources) and grid traverses (for the deeper sources) for anomaly by anomaly analysis. The wireframe traces in the image to the right show interpretations of a surface extrapolated along sections to define a basement surface through the tops of the source bodies. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 172

18 Waukarlycarly Basin, WA The images above show a faceted surface derived from those interpreted depths sections (left) and a gridded surface with a draped image of the vertical derivative of RTP. These are interpretive products, whereas the depth solutions themselves are essentially analytic solutions. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 183

19 Semi-Automated Workflow Select sweet-spot data Generate initial source solution Upgrade solution and convert to a model Use inversion to further upgrade the model and compute sensitivities Web-based delivery We envisage that various groups across Australia might undertake exercises using manual depth estimates, the results of which can be captured in a national database, but given the volumes of magnetic data in Australia an automated or at least semi-automated process is also required. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 194

20 AutoMag A moving-window profile-based depth estimator Based on an algorithm by Henri Naudy Further developed by Zhiqun Shi at Adelaide University Implemented in several software packages: ModelVision and Intrepid Superior because it generates solutions sufficiently defined to generate forward computation models AutoMag is a key step in our proposed workflow, allowing conversion of depth estimates to model source bodies available for subsequent inversion. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 205

21 Detail of the magnetic image Line spacing 400 m 7 km Terrain clearance 60 m This example from the Elkedra area of the NT illustrates the ability to generate bodies representing geological magnetisations from magnetic field data in a semi-automated process. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 217

22 AutoMag solutions on a flight-line Similarity coefficient TMI Depth solutions Troughs in the similarity coefficient cutting below a critical threshold generate solutions which can be considered seed bodies. In this case the analysis has been run on a vertical derivative filter of the TMI. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 228

23 Converted AutoMag solutions Vertical derivative TMI (black) and computed (red) Converted Depth solutions Bodies converted automatically from those solutions (including a strike correction) match the vertical derivative of TMI (top track). It mostly needs an appropriate regional field for those solutions to also fit the TMI (middle track). Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 239

24 AutoMag solutions converted to source bodies A perspective view of the source bodies from the inner anomaly around this refolded fold show that it an anticlinal structure. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 30

25 Web Delivery We have an appropriate delivery system already developed for the Remanent Anomaly Database We already have experience at developing suitable utilities to deliver the solutions from both manual and semi-automated analyses. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 255

26 Australian Remanent Anomalies Database Currently 250+ solutions Magnetization details and 3D source models The Australian remanent anomalies database in the Auscope portal has a structure that provides an excellent template for delivery of solutions from manual analysis. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 33

27 Australian Remanent Anomalies Database 30,000 solutions First step towards mapping igneous and thermal events beneath cover A second layer in the Auscope portal of automatically generated magnetisation estimates across Australia provides a suitable template for storage and delivery of automated or semi-automated depth solutions. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 34

28 Automated Mapping of Magnetization Direction Across Australia Inclination Rotation from the geomagnetic field Declination Regional Magnetization Mapping using a search algorithm Different displays of data from the remanent anomalies database automated layer. Similar multiple displays could be generated for the sharp focus magnetic depth solutions, reporting parameters such as depth, magnetisation, width, trend, strike length, etc. Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 35

29 Conclusions Magnetic field data only contains useful source information at specific locations above suitable subsurface magnetisations The optimum recovery of source depth information is to perform intensive individual inversions of those carefully selected data packets Non-uniqueness prevents derivation of true error estimates, but model sensitivity values can be used as proxy quality attributes Storing these results in a national database will provide a resource for interpreters to build their products from standardised, high-quality analytical solutions Making the most of regional magnetic field data Clive Foss, Tony Meixner, James Austin Page 36

30 Thank you CSIRO Mineral Resources Clive Foss James Austin Geoscience Australia Tony Meixner t e clive.foss@csiro.au MINERALS DOWN UNDER FLAGSHIP