Exploration Geophysics - Volume 35, Issue 3, 2004

Seismic exploration in the Northwest Shelf of Australia is hampered by the presence of strong, coherent noise that overprints the reservoir section and deeper intervals. The limited success of attempts to improve seismic data quality in this region suggests that complicated noise waveforms are involved. We gained insight into the noise problem using realistic synthetic seismograms, upon which processing methods could be fully tested and the results judged objectively using known primaries.

Full-waveform synthetics were generated using the reflectivity method at three wells in the Dampier Sub-Basin and tied to field records from nearby seismic surveys. In addition, we used a finite-difference modelling technique to separate individual multiple modes from primary events in the synthetic data. We identified interbed multiples as the most cumbersome of the various multiple modes. They have similar amplitudes to weak primary events and are generated within high reflectivity packages in the overburden. They have similar moveout to the primaries and are not suppressed by stacking over the available offsets. Other significant noise sources identified include both P- and S-wave guided waves and mode-converted arrivals generated between carbonates in the Tertiary section.

Processing tests were applied to the synthetics and the results were assessed visually and by correlation with the known primaries. Routine processing largely suppressed the guided waves and mode-conversions and, to a lesser extent, the water-bottom multiples. Gap deconvolution in the tau-p domain helped to suppress the interbed multiples. Poor image quality and velocity uncertainty could remain at the target level because of weak primaries and imperfect noise elimination over the available offset range.

The seismic stacking-velocity data in the Great Australian Bight are a useful dataset for calculating depths and sediment thicknesses on a regional scale. This work compares these data with P-wave velocities from sonobuoys and sonic logs from wells, and on this basis, a depth over-estimate 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.

A procedure for the semi-automatic interpretation of geophysical data is described. A simple model, such as the geophysical response from a layer or a point source, is first constructed. Parameters of the model are then varied simultaneously while being constrained in that the model‘s geophysical response must fit the observed data. Each observed data point contributes a ‘spray’ of solutions in parameter space. The region of parameter space with the greatest density of solutions corresponds to the model that best fits the data, and error bounds can be obtained by examining the distribution of the solutions about that point. The procedure is demonstrated on gravity, magnetic, and seismic refraction models. The method is simple and fast to apply, and can be used as a filtering procedure rather than an interpretation procedure by replacing the measured data with the forward model response of the most probable set of model parameters.

The sensitivity and resolution of surface resistivity and Induced Polarization (IP) surveys can be severely restricted by the conductivity and thickness of overburden material. Electrode arrays using a current electrode buried below the overburden can increase the sensitivity and resolution of surveys conducted in such environments. A current electrode is located down a drillhole, and resistivity and chargeability measurements are made on the surface along survey lines radial to the drillhole. Reconnaissance resistivity/IP surveys using the downhole radial electrode array were conducted in the Cambro-Ordovician Mt Windsor Volcanics near Charters Towers in Queensland, an area covered by electrically conductive unconsolidated sediments of the Late Tertiary Campaspe Beds. The surveys demonstrated the effectiveness of the array in detecting anomalies from buried base metal targets. Data were acquired from a number of surveys conducted around a series of adjacent drillholes to provide continuous coverage across the prospective area. The data from individual radial traverses show the sounding response of the ground as well as the ground response along the profile. A method was devised for removing the sounding response, which is local to each drillhole, from each radial traverse so that the data from each traverse could be merged to form a continuous data set. A number of issues have been identified for future research which include the optimum depth of the buried current electrode, removal of the sounding response, and interpretation of the survey data.

SkyTEM is a time-domain, helicopter electromagnetic system designed for hydrogeophysical and environmental investigation. Developed as a rapid alternative to ground-based, transient electromagnetic measurements, the resolution capabilities are comparable to that of a conventional 40 × 40 m² system. Independent of the helicopter, the entire system is carried as an external sling load. In the present system, the transmitter, mounted on a lightweight wooden lattice frame, is a four-turn 12.5 x 12.5 m² square loop, divided into segments for transmitting a low moment with one turn and a high moment with all four turns. The low moment uses about 30 A with a turn-off time of about 4 |as; the high moment draws approximately 50 A, and has a turn-off time of about 80 |J,s. The shielded, overdamped, multi-turn receiver loop is rigidly mounted on the side of the transmitter loop. This is essentially a central-loop configuration with a 1.5 m vertical offset.

