ASEG Extended Abstracts - ASEG2003 - 16th Geophysical Conference, 2003

I generate three starting models for the inversion of a set shallow seismic refraction data using wavepath eikonal traveltime tomography. Two models are generated with the generalized reciprocal method (GRM), which uses a multi-layer model with discrete velocity changes while the other is generated with a one dimensional (ID) inversion algorithm similar to the tau-p method, which uses a model with vertical velocity gradients. There is little correlation between the results in either the absolute depths, the relative depths or the refractor velocities.

I attribute the differences in the absolute depths to the occurrence of a velocity reversal in the overburden, the existence of which is indicated by the concave-upwards shape of the traveltime graphs and by the extremely rapid decrease in amplitudes.

I attribute the differences in the relative depths and seismic velocities to the poor starting model generated by the ID tau-p inversion algorithm, which is unable to resolve the fundamental ambiguity between seismic velocity and structure on refracting interfaces. The GRM-derived refractor velocities are compatible with the head coefficients determined with amplitude products.

The existence of velocity gradients is suggested by the convergence of the traveltime graphs with increasing shot-to-detector distance. I attribute this convergence in part to the large variations in signal-to-noise ratios, which affect the accuracy of the traveltime data.

The shot amplitudes decrease rather than increase with distance, which supports the occurrence of distinct interfaces with velocity gradients over relatively small depths. Therefore, while velocity gradients are likely to occur, the results generated with ID tau-p tomography are probably not representative of the subsurface conditions.

The considerable differences between the results generated with the GRM and those obtained with ID tau-p tomography are compelling demonstrations of the non-uniqueness of model-based inversion and of the importance of the selection of the initial model. Accordingly, all methods of model-based inversion of shallow refraction data could usefully include starting models generated with the GRM or the refraction convolution section.

We derive refraction statics for seismic data recorded in a hard rock terrain, in which the statics corrections range from less than 10 ms to in excess of 70 ms, over distances as short as 12 receiver intervals or 480 m. We compare statics values computed with a simple model of the weathering using the generalized reciprocal method (GRM) and the refraction convolution section (RCS) with those computed with a more complex model of the weathering using least-mean-square inversion with the conjugate gradient algorithm (Taner et al, 1998). The differences between the GRM model and that of Taner et al (1998), systematically vary from an average of 2 ms to 4 ms over a distance of 8.8 km. The differences between these two refraction models and the final statics model which includes the automatic residual values, are generally less than 5 ms. The residuals for the GRM model are frequently less than those for the model of Taner et al (1998). The RCS statics are picked approximately 10 ms later, but their relative accuracy is comparable to that of the GRM statics.

The residual statics values show a general correlation with the refraction statics values, and they can be reduced in magnitude by using a lower average seismic velocity in the weathering. These results suggest that inaccurate average seismic velocities in the weathered layer may often be a source of short wavelength statics, rather than any shortcomings with the inversion algorithms in determining averaged delay times from the traveltimes.

The significance of these results is that the RCS achieves improved accuracy through stacking to improve signal-to-noise ratios prior to the measurement of any traveltimes. Therefore, the RCS offers a new approach to determining more accurate statics with second arrivals such as shear waves, for which signal-to-noise ratios on shot records can be much less than those of the first arrival compressional waves.

Among geophysical exploration techniques, the application of the electrical resistivity method has recently increased for geo-environmental investigation such as in the detection of soil and groundwater contamination. The electrical resistivity method is used to identify permeable soil layers that are possible channels of contaminant transport. Geo-environmental investigation deals with important social issues and is a growing field of innovation.

This paper describes the present state of soil and groundwater contamination in Japan and the application of the electrical resistivity method to the identification of permeability structures that are possible channels of pollutants. Two field studies were carried out: an analysis of the infiltration of saline water and a measurement of the movement of saline water masses by resistivity monitoring.

The results are summarized as follows: (1) determination of the state of saline water infiltration by resistivity is effective in preventing contamination by saline water and in determining appropriate sites to bore wells where fresh groundwater is available, and (2) combination with the indirect method of resistivity monitoring, which roughly estimates the positions of saline water masses, sharply improves the efficiency of the salt tracer method. Therefore, the two methods should be combined to understand accurately the flow mechanisms of groundwater in contaminated districts.

