The aim of this atlas is to provide examples of the relationship between three-dimensional structure and potential-field response. We have used the Noddy modelling system, which was developed as a result of an AGCRC/AMIRA/ARC project. This allowed us to create a variety of structural models which allow interpretive skills to be developed, through the specific comparison of structures and their responses. These models also provide a starting point for the interpretation of actual survey results. All of the history files used to create these models are provided in digital form, so that in combination with the Noddy software, variations to the models can be easily examined. In addition this addition of the atlas contains wavelet transforms of the data so that the interpretive skills needed for this new visualisation technique can be learned.

In order to reduce printing problems, a PDF version of this Atlas is also available, and a PDF version with embedded links also exists.

This Table of Contents lists each page in text form, the Image Index (much slower to load) contains one example image from each page, and the Help page describes the meaning of each element in a page, and how to configure your browser to load the various file types. The atlas contains a complete set of images models calculated for both Southern and Northern Hemispheres, and each set can be accessed separately from the home page of the Atlas.

SECTION 1 BASIC INTERPRETATION PRINCIPLES In this section a number of basic intepretation principles are reviewed. The model geometries are kept very simple so that the effects of depth, latitude, and possible causes of potential-field anomaly asymmetries can be separated from the more complex issues of three-dimensional structures. Many of these principles can in fact be demonstrated in two-dimensions using profiles, and the reader is encouraged to draw profiles across the data sets in order to see these effects.

1.1 The effect of depth on anomaly dimensions in gravity data

1.2 The effect of depth on anomaly dimensions in magnetic data

1.3 A cross section through the gravity and magnetic fields

1.4 Vector components of a magnetic field

1.5 The effect of changing latitude on anomaly shapes in magnetic data

1.6 Asymmetries in magnetic and gravity data

1.7 Magnetic inclination and declination effects for complex structures

SECTION 2 SIMPLE STRUCTURAL TYPES In this section the potential-field response of simple structures is displayed. In some cases some earlier feature, such as a dyke, has been added to clarify the point being made. This chapter concentrates on contrasting different deformation geometries and demonstrating the effects of structurally controlled or field inclination controlled anomaly asymmetries.

2.1 FOLDS

2.1.1 Variation in fold profile

2.1.2 Variation in fold plunge direction of sinusoidal folds

2.1.2b Variation in fold plunge direction of sinusoidal folds (continued)

2.1.3 Variation in fold plunge of sinusoidal folds

2.1.4 Ambiguities in the interpretation of sinusoidal folds

2.2 FAULTS

2.2.1 Variation in fault geometry

2.2.2 Variation in fault dip direction of low susceptibility footwall faults

2.2.2b Variation in fault dip direction of low susceptibility footwall faults (continued)

2.2.3 Variation in fault dip direction of high susceptibility footwall faults

2.2.3b Variation in fault dip direction of high susceptibility footwall faults (continued)

2.2.4 Variation in fault dip

2.2.5 Interpretating fault offsets

2.3 UNCONFORMITIES

2.3 Unconformity Geometries

2.4 INTRUSIONS

2.4.1 Simple Plug Geometries

2.4.2 Variation in Dip Direction for a Thin Dyke

2.4.2b Variation in Dip Direction for a Thin Dyke (continued)

2.4.3 Variation in dyke dip

SECTION 3 COMPLEX STRUCTURES This section provides a number of examples of the interaction of two or more episodes of deformation, some derived from specific locations, others simply to demonstrate scenarios which may or may not be resolved by using the magnetic or gravity data sets.

3.1 Faulted dyke

3.2 Faulted Fold

3.3 Basin Setting (Flat-lying sediments)

3.4 Block faulted, rifted and folded region

3.5 Fold and Thrust setting

3.6 Dome and Basin setting

3.7 Fold Interference Patterns

SECTION 4 TOPOGRAPHIC EFFECTS This section provides two simple examples of the effects of topography on potential-field data. The two normal survey modes of draped and barometric flying are compared.

4.1 Horizontal stratigraphy

4.2 Dipping stratigraphy

SECTION 5 REMANENCE AND ANISOTROPY This section demonstrates the effects of a uniform or variable remanent magnetisation component, and a uniform or variably oriented magnetic anisotropy. A comparison of alteration haloes and remanent magnetisation haloes around igneous bodies is also made.

5.1 A remanently magnetised sphere

5.2 Remanence and folding

5.3 Anisotropy and folding

5.4 Concentrically zoned plugs

SECTION 6 ALTERATION ZONES In this section two examples are given which compare the effects results of having alteration haloes associated with igneous intrusion, for regions with pre-existing structure.

6.1 Depletion alteration halo around a dyke

6.2 Enrichment alteration halo around a plug

APPENDIX A: GEOLOGICAL MODELLING In this appendix the geometries resulting form each type of structural event are displayed for a chequerboard model.

Appendix A: Geological Modelling Events

APPENDIX B: Wavelet Transforms This appendix includes two papers describing the basis for the wavelet transforms models are given. In addition, a number of VRML models of 3D structures are provided which can be viewed interactively..

Appendix B: Wavelet Transforms

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Southern Hemisphere Edition