Exploration Geophysics - Volume 19, Issue 1-2, 1988

Minimum phase conversion is a common and often routine first step in the processing of zero phase correlated Vibroseis data.

Over the period of March 1983 to December 1985 the Cooper Basin joint venturers used five different recording contractors and as many as nine different processing contractors. Many misties, due to apparent phase differences, were noted and a study was initiated to determine the cause of these. A data set was sent to seven different processing contractors and the results compared. Major differences appeared during the correlation and minimum phase conversion of the data. The degree of difference depended primarily on the contractor but was also affected by the choice of sweep parameters.

The results of the study are presented in this paper. The minimum phase conversion operators are designed from the amplitude spectra of the Vibroseis pilot sweep. These differ from contractor to contractor due to the various levels of additive noise and base levelling of the amplitude spectrum outside the swept frequency range. The degree of phase rotation is dependent on the levels of noise and is not necessarily linear. Sweep non-linearity can also affect the minimum phase conversion procedure and the level of phase distortion caused by this is shown.

A standardisation of the minimum phase conversion to avoid phase shifts between the various contractors was introduced by the Cooper Basin operators in 1986 and 1987.

The potential for application of seismic methods in the evaluation and monitoring of nuclear waste repositories is immense. Past field studies have established that variations in velocity and attenuation indicate changes in the fracture density of the rock mass surrounding a storage facility. A study was undertaken to determine the effects upon velocity and attenuation of varying stress fields surrounding a crystalline repository. Laboratory experiments performed on fractured core samples found that amplitude and rise time are sensitive to stress applied parallel to travel path and velocity is not. Ultrasonic crosshole measurements were taken within the rock mass surrounding an underground research facility. The crosshole method was able to detect a zone of high fracture density caused by blasting near the surface of the room. The blast damage zone was characterized by relatively low velocities and amplitudes and large rise times. Moving outward into the rock mass, velocity stabilized while amplitude and rise time varied significantly indicating the existence of a stress concentration. This coincides well with the estimated stress distribution based on measured in situ stresses. Other significant variations in acoustic parameters were encountered and attributed to changes in fracture densities.

Interpretation of aeromagnetic data is greatly aided by the use of synthetic images generated from three-dimensional models.

A study of relatively simple models, varying parameters such as source geometry, height above the body, and magnetic field values, highlights some well-known characteristics of magnetic anomalies. The study also clearly demonstrates that resolution of anomaly sources and their shapes is difficult except for elevations less than the source dimensions.

A complex geological model was constructed, comprising a set of ring-structures (the host geology) and a set of random prisms (the target ore-bodies). The magnetic field was calculated at locations typical of modern aeromagnetic surveys and the resulting data sets interpolated using a variety of algorithms. These included methods based on distance-weighting, spline interpolation, minimum curvature and triangulation techniques. The gridded data sets were then subjected to downward continuation and latitude reduction procedures.

The resulting images demonstrate typical gridding problems, including: inadequate representation of the anomalies between flight lines; introduction of trends and features not present in the original data; and, distortion of the shapes of anomalies.

Thus, where geophysical data are to be presented as enhanced images, care is needed with the choice of gridding technique. Furthermore, survey specifications currently employed by the industry, may need to be revised.

For small angles of incidence, θ, the reflection amplitude of plane p-waves from a planar interface between two elastic media is nearly linear with sin² θ. For an NMO-corrected common-midpoint (CMP) gather, the linear fit of amplitude versus sin² θ at each time sample yields two new kinds of seismic trace. The trace constructed from the zero-offset intercept, the so-called ‘p-wave stack’, represents the response to changes in acoustic impedance at reflecting boundaries. The trace constructed from the slope, the ‘gradient stack’, represents the response to changes in s-wave velocity as well as p-wave velocity and bulk density. Under certain assumptions, the difference between the p-wave trace and the gradient trace reflects changes in s-wave velocity alone, and the sum reflects changes solely in Poisson’s ratio. Here, we demonstrate how the method of seismic lithologic modelling, a parameter-estimation technique that refines thin-layer models of p-wave velocity and density by iterative matching of synthetic data to CMP stack sections, can be extended to include s-wave velocity in the model. In this extended approach, a new form of synthetic data trace is matched to the gradient stack (or to either the ‘s-wave stack’ or ‘Poisson’s ratio stack’) as well. As with any other approach that attempts to obtain s-wave velocity structure from the offset-dependence of amplitude, success in application to field data depends heavily on the extent to which data can be conditioned so that they satisfy the assumptions upon which amplitude-versus-offset analysis is based.