Conventional predictive deconvolution is very good for suppressing normal incidence water bottom reverberations. Classic papers, some published in Geophysics, have provided rules of thumb for conventional seismic deconvolution processing. These rules have been invaluable in shortening field wavelets enough to allow structural interpretation of the subsurface. However, concepts of seismic deconvolution processing have evolved. Merely shortening the interpretation wavelet is no longer enough. In order to interpret rock properties, it is necessary to know the interpretation wavelet shape and to maintain its amplitude and phase spectrum throughout a seismic volume.

By utilizing an example seismogram synthesized with a finite impulse response (FIR) wavelet kernel, these rules can be exemplified and refined:

1. Predictive deconvolution can suppress reverberations as long as the lag is less than or equal to the minimum time of the water bottom reverberation sequence.
2. An isolated reflection's signature is not distorted by predictive deconvolution, as long as the lag is larger than the length of the wavelet kernel.
3. The dereverberation filter changes shape whenever the lagged interval includes a significant portion of the wavelet kernel's autocorrelation.
4. xsPlacing the lag at the second zero crossing is a reasonable compromise but the reflection's signature will be distorted and it can extend beyond the lag.

The output signature can vary significantly with the value selected for the prediction distance (lag). Relative entropy deconvolution concepts can provide consistent dereverberation filters for lags shorter than the length of the wavelet kernel. Actual field signatures can be compensated to any convenient wavelet shape, including those with infinite impulse responses, before applying a relative entropy predictive deconvolution.

As part of a project to investigate long-term exposure of low-level earth currents on dairy cows, an unusual multiple-site, remote-reference audio magnetotelluric (AMT) data set was acquired on four Wisconsin dairy farms and two remote sites in the summer of 2001. At each dairy, several five-component sites were recorded along with a remote located in a wildlife refuge, at a distance of roughly 30 miles from the farms. EMI MT24 systems were deployed under power line transformers, above and below three-phase transmission lines, in close proximity to machinery such as compressors, pumps, electric fences, and large fans. Data were recorded at the dairies with 20 m dipoles to capture the natural signal while not overloading the instrument electronics with high cultural signals. Sampling frequencies of 48, 9.6 and 2 kHz were employed.

To accurately determine the characteristics of the earth's fields, including the magnitude of the very narrow band-nature of 60 Hz signal and its harmonics, normal impedance estimation techniques using relatively short segments of the time series were found inadequate. Thus, the data were processed with a Fast Fourier Transform of length 524,288 applied to each of the 500,000 data point segments. Electrical resistivity data were acquired with Wenner array spacings of 1 to 10 m. Earth current densities were estimated from the 1 m resistivity values and the electric field data from the MT24.

Short recording times were used to determine earth currents under different conditions, not to estimate the impedance. Hence, we present characteristics of various cultural noise sources to improve the knowledge base of what practitioners must deal with in noisy environments. We are not evaluating processing technique, but have applied some robust processing to observe if it is possible to attain impedance estimates in the presence of coherent noise and low signal strength.

A field instrument and analysis method was developed to estimate the vertical distribution of hydraulic conductivity, K, in shallow unconsolidated aquifers. The field method uses fluid injection ports and four pressure transducers in a hollow auger that measure the hydraulic head outside the auger at several distances from the injection point. A constant injection rate is maintained for a duration time sufficient for the system to become steady state. The novelty of this method lies in the fact that K is determined while the drill string is in the ground and the change in hydraulic head is monitored in the same drill stem. Dense vertical sampling and the application of four transducer offsets provide a detailed resolution of the vertical variability in hydraulic conductivity. Exploiting the analogy between electrical resistivity in geophysics and hydraulic flow, data are processed with a 1-D inversion algorithm for the pole-pole resistivity method resulting in models consistent with the known geology.

The injection methodology, conducted in three separate drilling operations, was investigated for repeatability, reproducibility, linearity, and for different injection sources. Repeatability tests, conducted at ten levels, demonstrated spreads of generally less than 10%. Reproducibility tests conducted in three closely spaced drilling operations showed a spread of less than 20%, which may be due to lateral variations in hydraulic conductivity. Linearity tests, made to determine dependency on flow rates, showed no indication that the applied flow rate biased the measurements given the uncertainty of repeated measurements. In order to obtain estimates of the hydraulic conductivity by an independent means, a series of measurements were made by injecting water through screens installed at two separate depths in a monitoring pipe near the measurement site.

