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

Existing packages that allow plate scale reconstruction deal with a host of individual objects, abstracted from geological and geophysical data. The typical approach is to show existing continental outlines and to represent their movement through a few hundred million years of history, providing a reconstruction based on rigid plate motion. This work presents the release of a new reconstruction package, PlatyPlusPlus, designed to provide a new way to carry out tectonic reconstruction. Based on the first version of PlatyPlus, PlatyPlusPlus has been designed with a client-server architecture, allowing reconstructions to present a degree of complexity not reached before. In this paper we will present (1) the architectural complexity of the software; (2) some of the new features; (3) analyse their merits in comparison with classical reconstruction (4) the first results of this new technology.

Woodside holds significant equity in the VIC/RL2 and VIC/RL6 permits, which are located in the Gippsland Basin, Australia. These permits contain several fields that are spread over an area of 300 km . This area is covered by two overlapping 3D seismic surveys within which 11 wells have been drilled. Our recent reprocessing, merging and PreSDM of these surveys produced a single seismic data volume of high quality over the study area.

During the reprocessing we were afforded the chance to remove striping effects and suppress coherent noise more strongly than before. Full-waveform synthetics were computed from wells in the survey and used to optimise the design of a Radon filter with more ‘p traces’ than used in the initial processing only a few years before. When compared to the field gathers, the synthetics revealed persistent multiples in the deep section and often these had been inadvertently picked during previous velocity analyses. Careful QC of the velocity picking also resolved a major problem in the overlap zone between the two surveys.

PreSDM clearly improved fault definition over both the PreSTM and PosTM applied to the respective surveys during the initial processing. Time-variant spectral whitening helped to restore the bandwidth after migration and deliver a large improvement in temporal resolution. Interval velocities from the PreSDM depth model were used to shape the depth conversion away from well control. Subsequent re-interpretation, amplitude studies and seismic inversion have identified new prospects and have narrowed the large range of uncertainty in developing this area.

As the saying goes, you can never be too rich, too thin,… or have too much bandwidth. Consequently the desire for better bandwidth has been never ending.

Actually, the term “bandwidth” is sometimes used a little loosely. For instance, “bandwidth” and “resolution” are often used interchangeably, but actually there is a fine distinction. Bandwidth relates to the range of frequencies in the spectrum, and thereby refers both to Fmin and Fmax. This is an attribute relevant for discussions in inversion. Resolution refers to the ability to delineate things like thin beds and pinchouts in wiggle trace displays. Although opinions vary, from a pragmatic and empirical perspective, many geoscientists feel this is tied chiefly to Fmax. That is, the specific value of Fmin is not particularly relevant to resolution – as long as at least two octaves of “bandwidth” are present.

Causes that limit recoverable bandwidth and resolution are many. They include ghosts, multiples, noise and arrays. The industry has had some success at addressing these causes by changing how the wavefield is sampled. For instance in seabed surveys, sampling both the pressure and particle velocity components of the wavefield allows signal to be acquired more continuously from Fmin to Fmax – thereby improving bandwidth. This is because the corresponding ghost notches are staggered.

On the other hand, by sampling just one component of the wavefield, but doing so more finely in a spatial sense, noise trains can be suppressed better. This is because the noise trains are no longer aliased and can therefore be easily attacked by various FK or adaptive filters. This is the strategy in single sensor surveys. In the case of ground roll in onshore surveys, this can have benefits at both ends of the temporal spectrum. For instance, direct-travelling ground roll is often strongest at low frequencies; so removing it from a data set can allow the low frequency content in the underlying signal to be seen – thereby effectively lowering the Fmin value. However, if scattering is prevalent, the contamination of the spectrum at high frequencies can be significant too. This is because the amplitudes of scattered waves are proportional to the square of the temporal frequency. Hence, in such cases, being able to remove scattered ground roll from a data set can allow us to see the high temporal frequencies in the signal -thereby increasing the effective Fmax.

Analogously in the marine case, by sampling the swell noise more finely along the streamer, we are able to remove it more successfully than in the past. This allows us to tolerate the acquisition of more noise in the raw data. Therefore, the streamers can be raised to shallower (noisier) depths. By doing this, the receiver ghost notch in the signal is shifted to a higher temporal frequency often permitting dramatic improvements in resolution.

Finally, improvements in data processing and interpretation have also permitted improvements in the drive for better bandwidth. One case in point is the full waveform prestack inversion as enabled by formulations that exploit the genetic algorithm. This method simultaneously finds the correct NMO velocity and the correct amplitude variation with angle. Consequently both low- and high-frequency model components are derived by the inversion.