Vertical electrical sounding Is applied to a wide variety of objectives, including mineral resources, thermal energy, and water resources. Thus, there is a great need for electrical exploration instruments that minimise the effects of noise from stray currents or electromagnetic coupling, as well as from anomalies such as local variations in shallow underground soil or changes in topography. In addition, there is a demand for more rapid measurement capability, cost reduction and instruments that can conduct in-situ measurement automatically.

Methods that have been proposed for improvement of S/N ratio include the Offset Wenner method, proposed by Barker (1981), and the synthetic potential method using the equidistant spacing of 4 electrodes, proposed by Sakayama and Shima (1985). In the latter case, they check noise contribution and eliminate the effects from local anomalies. The Offset Wenner method and digital stacking resistivity meter can be applied to the problem of achieving more rapid measurement and cost reduction.

In regard to analysis, the appearance of the linear filter method has simplified forward calculation. The development of a stable algorithm using a non-linear type method of least squares has made the use of mini-computers, to conduct inversion for vertical exploration, commonplace.

With the background of this kind of technological advance, we have developed a new generation all-in-one type electrical resistivity apparatus, the McOHM II.

The McOHM II combines in a single package an electrode switchover scanner, a resistivity meter, a vertical exploration analyzer and a printer/RS-232C output section. The operation of each section is controlled by a microprocessor. Measurement is automatic, with measurement error monitored by the microprocessor, always maintaining optimal settings. The McOHM II does not require a mini-computer or personal computer; its ROM base determines resistivity structure from measurement values automatically.

Measurement data and analysis results are stored in the memory, and can be output on 80 mm wide paper from the printer.

With the features described above, the McOHM II merits the title ‘new generation electrical exploration apparatus’. The paper summarises specifications and features of the McOHM II, and gives examples of its use in field surveys.

Graphics workstations, considered a novelty a few years ago, are now justifiably accepted as productive geophysical tools. This paper is aimed at highlighting the true significance of the workstation, particularly in the light of potential developments in the area of network computing. Emergent technology in this area will provide user-transparent facilities for high-speed distribution of both data and tasks across a heterogeneous network of workstations, mainframes and file servers. Such technology would facilitate the introduction of truly interactive processing, providing transparent access to diverse high-performance resources via the convenience of a workstation interface.

It has been recognised for a long time groundroll has an elliptical, vertically polarised particle motion. Body waves have a rectilinear particle motion. Interference between bodywaves can result in an elliptical type motion. Particle motion records provide a means of identifying these seismic waves and understanding the result of their interference upon particle motion. This is useful when assessing the results of filter techniques based upon either the ellipticity, or phase difference between components, of groundroll motion.

Wide frequency range complex impedance measurements obtained with new RAS-1 field equipment, reveals the ground response near a dipole energy source after the reduction of coupling and other measurement errors to insignificant magnitudes. The observed field impedance data has been modelled by the radial and tangential electric fields of a radiating dipole in full-space. Qualitative agreement is obtained for the amplitude measurements up the medium frequency range with qualitative agreement of the phase.

Good values of the IP and conductivity can be obtained, and since the position of the attenuation is sensitive to small permeability changes, permeability will be a measureable quantity in many geological environments and may be very prominent in association with mineralization. Permittivity is measureable in more resistive environments at high frequencies. The availability of more accurate measurements is a prerequisite to the use of more refined models of the source and the half-space, taking into account the complex nature of the conductivity, permeability and permittivity, for the purpose of more accurate delineation of inhomogeneities in the ground.

Magnetic surveys are used in both mineral and hydrocarbon exploration to map the characteristic patterns of magnetisation of crustal rocks. However there is a part of the magnetic field of the earth which fluctuates with time and changes over seconds, minutes and hours. This fluctuating part of the magnetic field is due to the flow of electric currents both internal and external to the earth.