A field instrument and analysis method was developed to estimate the vertical distribution of hydraulic conductivity, K, in shallow unconsolidated aquifers. The field method uses fluid injection ports and four pressure transducers in a hollow auger that measure the hydraulic head outside the auger at several distances from the injection point. A constant injection rate is maintained for a duration time sufficient for the system to become steady state. The novelty of this method lies in the fact that K is determined while the drill string is in the ground and the change in hydraulic head is monitored in the same drill stem. Dense vertical sampling and the application of four transducer offsets provide a detailed resolution of the vertical variability in hydraulic conductivity. Exploiting the analogy between electrical resistivity in geophysics and hydraulic flow, data are processed with a 1-D inversion algorithm for the pole-pole resistivity method resulting in models consistent with the known geology.

The injection methodology, conducted in three separate drilling operations, was investigated for repeatability, reproducibility, linearity, and for different injection sources. Repeatability tests, conducted at ten levels, demonstrated spreads of generally less than 10%. Reproducibility tests conducted in three closely spaced drilling operations showed a spread of less than 20%, which may be due to lateral variations in hydraulic conductivity. Linearity tests, made to determine dependency on flow rates, showed no indication that the applied flow rate biased the measurements given the uncertainty of repeated measurements. In order to obtain estimates of the hydraulic conductivity by an independent means, a series of measurements were made by injecting water through screens installed at two separate depths in a monitoring pipe near the measurement site.

Geophysical methods have been used at Panshanger Estate near Cressy, Tasmania to map the distribution of salt affected areas, and the underlying catchment structure.

Frequency domain electromagnetic (FEM) techniques were used to detect near surface salt affected zones on the basis of their high conductivity. Areas visibly affected by salt were found to coincide with areas displaying the highest conductivities. Other areas displaying moderate to high conductivity did not appear to be salt affected but may represent areas of future salt risk.          Magnetic, gravity and time domain

electromagnetic (TEM) techniques, in conjunction with drill logs from holes near the field area, indicate that the western half of the field area is underlain by shallow Jurassic dolerite, while to the east and north east, the dolerite is covered by a thicker layer of sediment. In the western part of the survey area, high conductivities occur close to the edges of bedrock highs, indicating that bedrock topography may be controlling groundwater movement. These bedrock highs generally correlate with surface topographic highs, and the elevated conductivities, indicative of high salt storage, are measured over depressions and creeks.

A sinuous conductivity anomaly in the north east of the field area occurs on slightly elevated ground, in an area where Tertiary cover is thick. There was no visible evidence of salinity at the surface over this feature, and it was interpreted as a shallow palaeo-channel on the basis of EM measurements. An investigation borehole intercepted an upwardly fining sandy clay sequence, representative of a meandering fluvial system. EC 1:5 analysis of the sediment core and water analysis indicates that the palaeo-channel may act as a mechanism of salt transport and storage.

The seismic stacking velocity data in the Great Australian Bight are a useful dataset for calculating depths and sediment thicknesses. This work compares these data with P-wave velocities from sonobuoys and sonic logs from wells, and on this basis a depth overestimate of at least 15% can be expected from the depths derived from stacking velocities. Megasequence boundary depths are calculated for the Ceduna Terrace to further illustrate data quality. The database makes available the unfiltered stacking velocities using conventional and horizon-consistent formats.

Remote sensing instruments have increasingly been applied to assist police locate buried murder victims, particularly during the last ten years. For the forensic investigator it is still a problem to identify the exact location of buried human skeletal remains within a large targeted search area. This presentation describes unique Australian research in forensic anthropology that is currently examining the application and effectiveness of geophysical instruments for the detection of buried skeletal remains.

Three aspects of the research are covered in this discussion: a national survey of Australian police jurisdictions conducted to establish how clandestine graves are located and the extent of the application of geophysical instruments in forensic case situations; the controlled experimental gravesites used to test the effectiveness of selected geophysical instruments representative of forensic case scenarios; and a situation in which resistivity was successfully applied to the detection of a 150 year old burial.

The national survey of police investigation section reveals that no reported locations of bodies have used geophysical instruments. This research will have significant impact on being able to provide data on reliable techniques for criminal investigations.

The experimental gravesites contain kangaroos, pigs and cadavers, and on the animal graves, ground penetrating radar and electromagnetic induction have presented positive results in locating the burials. This is the only known international research in this field that involves the burial of cadavers and presents a unique opportunity to establish a base of working field knowledge.

Two basic questions lay at the heart of AEM surveys, one concerning the basic physics of the process, the other the interpreter's capability. Is the equipment capable of exciting the target to the extent that it will produce a detectable signal above background response? Can the presence of a target be deduced from this signal?

