Exploration Geophysics - Volume 36, Issue 4, 2005

The accurate prediction of seismic traveltimes is required in many areas of seismology, including the processing of seismic reflection profiles, earthquake location, and seismic tomography at a variety of scales. In this paper, we present two seismic applications of a recently developed grid-based numerical scheme for tracking the evolution of monotonically advancing interfaces, via finite-difference solution of the eikonal equation, known as the fast marching method (FMM). Like most other practical grid-based techniques, FMM is only capable of locating the first-arrival phase in continuous media; however, its combination of unconditional stability and rapid computation make it a truly practical scheme for velocity fields of arbitrary complexity.

The first application of FMM that we present focuses on the prediction of multiple reflection and refraction phases in complex 2D layered media. By treating each layer that the wavefront enters as a separate computational domain, we show that sequential application of FMM can be used to track phases comprising any number of reflection and transmission branches in media of arbitrary complexity. We also show that the use of local grid refinement in the source neighbourhood, where wavefront curvature is high, significantly improves the accuracy of the scheme with little extra computational expense.

The second application of FMM that we consider is in the context of 3D teleseismic tomography, which uses relative traveltime residuals from distant earthquakes to image wavespeed variations in the Earth’s crust and upper mantle beneath a seismic array. Using teleseismic data collected in Tasmania, we show that FMM can rapidly and robustly calculate two-point traveltimes from an impinging teleseismic wavefront to a receiver array located on the surface, despite the presence of significant lateral variations in wavespeed in the intervening crust and upper mantle. Combined with a rapid subspace inversion method, the new FMM based tomographic scheme is shown to be extremely efficient and robust.

The three main variables that determine the elastic response of sediment within a clastic environment are porosity, mineralogy, and pore fluid. The number of the elastic variables offered by seismic data does not exceed two (e.g., the impedance and Poisson’s ratio). Therefore, resolving seismic data for reservoir properties is formally unattainable, even if a perfect elastic rock physics model exists. We solve this problem by utilising an additional petrophysical link, the one between the clay content and total porosity, which is often observed in a dispersed-shale environment. First, we establish a rock physics model. Next, we identify the reservoir through a combination of impedance and Poisson’s ratio. Then, within the reservoir, we invert the P-wave impedance for both the total porosity and clay content by using two relations: (a) the rock physics model that links the P-wave impedance to the total porosity and clay content; and (b) the petrophysical model that links the clay content to total porosity. The results are accurate when the method is applied directly to well-log data. Most importantly, we obtain accurate reservoir property estimates by using the method with upscaled well-log curves, which demonstrates its relevance to real seismic data.

The regolith obscures much of Australia’s bedrock geology, posing problems for mineral exploration under cover. Gravity and magnetic gradient tensor data can provide significant improvements over established potential field techniques by producing maps showing subtle variations in field data which relate to the subsurface geology, but which are hidden to standard total or vertical field measurements. We examine the forward potential field response of a complex three-dimensional regolith model that contains targets such as palaeochannels, land mines, and mineralisation. By summing the field responses from many small elementary cubes, it is possible to represent complex structures to yield the full gradient tensor response at a specified height above the ground.

The gravity gradient tensor data from the model ranges over values from –0.12 to 0.2 Eövös, and the magnetic gradient tensor data for the model ranges from -1000 to 600 nanoTeslas per metre (when measured at the surface), over typical regolith-related features. When a flight height of 80 m is used, the responses diminish considerably, and only the long-wavelength features of the model are detectable.

Model response values are compared to the measurable responses from existing acquisition systems, and it is shown that the magnetic gradient tensor case is most suited to regolith investigations. The resolution required for the gravity gradient tensor appears to be higher than is possible with current gravity gradiometers.