The fluctuating part is anomalously strong at coastlines, a phenomenon known as the ‘Geomagnetic Coast Effect’ which extends for distances of order 100 km either side of the coastline. A significant portion of the coast effect can be explained by large scale induction of electric current in the seawater. The electric conductivity structure of the continental margin will also influence the flow of electric current.

The south-east Australian coast is an ideal location for investigating this phenomenon due to its simple two-dimensional character and narrow continental shelf. Measurements of the fluctuating magnetic field were made using stationary magnetometers on land and seafloor during the Tasman project of Seafloor Magnetotelluric Exploration and the more recent Continental Slope Experiment. Within the accuracy of the station spacing, the coast effect has a maximum amplitude halfway across the continental slope.

The estimation of the seismic velocity field in two or three dimensions, by modelling the travel times of particular seismic phases or by matching observed and computed seismograms, represents a large-scale nonlinear inverse problem. The solution can be obtained by determining the minimum of a misfit function between observations and theoretical predictions, subject to some regularisation conditions on the behaviour of the model parameters. The minimisation can be achieved without the inversion of large matrices by using a search scheme based on the local properties of the misfit function. At each step in the iterative process, a subspace of a small number of directions is constructed in model space and then the minimum sought in a quadratic approximation on this set. At least two directions are required for rapid convergence. This approach is very suitable when the model parameters are of different types, since partitioning by parameter class avoids dependence on scaling.

If the model is to remain close to a reference then the regularisation term is particularly important and different types of a priori information (e.g. geological) can be introduced via the character of this term. When fit-to-data is emphasised there is the chance of finding features suppressed in a more conservative approach, but at the risk of introducing spurious detail.

A deep-towed seismic profiling system is being developed to obtain detailed subbottom structures in deep seas around island arcs like Japan. Features of the system are as follows:

1. Off-line towing: both a hydrophone streamer and a recording unit with power supply are equipped onto a deep-towed frame, whereas the sound source is towed at the surface. This means that the underwater system can be towed by wire without using any special cable.
2. No depth limitation: all electrical units except the streamer are kept in the pressure vessels (depth limit 10 000 m), and unlimited depth hydrophone elements are used for the streamer. Thus the system can be used in the deepest seas such as the Japan Trench, the Izu-Ogasawara Trench, etc.
3. Digital recording: an A/D converter with a floating point amplifier and magnetic bubble memory are used to obtain high quality and high volume data. This provides 120 db in dynamic range with 12 bit significance and 16 megabytes data space.

The prototype of the system hardware is complete, and the control software for the system is under examination. Field tests have proved the operability and functional ability of the system.

The geometrical patterns encountered in geophysics are irregular and fragmented at all scales. This expository talk reviews the possible applications of ther novel ideas of fractal geometries (Mandelbrot, 1982) to diverse fields of applied geophysics and rock physics. Examples include the internal surfaces of porous rocks, the irregular shape of earth materials and the fine structure of sedimentary sections.

A part of the Tasmanian Mines Department’s Mount Read Volcanics Project involved the collection of IP data from a variety of materials to define the expected signatures of massive sulphides, barren sulphides, alteration zones and relatively unaltered host rocks. In all data were collected from some 70 sites using time domain equipment. These data provide a unique uniform collection of in situ property measurements for western Tasmania.

The possibilities of mineral discrimination by fitting Cole-Cole models to the in situ data appear excellent. The economic massive sulphides are characterised by a distinct field of m-tau values bounded on the lower m side by a class of black shales and on the low tau side by other sulphide mineralization.

The information contained in amplitude-versus-offset (AVO) effects can be extremely useful in exploration/development work. A specialized set of techniques and software, however, is needed to extract and use this information properly. A systems approach has been developed at Berrong Enterprises Ltd to accomplish this goal. The system has as major components (1) AVO feasibility studies, (2) proper AVO data processing, (3) AVO scanning, (4) detailed AVO modelling supported by well logs, and (5) elastic model inversions at prospective locations. Each of these areas is discussed from an application/results viewpoint with practical examples from actual development problems. These examples will demonstrate the degree of information that is available in proper AVO information processing. Interpretation techniques to use this information are also presented.

