An interactive approach to forward modelling and inversion

Presented at Exploration17 by Glenn Pears, Principal Geophysicist, Mira Geoscience

Exploration is becoming harder, with greater focus at depth or under cover. Decisions must be made to eliminate ambiguity and maximize the benefits from various types of collected data. In terms of integrating geological and geophysical data, the essential goal is to interpret the available geophysical data based on a 3D geological model populated with physical properties. The key is to develop an understanding of the relationships between geology, geophysical responses, and rock properties. That information can then be used to create 3D geological domains with very little ancillary information, and often no drillhole data whatsoever.

Geologically-based forward modelling and inversion of geophysical data plays a vital role in quantifying these relationships, but it is important to emphasize that inversion is only one part of the interpretation process. Integrated interpretation requires an interactive approach to forward modelling and inversion. Therefore, the process demands a shift in mindset when it comes to geophysical inversion. Rather than inverting a geophysical data set once, forward modelling and inversion are performed according to the required geological hypotheses that need to be tested. Integrating geological and geophysical data, particularly in cases with limited subsurface control, also requires a practical, adaptive, and objective-driven approach to interpretation.

First geological model with anticline fold

Updated synclinal geological model.

Case study: Cave Rocks nickel and gold project area, Australia

This is a great example of producing an integrated 3D geological model based on existing geological mapping, very sparse exploration drilling, and airborne geophysical data. When tested against geophysics, the gravity, magnetics, and AEM data conflicted with the originally modelled geology. The objective was to first combine the modelled geological contact and structural surfaces, and overburden cover, in a 3D geological block model, attribute it with homogeneous properties, optimize it through inversion, and assess the computed gravity and magnetic responses. The first impression was that the correlation between the measured and calculated responses was reasonable, but areas existed where the geophysical data conflicted with the modelled geology. The originally supplied geological information depicted the main, central fold as a south-plunging anticline, but from modelling the geophysical data, it became clear that the overall plunge direction was a north-plunging syncline. Considering that a key aim was attempting to identify the basal contact of the ultramafics, this overall change in geological understanding had a major impact on the targeting strategy and any subsequent drillhole planning. Surfaces were remodelled, the geological model was attributed with optimised homogeneous properties, and the computed response was assessed. Comparison between observed and calculated improved, particularly where the syncline plunges towards the north.

James Reid – Principal Geophysicist with Mira Geoscience

James worked closely on the above project with Tim Chalke and Daniel Eden. He has extensive expertise in the planning and quantitative interpretation of electromagnetic and electrical methods, particularly the application of airborne electromagnetics to mineral exploration. He has broad experience in airborne, surface, and downhole geophysics, including survey design, data acquisition, processing, and interpretation using GOCAD, Geoscience Analyst, and the UBC and VP Suite inversion codes.

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