This study examines the response of a 400 siemen sulphide lens in an altered ultramafic lying at the boundary of a faulted host under a moderately conductive saprolite cover. In the easy case, the surface is flat and in the other, the whole lies under moderately steep topography. The study compares the ability of fixed wing time-domain, helicopter time-domain and helicopter frequency-domain systems to differentiate between when the lens target is present and when it is absent. Constant receiver ground clearance is assumed. In general the effect of the lens was to smooth the response horizontally so that a naive interpreter might mistake the barren response for the target response and vice versa. For the time-domain systems, the vertical component showed the greatest differentiation whereas the in-phase, in-line coaxial component was the most effective for the idealised HEM system. HEM systems showed better anomaly localisation than did fixed wing systems due to shorter transmitter-receiver separation and lower flying heights.

Conductivity depth images and other interpretations based on one-dimensional earth models yield misleading information for the type of model studied.

Electromagnetic drill-hole measurements are an important tool for locating conductive orebodies in detailed exploration scale. Dipole-dipole systems have good spatial resolution, especially if all three magnetic components are measured over a wide frequency range. Interpretation of field data requires the use of sound 3D modelling tools capable of dealing with high conductivity contrasts and the close proximity of boundaries to the source or receiver.

We modelled the response of the SlimBoris dipole-dipole drill hole system going through a block target using Loki, a 3D, full-domain, edge finite-element approach based on vector shape functions and scalar unknowns. This method achieves considerable speed advantage over conventional finite-element methods since only the one tangential component rather than three orthogonal components need be solved at each "node". Moreover, since no boundary conditions are violated in this approach, contrasts in excess of one million to one can be modelled accurately as verified through semi-analytical layered earth solutions. Computation time can be reduced by a further factor of five by solving initially for Schelkunoff potentials rather than electric or magnetic fields. This is due to the superior condition number of the resulting matrices. Accuracy is maintained using Green's function projectors to obtain the fields at the receivers rather than differentiating the potentials. A 30,000 cell model takes 20 seconds on a 1.9GHz Intel chip per frequency per transmitter position. Control files for complex models can be set up rapidly using either the Encom EMGUI or Maxwell from EMIT.

The results were in close agreement with scale model results at all contrast ranges. Another check was made using a 3D integral equation program. As expected, close agreement was obtained at contrasts of less than 300 but the integral equation results deteriorated at high contrast.

As mineral exploration activity shifts to regions at low magnetic latitudes, interpretation skills acquired at high latitudes become harder to apply, thus leading to under-utilisation of expensive magnetic survey data.

Changes in anomaly shape, reduction in overall amplitude and changes in map textures make the ready interpretation of geology from magnetic data difficult. These problems are worst for magnetic inclinations within 20° of the equator.

Reduction to pole (RTP) is the best theoretical solution because it removes the effect of induced magnetisation and strike while preserving dip information. But, in practice, the standard RTP transform is difficult to apply at very low latitudes and produces poor-quality maps dominated by declination-parallel artefacts. Additionally, the transform cannot completely reconstruct NS-trending anomalies.

The 3D Analytic Signal is a function of magnetic gradients and is easy to compute at all latitudes. It is almost but not entirely independent of magnetisation direction. It can be computed easily and accurately for any ambient and source magnetisation. However, it lacks the resolution that derivative maps provide and lacks the dip (and therefore structural) and textural information that total magnetic intensity and RTP maps contain.

Marine siltstones and fine sandstones of the Brachina Formation and olive-green siltstones of its more distal equivalent, the Ulupa Siltstone, were deposited during the Marinoan in the Adelaide Geosyncline. These sediments have since been deformed and metamorphosed during the Ordovician Delamerian Orogeny.

In the Flinders Ranges, the relatively unmetamorphosed sediments, though iron-rich, are only weakly magnetic. In the Mount Lofty Ranges, where peak metamorphic grades were reached, the Brachina Formation and its equivalent, the Ulupa Siltstone, are extremely magnetic. Magnetic markers within these formations have been used to trace the macrostructure of the Southern Adelaide Foldbelt and have been especially useful where outcrop is limited.

While the magnetic markers are consistent with respect to their continuity and presence, the amplitude and character of the anomaly changes significantly along strike. This is basically a function of metamorphic grade as anomalies at lower grades can be successfully interpreted (since both depth and dip are often known) assuming induced magnetisation only. At the highest grade, where the rocks are andalusite schists, the magnetic anomalies are inconsistent with the direction of the present magnetic field, and remanence must represent the greater component of the anomaly.