Since the development of a technique called Depth Image Processing (DIP) for direct transformation of multifold, step- response EM data into an imaged conductivity-depth section, a number of test and production surveys have been carried out. The results demonstrate the effectiveness and accuracy of the technique when the geological section is quasi-layered. Two recent case studies show very good correlation with available drill-hole results.

Scale and computer model data have been generated for a number of conductive/resistive inhomogeneities in a layered earth, and this data base is being studied with a view to improving the ability of the DIP process to better image these non-layered features. The results from this will be reported at the conference.

Described and illustrated are seismic inversion formulas derived by Tarantola (1987) and Mora (1987d). The formulas are based on nonlinear least squares iterations to find the elastic properties of the Earth corresponding to the synthetic wavefield that best matches the seismic observations. Primary P- and S-wave reflections, mode converted waves and Rayleigh waves are all theoretically useful in the inversions. Beginning with a starting guess, the Earth properties are iteratively updated using a preconditioned conjugate-gradient algorithm. The gradient direction is cast in terms of two wave propagations which can be easily implemented on fast fine grain parallel computers such as the ‘Connection Machine’ (see Hillis (1986)).

The method is tested by inverting a shot profile. The wave propagations are done, using elastic finite differences to allow for Earth models of realistic complexity. The results verify the theoretical predictions of Mora (1987c) that the iterative elastic inversion formulas are capable of obtaining all wavenumbers of the compressional and shear wavespeeds that are resolvable separately by prestack elastic migration and elastic reflection tomography.

Because of the complicated nature of earthquake induced ground motions and the corresponding transient response of structures to such motions, the use of response spectrum has achieved wide acceptance, in the field of earthquake engineering, as a meaningful measure of the intensity of an earthquake. Thus, it is useful to investigate the application of the digital computer to characterise real earthquake motion (in the form of digitised acceleration time histories) by means of response spectra. The conceptual development of the response spectrum (which should not be confused with the ground motion spectrum) and its application to the analysis of transient oscillations in elastic systems is attributed to Benioff (1934), Neuman (1936), and Biot (1943). Its engineering significance lies in the fact that once the spectrum is known for a one-degree-of-freedom system, it is possible to compute the value of the maximum shear produced by an earthquake. Further, extension of this concept of response spectra to multidegree of freedom systems can be done using the modal superposition method of dynamic analysis. The mathematical formulation for performing response analyses of a single-degree-of-freedom system is explained.

CSAMT method is widely used in Japan for mining and geothermal exploration. For geothermal applications we need to study the subsurface structure as deep as 2 km or more. This shifts the CSAMT survey to a relatively lower frequency range, where the CSAMT method tends to become less reliable and less efficient. For this reason we are developing a time domain version of CSMT method at the Geological Survey of Japan. Using a controlled source excitation with a period of four seconds the waveforms of electric and magnetic fields are measured. Noisy data are rejected by visual inspection, and only the high quality data are stacked. The resultant wave-forms are Fourier analysed to get the spectra in the frequency range between 1/32 and 8 Hz. This technique not only gives very reliable data of high repeatability, but also makes the measurement time much shorter than CSAMT methods where the time-consuming measurement needs to be repeated for many different excitation frequencies.

A numerical solution has been developed for transient electromagnetic (TEM) responses of prisms in a layered host. The solution is based on the method of integral equations, where the prisms are replaced with an equivalent scattering current. This scattering current is approximated with pulse and divergence-free basis functions. The divergence-free functions model eddy currents closed in the prisms, and hence simulate the inductive responses of the prisms in a very resistive host. Checks on the solution show that it is valid for conductive and resistive hosts, including a free-space host.

The masking effect of conductive overburden is shown to delay and suppress the three-dimensional (3-D) TEM response of a conductor. After the overburden response is removed, an interpretation of the conductor with free-space models is a poor approximation when the basement rock is conductive. Instead of an exponential decay at late times, the conductor’s response decays as an inverse power law. When the basement resistivity is increased, the conductor exhibits an exponential decay at late times.

We have used the solution to gain interpretational insight into the lateral resolution of multiple conductors with different TEM surveys. The resolution of multiple conductors is very poor in a fixed-loop survey, but in a central-loop survey the resolution is better, provided the data are interpreted at early times. At later times, multiple conductors may not be resolvable and interpretational ambiguities could arise in a central-loop survey.