No abstract

Quantitative estimates of denudation rates in different tectonic and climatic environments are of fundamental importance to an understanding of long-term landscape development. Little quantitative information is currently available on minimum denudation rates, although this is important in placing constraints on the maximum survival potential of individual landforms or erosion surfaces in terrestrial environments. The persistence for up to >15 Ma of a hyper-arid, polar climate in the ice-free Dry Valleys area of southern Victoria Land, Antarctica, makes this a highly probable environment for minimum terrestrial denudation rates. 40 Ar/ 39 Ar dating constraints on the mimimum formation ages of individual landforms indicates generally little modification of even minor features over the past few million years. Relatively, the highest rates of denudation occur at low elevations near the Ross Sea coast. By contrast, rates of landscape change are exceedingly slow at elevations above c. 1500 m in the western Dry Valleys where landforms with a relief of a only a few metres have survived with minimal modification since the mid-Miocene. Measurements of in situ -produced cosmogenic isotopes indicate that even on rectilinear slopes maximum denudation rates may be only c. 1 m Ma −1 , with rates falling to <0.2m Ma −1 at some locations on low relief surfaces at high elevation.

SUMMARYThe presence of chargeable materials can significantly impact the data in electromagnetic (EM) surveys. This affected data has traditionally been treated as noise that must be removed prior to interpretation or inversion. The ability to extract induced polarization (IP) information from an airborne platform would be a valuable tool in the mineral exploration industry, and thus the pursuit of this ability has recently led to significant interest in the interpretation of IP effects in airborne data. A variety of interpretation methodologies have been proposed to aid in the identification and extraction of information from time domain EM data containing IP effects. Any interpretation scheme needs to be thoroughly tested on realistic synthetic examples so that the strengths and weaknesses of the method are well understood.In this work, we present a methodology for accurately and efficiently simulating the response of a time domain EM experiment by modelling the convolution that occurs in Ohm's Law in the presence of a frequency dependent conductivity. This method is free of any assumptions about the dimensionality or frequency dependence of the chargeable material and can be used to simulate the response of any time domain system.

The southern McArthur Basin in Australia's Northern Territory is host to some Tier-1 sediment-hosted base metal mineral deposits including the McArthur River Zn-Pb-Ag mine. Airborne electromagnetic (AEM) data sets have been employed as a key exploration technology in the search for these mineral systems. A geological interpretation of results arising from the use of different inversion techniques, including a 1, 2.5 and 3D methods, was undertaken on a helicopter EM data set acquired over a structurally complex sediment package in the Batten Fault Zone north of the McArthur River Mine. The exploration targets were conductive, mineralised units (HYC pyritic shale member) associated with the Barney Creek Formation. Results from this study suggested that although the model fits were good, the derived conductivity models for the 2.5D and 3D inversions appeared to be smooth representations of geological reality, particularly when compared with data from drilling and surface geological mapping. Superficially, the 1D smooth model layered Earth inversions appear to map geological variability and structural complexity in greater detail even though the structures are more 3D in nature. IP effects are observed in the data and influence the modelled structure, but can be accounted for and complement the non IP 1D inversion results. The outcome of this study also indicates that when employing higher order inversion methods in the interpretation of AEM data sets, there may be significant benefit in asking a contractor/consultant for 1D inversion results as well. In the resulting interpretations if conductors appear in one but not the other, it is worth asking the question why?

SUMMARY3D airborne electromagnetic (AEM) inversion has routinely been applied to frequency and time-domain problems over the past few years, however this research field continues to undergo rapid improvements with the implementation of new ideas and faster computational resources. To keep pace with these developments, we have rewritten our 3D AEM inversion software suite to leverage the rapid growth in parallel processing, and to create a flexible inversion framework capable of standard inversion plus many additional types: joint, cooperative or parametric, all on semi-structured octree meshes. Our resulting framework further improves recent key ideas such as the decoupling of forward meshes from the inverse mesh, to allow the forward problem to be easily distributed on separate nodes of a cluster for fast and efficient modelling of the fields.We present two large-scale field examples, one in the frequency domain and one in the time domain. The frequency domain survey demonstrates our ability to recover thin conductors, in this case representing orogenic gold targets, across a large region (40km x 35km). The time domain example focuses on a smaller area within a larger survey area where mapping groundwater resources is the primary goal. Here the fine-scale results are compared to a 1D inversion, and we see a good correlation between the 3D and 1D results due to an approximately 1D layered-earth environment. However we see a removal of 1D artifacts in the neighbourhood of vertical conductors and topographic changes in the 3D result with the added bonus of information between lines in which decisions regarding groundwater management can be made.