The Beruang copper-gold deposit is located very near the centre of Kalimantan, in Indonesia. It is a diorite porphyry hosted deposit currently being explored by PT Kalimantan Surya Kencana, which is a subsidiary of Kalimantan Gold Corporation Limited of Canada. Extensive IP (Induced Polarisation) coverage was obtained over the Beruang deposit and surrounding area using the 150m dipole-dipole configuration, during 2001. 2D Inversion of the IP data was achieved using various smooth modelling 2D inversion algorithms. A number of 2D and 3D images have been generated to provide presentations of the model results. Drilling of a number of targets derived from the inversion modelling has resulted in some significant copper-gold intersections. Exploration in the project area is continuing.

The sand covered Muweilah archaeological site in the United Arab Emirates (UAE) is a unique Iron Age site, and subject to intensive excavation. However, this time consuming process would require up to 20 years to complete. This paper presents preliminary results of geophysical surveys with the ultimate objective of characterising the site far more quickly.

Ground penetrating radar (GPR) was trialed as a primary imaging tool with a very shallow time domain EM (MetalMapper) system in a supporting role. Dense 3D GPR datasets were migrated at 10 cm intervals to produce horizontal (plan view) slices which are conceptually similar to the excavation methodology used by archaeologists. The objective here was to delineate extensive linear and planar features. In addition, isolated scatterers were classified. Finally, MetalMapper images were used to discriminate between metallic and non metallic scatterers.

The highly resistive sand cover provided GPR depth penetration of up to 5 m. GPR successfully mapped floor levels, walls and isolated anthropogenic activity, although in some cases crumbling walls were difficult to track. From this study two possible courtyard areas were recognised. The MetalMapper was less successful due to the limited depth penetration of 50 cm. Despite this, the system was still useful in detecting modern day ferruginous waste and bronze artefacts.

The results (subject to ongoing ground-truthing) indicated that GPR was optimal for sites like Muweilah, which are buried under a few metres of sand. The 3D survey methodology proved essential to achieve line to line correlation for tracking walls. Although MetalMapper surveys were not as useful as hoped, they certainly indicated the value of including other geophysical data to constrain interpretation of complex GPR features. Practically, a significant improvement in data quality accrued when survey areas were flattened and de-vegetated.

In the wine district of Clare Valley in South Australia, the natural-voltage SP-signal generated by fluid flow in fractured rock during a pumping test was carefully monitored over time. From ten electrode-locations surrounding the pumping well logged every 15 s, the drawdown cone produced by pumping was determined on basis of the SP measurements together with laboratory measurements of the grain-boundary ζ (zeta) potential. Such measurements allowed calculation of the fractured-rock aquifer’s transmissivity and average permeability. Results were confirmed by piezometer measurements to the extent that data were available.

The study has revealed that SP-signals generated during pumping tests are of complex nature. However, if the pumping test is sufficiently long to allow the signal to stabilise, and careful field procedures are in place, then SP-measurements have the potential to add significant hydrogeological information. SP-measurements are relatively easy and cheap, and are, contrary to traditional hydrogeological methods, not restricted to the locations of existing piezometers, which is particularly useful in fractured-rock aquifers.

In recent years significant improvements in survey design, acquisition and processing technologies have advanced high resolution 3D seismic reflection imaging. However, shallow coal applications require high-resolution imagery that is more costly than conventional petroleum-scale reflection.

The 3D seismic grid design fundamentally controls the effective spatial sampling and signal-to-noise ratio of the final seismic volume. Whilst orthogonal, brick and slant-line shot-receiver geometries have all been employed, the latter has been found to produce fewer imaging artefacts, at a lower shot density and line-kilometre preparation cost. For targets at depths typical of underground coal-mining operations (70 to 700 m), a CMP bin size of approximately 50 square meters provides an optimal trade-off between spatial resolution and acquisition cost. If the bin size is increased beyond this point, the definition of subtle structural features in the target coal seam is compromised.

In typical coal imaging situations, dynamite charge sizes are in the range of 150g-400g which are small enough to provide necessary bandwidth, whilst still generating acceptable signal levels. Practical experience has determined that optimum source coupling is achieved by locating the charge at least 2m below base of weathering, and at least 12m below surface. To minimise the reduction in high-frequency fidelity, geophones with a natural frequency of at least 30 Hz are best.

The current and planned dedicated satellite gravity field missions (i.e., GRACE, CHAMP and GOCE) will make significant improvements to long-wavelength global models of the Earth’s gravitational field. Used together, they will provide a more homogeneous, accurate and near-complete spatial coverage than was ever-achievable using ‘classical’ techniques. GRACE and CHAMP will also measure temporal variations in the gravitational field. These data will offer considerably improved constraints on global and regional geophysical models, and the time varying components will offer a totally new dataset on contemporary geodynamic processes. This paper describes the concepts of dedicated satellite gra-vimetry and summarises the GRACE, CHAMP and GOCE mission parameters. Using published syntheses of the error degree variances of these new data, attempts will be made to quantify the level of improvement offered. This will then give an indication of how much more weight can be placed on these new data in geophysical studies. Finally, a strategy will be proposed to integrate these new data with terrestrial gravimetry using spherical harmonic filters.