The results of the rock magnetism study show a close correlation between metamorphic grade and the variation in magnetic mineralogy and natural remanent magnetisation. The rocks display multiple components of NRM including a weak primary detrital component acquired during deposition and a much stronger thermal or chemical component acquired during the Delamerian Orogeny. The secondary component has been related to the NRM direction indicated by the nearby Black Hill Norite which is believed to have intruded during the waning phases of the Delamerian Orogeny.

The results of the rock magnetism study have been interpreted in terms of the geologic history of the Ulupa Siltstone and have been used in the interpretation of the aeromagnetic anomalies it causes.

The area under study is located in the southern part of the Central Iranian Volcanic Sedimentary belt and covers an area of about 600 sq. kms. The Sar Cheshmeh,        Darrehzar and areas with known

mineralization and alteration are chosen as control areas. Airborne geophysical data - radiometry and megnetometry - has been integrated and analysed using fuzzy classification. This type of classification is suggested for remote sensing data, but it can also be used for classification of airborne geophysical data. In this study Landsat and airborne geophysical data are integrated and analysed by fuzzy classification method. After defining the training areas(Sar Cheshmeh and Darrehzar areas), the entire region is classified into altered and unaltered areas. This analysis is found useful for exploration of porphyry type deposits in the Central Iranian Volcanic Belt, where most parts of this belt is surveyed by airborne geophysics.

The three-dimensional modelling program MARCO_AIR has been used to calculate the response of idealised pressure ridges to practical airborne and shipborne electromagnetic systems. The model results clearly show the superior resolution of the horizontal coplanar ship-borne system compared to airborne measurements. However, sea ice keel thicknesses estimated via inversion of ship-borne single-frequency electromagnetic data are strongly dependent on relatively small variations in survey altitude.

Inversion of helicopter electromagnetic data over 3D pressure ridge models shows that the maximum keel thickness is consistently underestimated, although airborne EM methods yield reliable thickness estimates over level ice. The vertical coaxial coil survey geometry offers excellent lateral resolution of multiple targets, but the anomalies of typical Antarctic sea ice pressure ridges would be too small to detect in practical surveys using a close-coupled (2 - 3 m) geometry. For a system with a coil separation of 8 m, the vertical coaxial responses are larger, and lateral resolution of the measurements at a flight height of 20 m is superior to a close-coupled system flown at an altitude of 10 m.

Recent regional scale helicopter electromagnetic (HEM) data acquired on the west and northwest coasts of Tasmania have demonstrated the powerful geological mapping capabilities of the HEM technique.

Quality control for the survey was provided by analysis of repeat data from daily test lines, and comparison with surface geophysical data. Data have been interpreted by conductivity-depth imaging, complemented by layered-earth inversion of selected lines. The maximum depth of investigation for the HEM survey has been determined to be around 150 m, based on both theoretical calculations and the results of conductivity-depth imaging.

A preliminary interpretation of the HEM data is presented for the Balfour region. The HEM survey has clearly differentiated previously unmapped units within the Proterozoic Rocky Cape Group, and has defined several major structural trends within the survey area. A large number of bedrock conductors have been identified in the HEM data, particularly close to the historic copper mining centre of Balfour.

The in situ stress field and consequent risk of reactivation has been evaluated in the Bight Basin in order to assess the risk of fault seal breach at seismically mapped prospects. Borehole breakouts interpreted from dipmeter and image logs in five wells in and around the Bight Basin indicate a 130°N maximum horizontal stress orientation. The large variation in water depths across the Bight Basin required the use of effective stress magnitudes. A depth-stress power relationship is used to define the effective vertical stress based on density log data from 10 wells. The effective minimum horizontal stress was estimated at 6 MPa/km using effective pressures from leak-off tests. An upper bound (18.7 MPa/km) for the effective maximum horizontal stress was determined using frictional limits to stress. Pore pressure in wells in the region is hydrostatic except in Greenly 1 where over pressure occurs below a depth of 3600 m.

The risk of fault reactivation in the Bight Basin was evaluated using the FAST technique. The risk of fault reactivation and consequent seal breach is expressed in terms of the pore pressure increase that would be required to induce failure for three different in situ stress cases. In all three cases faults striking 40°N (+15°) of any dip are the least likely to be reactivated.