The full potential of airborne electromagnetic (AEM) applications requiring sub-metre depth accuracy cannot be achieved if variable geometry between the receiver and transmitter is neglected. Methods involving photography, laser ranging, and aerodynamic computations have too many limitations to be of general use. We utilise the primary-field technique for estimating the dynamic bird position, assuming a free-space approximation. This procedure is expected to be accurate at high altitude or over a very resistive ground. In these circumstances, averaged bird offsets, relative to the transmitter, can be compared to the nominal receiver bird offset. When using AEM for bathymetric mapping of shallow seawater, the primary field is distorted by conductive seawater, and an inductive-limit approximation is used to correct the primary field. We compare the bird offsets obtained from both uncorrected (free-space) and corrected (inductive-limit) primary fields from survey data recorded over seawater, in order to understand the limitations of these two methods.

Primary field values at the receiver increased when flying from a resistive ground to a conductive ground. The increment was greater for the vertical component than the inline component and was more pronounced at 25 Hz than at 12.5 Hz. Our analysis shows that for 25 Hz survey data over seawater, the inductive-limit approximation is an improvement on the free-space approximation, but still does not give the same range of receiver offsets as those obtained from high-altitude primary field data. Averaged bird offsets at high altitude were found to differ from the expected nominal receiver offsets. At 25 Hz, there is only agreement within the upper limits of experimental error. At 12.5 Hz, only the horizontal offsets are in agreement within experimental error. The accuracy of bird offsets at survey altitude is estimated by comparing them with those obtained at high altitude. Differences greater than several metres were observed. If errors greater than several metres in the bird position cannot be tolerated, then the values derived from the primary field method should not be used and alternative methods must be investigated.

A simple, useful, and practical tool is proposed for the interpretation of moving-loop time-domain electromagnetic (TEM) surveys over two-dimensional (2D) sheet-like conductors embedded in a resistive host. It is based on the relationship observed between attributes of the displayed responses and the geometrical and electrical parameters of the conductive ore body. The ore body is modelled as a plate conductor for which the depth, dip, and conductance parameters are estimated. Numerical and scale modelling are used to establish the interpretative expressions. Responses computed for the various plate parameters are classed according to the following response attributes: time constant, asymmetry, and peak-to-peak distance. Three expressions relating depth, dip, and conductance to the response attributes are determined using multiple linear regression. The relationships are validated using scale-modelling data. The method allows the determination of the plate depth, conductance, and dip with an accuracy of ±10%, ±10% and ±5° respectively. The method is tested on SIROTEM survey data from Chutes-des-Passes in Quebec (Canada), where drill hole information is available. The results show that the regression relationships provide accurate estimates of the basic characteristics of the deposits.

We interpret airborne EM response data recorded by the GEOTEM 12.5 and 25 Hz systems flown over shallow seawater, using conductivity-depth imaging (CDI) to estimate sea depths to 65 m. We observed non-monotonic decay in the vertical component of the 25 Hz data recorded at survey altitude. Nonmonotonic decay had an adverse effect on the CDI results when the processing was run with positivity constraints. The removal of positivity constraints resulted in significant improvements in the quality of CDIs for interpreting water depth. Layered-earth modelling further showed that this non-monotonic decay in the 25 Hz data was due to variations in the transmitter-receiver geometry over the highly conductive seawater. Currently, 12.5 Hz airborne EM is unsuitable for bathymetric mapping unless latetime system noise can be reduced. CDIs from dBx/dt component data at 25 Hz provide the most accurate interpreted sea depths. For this dataset, the use of B-field responses computed from dB/dt observations does not offer any clear advantage over dB/dt data for interpreting sea depths from CDIs.