A wide-angle reflection seismic survey coincident with a regional transect through Northeastern Yilgarn Craton focused on the Leonora-Laverton Tectonic Zone, Western Australia, was carried out to supplement deep seismic reflection studies. The major objectives were: to collect high-density refraction information for offsets of up to 60 km; to carry out a comparative study of near-vertical and wide-angle seismic images of the crust in the study area; to obtain velocity information for the upper crust. The survey deployed 120 short period recorders with a 500 m spacing. Acquisition parameters used for the wide-angle reflection experiment were selected so that it would to fit into the schedule and technology of the conventional reflection survey. The same vibrations were recorded in both surveys simultaneously. The major challenge in processing the wide-angle data was to manage the huge volume of information. The processing sequence included sorting into receiver and source gathers, cross-correlation with reference sweeps and stacking original seismic traces to form single source point traces, producing seismograms from individual traces and finally creating seismic record sections from separate seismograms.

High amplitude seismic signal from vibroseis sources was recorded at least up to 45 km offsets in the first arrivals, and later arrivals were observed down to 12 s next to sources. A preliminary upper crustal model developed from the wide-angle data shows that the thickness of a high velocity layer, corresponding to the greenstone rocks, is 4.0-4.5 km. The boundary separating this layer from a low velocity layer below it is possibly a compositional boundary between greenstones and underlying felsic gneisses. There is no evidence for high velocity material below this boundary. Assuming the Moho belongs to the deepest reflections modelled, total crustal thickness in the region can be speculatively estimated in the range 32-37 km. This model will be refined when more processed data become available for modelling.

To maintain self-sufficiency in oil, Uzbekistan is systematically exploring all oil-prospective basins in the country, including the Bukharo-Khivinsky paleorift (BKP) in central Uzbekistan. The BKP has a prospective area of 14 100 km² and contains an estimated 63 000 km³ of prospective Paleozoic sediments below gas-producing Mesozoic sediments. The “Paleorift” project (begun in November 2001) is an integrated project comprising ten regional profiles 80-90 km long that will include seismic (using new 3D equipment), MT, gravity, magnetics, and thermometry. The field work began with MT because MT costs less than seismic, is easy and quick to use and interpret, causes minimal disturbance to agricultural land, and is definitive in this application. Uzbekneftegaz has long experience with MT and other electrical methods and routinely uses them before seismic. Approx. 350 MT stations were measured on three of the regional profiles (1 km station spacing) and on two “prospecting” profiles (500 m station spacing) over known Mesozoic gas deposits. The profiles mapped the boundaries of the BKP central graben, a crustal conductor in the north part of the BKP, a gas-producing Jurassic structural high, and a resistivity low associated with the gas deposits.

Constrained 3D gravity and magnetic inversion has been applied to an area of approximately 11 km x 4.5 km straddling the Pillara gravity ridge, Lennard Shelf, Western Australia. The main aim was to better define the depth to top limestone. The starting point for inversion was a simplified geological model based on three generalised litho-stratigraphic units:

Shale/Siltstone, Limestone, and Basement.

A staged inversion procedure was adopted. First the effects of the large property contrasts were accounted for, most notably the density contrast between limestones and elastics. Subsequently the residual gravity and magnetic data were inverted to define more subtle contrasts within the sediments.

Gravity inversion involved adjustment of the Limestone contact geometry as well as model densities. The contact was fixed where pierced by drill holes, and a priori upper and lower bounds were imposed on the densities of the geological units. The inferred Limestone contact is a strong determinant of prospectivity, both in terms of depth and in terms of fault displacements. Final stage inversion highlighted coherent intra-sedimentary density trends oriented NE and NNE; these features could be associated with mineralising faults.

Aeromagnetic inversion defined a basement susceptibility distribution generally decreasing from the SW to the NE, reflecting the character of the TMI data. More subtle susceptibility trends attributed to the sediments may reflect the underlying structural fabric, though the most pervasive residual gravity features are not strongly developed in the residual magnetic data.

3D gravity inversion is effective the Lennard Shelf as a means for defining the depth to limestone. The reliability of the inversion will be enhanced in areas where the gross geometry of the Limestone contact and basement unconformity are constrained by sparse drilling and/or seismic, and where the densities are well known from drill core determinations or wireline logging. Magnetic inversion can play a supporting role, insofar as it defines the basement topography.

