Inversion of AEM data at a survey scale utilising new inversion solver and forward model

Transcription

1 Inversion of AEM data at a survey scale utilising new inversion solver and forward model Jovan Silic (), Rod Paterson (Intrepid Geophysics), Des FitzGerald (Intrepid Geophysics),

2 Outline: Describe the history of the 2.5D (2D Section, 3D Source) AEM development Outline the new developments and the advantages of the new GSVD adaptive inversion solver Report on parallelisation and performance Illustrate how AEM inversion is integrated with the 3D geological modelling software Show differences between 1D, 3D and 2.5D inversions on real survey data and highlight the benefits of 2.5D.

3 Why 2.5 D? 1D creates a number of artefacts if the section significantly deviates from 1D layering (e.g. dipping conductors, contacts, complex). 3D requires solving for millions unknowns for a large 3D model. Inversion at a survey scale is currently (may be) impractical. Depending on line spacing for many surveys true 3D data is not collected. 2.5D requires solving for only 10 s of thousand unknowns and is accurate for geology that is reasonably 2D within the footprint of the AEM system. It is feasible to invert data on a survey scale.

4 The Forward Model Algorithm

5 Maxwell s Equations Solutions 1D solutions are analytical 2.5D (2D geology, 3D source) solutions are numerical Uses Finite Element Method (FEM)

6

7

8 No New Inversion Algorithm Mapping Known Deposits Geology

9 No Mapping Known Deposits Geology

10 Inversion Equation Solution : GSVD on G and L matrices and use of Relative Singular Value Truncation parameter ( RSVT ) : Decomposition : Data / Model Transformation No = Regularization Parameter Mapping Known Deposits Geology

11 Example of inversion / modelling algorithm No Mapping Known Deposits Geology Survey Data over a syncline, using X and Z Components

12 Using Z component only No

13 Using X component only No

14 Using X and Z components No X and Z components give a more constrained solution.

15 Mapping known deposits Mapping Known Deposits Geology No and geology

16 Mapping Known Deposits Geology No Not all deposits have simple geometries.

17 No Mapping Geology Comparisons with CDI, 1D and 3D Mapping Known Deposits Geology

18 No Mapping Known Deposits Geology

19 PATERSON AEM TEMPEST - Kintyre Example Geoscience Australia s Onshore Energy and Security Program Fitzpatrick, A., ASEG Workshop, 2012 TEMPEST 25 Hz system 120 metre flying height 1 km spaced EW Lines Cameco sponsored infill at 200 metre line spacing

20 PATERSON AEM TEMPEST - Kintyre Example Inversion Comparisons GM 1D and 2.5D with GA LEI 1D and EMVision 3D

21 Managing Water Resources

22 Managing Water Resources Example SkyTEM example from Menindee Lakes, Broken Hill Fault Offsets

23 Managing Water Resources Fine Detail & Topography Peace River - Alberta Fault? 2.5D Shows big improvement in steep topography and greatly improves definition of second deeper conductive layer

24 No Imaging Very Deep Mineralized Conductive Targets Mapping Known Deposits Geology

25 Conductivity Section 10130, Lalor Lake

26 2.5 D Conductivity Sections with drill holes Drilled geology suggests that the top of the broad low dipping conductive feature is related to the Hanging Wall Fault or Lalor-Chisel contact/unconformity, Caté et, 2014, below which a zone of strong alteration hosts the known VMS deposits at Lalor and Chisel Lake. The conductive feature is associated with the first appearance of logged schistose and fragmental rocks in drilling i.e. CH00305.

27 A 3D Perspective View of Conductive Horizon Relative to Lalor Deposit ( 2.5 D ) The published 3D inversions show less structural detail, more coarsely defining the general Lalor deposit and alteration halo.

28 Results from inverting data at a survey scale Mapping Known Deposits Geology No

29 VTEM : Line No Mapping Known Deposits Geology 2.5 D Inversion Section : All late time features are bedrock conductors

30 2.5 D Inverted Sections : View from NW No Mapping Known Deposits Geology

31 2.5 D Inverted Sections : View from SW No Mapping Known Deposits Geology

32 2.5D AEM inversion in 3D geological mapping using new adaptive inversion solver and forward model Conclusions: Examples indicate that 2.5D inversion can add significant accuracy and detail even where the 1D assumption is mostly met. The ability to image 3D structure in 2.5D inversions is compelling from a geological perspective. 2.5D Inversions do not suffer from the loss of resolution common in 3D inversions and can be used to invert data on survey scale. For many AEM surveys, the line spacing is too broad to allow sensible 3D inversion making it difficult to invert whole survey data sets. The time penalty between 1D and 2.5D inversions is high but our efforts to take advantage of modern parallel architectures improve this greatly; speed can be improved further.

33 2.5D AEM inversion in 3D geological mapping using new adaptive inversion solver and forward model ACKNOWLEDGMENTS The authors wish to thank CGG for providing the HELITEM survey data from Lalor. Hudbay Minerals Inc. for providing helpful comments and some relevant drillhole geological data for Lalor. We also wish to thank Minotaur Exploration Ltd for providing their VTEM max survey data and permission to publish results.