Separation or layer filtering of regional and residual magnetic fields is an important component of magnetic interpretation. Separation filtering depends fundamentally on the concept of random distributions of sources within discrete layers, and assumes that there is no statistical difference in response along each ideal layer and no correlation between the distributions in different layers. Separation filtering becomes very difficult when there is considerable overlap in the spectra of individual depth ensembles. The degree of separation achieved depends on the depth differences and the spectral b/B ratio, the ratio of the amplitudes of the shallow and deep ensembles. A high b/B ratio is needed to separate the effects of shallow sources with minimum contamination by deeper sources. In practice, it is usually impossible to achieve complete separation because the problem is non-linear and spectra have too much overlap, but the results are still a very powerful interpretation aid when used qualitatively (Jacobsen, 1987). Selection of a suitable filter depends both on the magnetic signature of the target source and on the data quality. Both tonal (amplitude) and textural information are needed to recognise type patterns characteristic of the magnetic expression of geological features of interest.

It is well known that derivatives of potential fields enhance the field component associated with shallow features and de-emphasise the field from deeper sources. Fractional vertical derivatives provide an objective, flexible approach to shallow-layer separation filtering, as the order of the fractional derivative can be selected to match the data and optimise enhancement of the shallow field component. The method avoids the uncertainties in selecting spectral matched filter parameters. Fractional derivatives of different order can be combined to produce RGB images and this can be a significant aid to the interpretation of the data. Finally, the order of the vertical derivative can be varied across the dataset, based on local statistics, to produce balanced derivative images that show detail in both ‘smooth’ and ‘rough’ regions simultaneously.

The application of fractional-derivative separation filtering is illustrated using high-resolution aeromagnetic data covering the Ghanzi-Chobe fold belt in Botswana. Total magnetic intensity data are dominated by crystalline basement anomalies. Progressively increasing the order of fractional vertical derivatives provides rejection of deeper basement anomalies and provides better resolution of subtle supracrustal anomalies than the conventional vertical gradient. Varying the order of the vertical derivative across the dataset provides a local rather than global filtering capability, unlike conventional matched filters.

Imaging spectrometry (hyperspectral imagery) has been acquired for the Broken Hill Block. The data were acquired using the HyMap scanner on behalf of the New South Wales Department of Primary Industries (former Department of Mineral Resources). Interpretation of the imagery for selected test sites demonstrates the capability to map a wide range of lithologies and minerals. Minerals mapped include iron oxides, garnets, micas, clay minerals, and amphiboles. Lithologies mapped include all the lithologies present in the region, and the surficial sediments. HyMap is also able to map certain lithologies critical to mineral exploration in the region, such as gahnite-bearing quartz, manganese-bearing garnetiferous rocks, and sodium-rich micas indicative of likely mineral alteration. The mapping capabilities of the HyMap scanner are demonstrated using selected test sites, but the lithology and mineral mapping has also been extended to cover a whole 1:25 000 map sheet area.

It is suggested that the HyMap-derived information will form the vital surficial component in a three-dimensional view of the crust provided by geophysical data including gravity, magnetic, radiometric, and imaging spectrometry data.

The Pajingo Epithermal System is an area of low-sulphidation epithermal veining and alteration, covering about 150 square km at the northern margin of the Drummond Basin. Tertiary and younger conductive sediments cover about 80% of the area. Gold mineralization occurs in thin quartz veins (generally 0.5–3 m wide), and most of the ore bodies discovered to date are along the north-west trending Vera-Nancy structure

The host intermediate volcanics are magnetic, but the epithermal alteration that extends up to 50 m from the veins along the Vera- Nancy structure is magnetite destructive. Results of a high-resolution magnetic survey clearly delineate the major structures, including the Vera-Nancy structure. The quartz veins occur within broader zones of silicification, and gradient-array resistivity surveying has been successfully used to map these zones. Generally, the high resistivity zones due to silicification are coincident with the structures identified in the magnetics. Gravity and seismic surveys have aided the interpretation of regional structure. These techniques along with stratigraphic data indicate that the Vera-Nancy structure is a major north-west striking extensional fault.

High resolution magnetics and resistivity continue to be the most useful geophysical tools in the ongoing exploration for additional mineralised structures within the Pajingo Epithermal System.