Cross-hole frequency domain electromagnetic (EM) methods are most commonly applied at high frequencies (> 1 kHz) for definition of petroleum and coal deposits. Interpretation is often based on tomographic reconstruction of the data, assuming far-field, ray-like behaviour. In some cases these assumptions are untenable. Conventional tomography is also questionable if data are recorded at depths which are small in comparison to a wavelength, i.e. if surface reflections are appreciable.

In order to address these limitations of tomography, a program has been written to invert cross-hole data from a vertical magnetic dipole transmitter in a layered earth. ID inversion and conventional tomography are applied to the same radio frequency data set to illustrate the advantages of the more rigorous approach at shallow depths and short ranges.

The vertical distribution of conductivity in the ground exhibits varied characteristics. Abrupt changes may occur at geological unconformities, and gradational changes are also common with saturation and salinity gradients, or with clay content through the weathering profile.

This paper describes a rapid, automatic method of picking probable vertical locations for abrupt changes in conductivity using the conductance-depth curve derived in an approximate conductivity-depth transform of airborne EM data. The method is based on identifying the location of slope changes.

The location of probable layer boundaries shows good agreement with synthetic data when the conductance contrast is adequate and boundaries are well separated. Correspondence of structures picked on separate x and z components increases the confidence of interpretation on a conductivity depth image (CDI).

In field data, the quantitative usefulness of including probable layer boundaries on a CDI display is yet to be determined, qualitatively however it appears to provide very useful information where conductivity contrasts are poorly imaged by choice of colour bar.

Much HEM survey data suffers from low (spatial) frequency along-line variability in amplitude caused by slow changes in the system geometry and electronic drift. Even though the system is periodically taken up to a sufficient altitude to re-zero the primary field correction, the errors are not totally removed. This paper describes a procedure to reduce the effect of these errors.

Each flight line is corrected by subtracting a slowly varying function of time that has been chosen so that the between-line differences over the whole survey area are minimized. The parameters of the correction functions are estimated using weighted, damped least squares. The procedure produces a marked improvement in the quality of the images of low frequency, in-phase channels that have been corrupted by drift noise.

Numerical experiments have been performed to test three inversion schemes for the imaging of surface seismic data. Three shallow subsurface models were considered: buried karst topography, dipping blocks, and isolated waste ponds. The synthetic experiments involved only a limited number of ‘surveys’ with just 8, 3, or 1 shots into variable length geophone arrays. In all cases, conventional seismic data processing hardly recovers the structure.

The inversion algorithm works in the frequency domain and relies on a finite element method to do the 2D/2.5D acoustic wave modelling. Our original inversion scheme used the known source signature (KSS), whereas the other two schemes either estimate it from the various shot gathers (ESI), or perform a normalised data inversion (NDI). The spectral inversions typically worked over 50-400 Hz and involved about 3500 data points. The features in the models typically have dimension of 5-20 m, which can be compared with the geophone spread length of 40 m (8 shots) to 100 m (3 or 1 shots).

The 8 shot inversion performed best for all the models. In fact, it was not possible to recover the karst structure with just 1 or 3 shots, but the other models were recovered, albeit with less accuracy, using such a small number of shots. ESI and NDI perform remarkably well on the dipping block and waste pond models. KSS yielded superior results to the other two schemes. The estimated source spectra were close to but did not exactly match the known wavelets.

One of the main issues in the characterisation of fractured reservoirs is the ability to predict the effect of fluid properties on seismic characteristics. Background porosity can significantly affect the elastic properties of fractured rocks. This effect is studied using the model of fractures as linear-slip interfaces in an isotropic porous background. Such a medium represents a particular case of a transversely isotropic (TI) porous medium, and can be described by equations of anisotropic poroelasticity. An analysis based on these equations yields explicit analytical expressions for the low-frequency elastic constants and anisotropy parameters of the fractured porous medium saturated with a given fluid. The five elastic constants of the resultant TI medium are derived as a function of the properties of the dry (isotropic) background porous matrix, fracture properties (normal and shear excess compliances), and fluid bulk modulus.

Analysis shows that: (1)- for penny-shaped cracks in a non-porous host medium the results reduce to the classical equations for isolated cracks; (2) for the same case of penny-shaped cracks but with background porosity, the expression for P-wave anisotropy parameter e has the form similar but not identical to that given by the model of Thomsen; (3) the compliance matrix of the fluid-saturated fractured medium with considerable background porosity is not equal to the compliance matrix of any solid medium with a single set of parallel fractures. This effect is caused by the wave-induced flow of fluid between pores and fractures.

These results can be used for fluid substitution in porous rocks with parallel fractures, which is important, in particular, for AVO analysis in naturally fractured reservoirs.