In vertical hover mode the SkyTEM responses were within 2% of those from a conventional ground-based system. Instrument bias level is not a concern as high-altitude tests showed that the background noise level is higher than the instrument bias level. By inverting a sounding from a test site to a standard model and then applying the SkyTEM system parameters to compute the forward response, conventional measurements were within 5% of SkyTEM responses for flight heights of 7.25, 10, and 20 m. Standard field operations include establishment of a repeat base station in the survey area where data are acquired approximately every 1.5 hours, when the helicopter is refuelled, to monitor system stability. Data acquired in a production survey were successful in detecting and delineating a buried-valley structure important in hydrogeophysical investigations.

Conductivity-depth images (CDIs) are finding application in salinity, groundwater, and environmental mapping. Hydrological modelling demands are for a much higher vertical resolution than the 10+m accuracy that was adequate in CDIs used for mineral exploration. Contractors are increasingly confident of system waveform, geometry, and some provide corrections for factors such as pitch, roll, and yaw. This increased system accuracy is the trigger for efforts in increasing the accuracy of processing.

The CDI process makes a number of approximations in order to increase the speed of processing. One of the most critical in program EMFlow is an assumption that the transmitter and receiver are entirely within the current system induced in the ground at all delay times. This assumption equates to all components of the secondary field decaying monotonically with time. For typical fixed-wing AEM geometries, this assumption is poor for the z component of the response, and in fact on a CDI, z component data may predict the top of a surficial conductor to be several metres in the air. Allowing for part of the induced currents to lie between the transmitter and receiver, when coupled with an Inductive Limit constraint, leads to an accurate prediction of surficial conductors to lie at or below surface.

The minimum size for shallow horizontal structures to be detectable and resolvable with airborne electromagnetic (AEM) systems is discussed, using synthetic modelling results and analysis of survey data.

Synthetic data were generated for the helicopter frequency-domain system RESOLVE and the fixed-wing time-domain systems TEMPEST and GEOTEM. The modelled scenarios include conductive prisms in a resistive host and resistive prisms in a conductive host. The EM responses of these models were computed for a range of prism thicknesses, side-lengths and host conductivities. Gaussian noise in amplitude comparable to actual system noise levels was added to the synthetic data followed by the derivation of conductivity-depth sections via three-layer inversion and EMFLOW conductivity-depth imaging. Where these ID algorithms failed to indicate the presence of the prism, the data were inspected for prism anomalies in order to evaluate whether 2D or 3D algorithms might be able to map these structures.

Results from the modelling of synthetic data indicate that beyond a minimum prism conductance (conductivity-thickness product), prism thickness is much less important for prism detectability than side-length. The minimum prism conductances determined for the RESOLVE, TEMPEST, and GEOTEM system are 1, 2, and 8 S, respectively. For horizontal prisms to be detectable with the RESOLVE system on response profiles and ID conductivity-depth sections they have to be at least 12 m wide. In order to resolve their thicknesses and conductivities, the prisms must be at least 120 m wide. For horizontal prisms to be detectable with the TEMPEST and GEOTEM system on ID conductivity-depth sections they have to be at least 150 m wide. In order to resolve their thicknesses and conductivities, the prisms must be at least 450 m wide. Profile inspection shows that horizontal structures as narrow as 24 m might be detectable with more advanced algorithms. However, the minimum size for resolving a prism‘s parameters is not a result of the application of ID algorithms but is defined by the system’s minimum footprint, which is a function of the system‘s geometry.

Some results of the synthetic data were confirmed by analysis of recent RESOLVE and TEMPEST survey data, which suggests conductive structures have to be at least 10-20 and 100-160 m wide, respectively, to be mapped on EMFLOW sections.

This study evaluates the in-situ stress field and the potential risk of fault reactivation and seal breach in the Mutineer and Exeter fields, Australian North West Shelf. Stress determinations are undertaken using pumping pressure test, rock mechanical, and log data from twelve wells. Subsequent geomechanical modelling uses the stress data to assess pore pressure changes that may induce slip on mapped faults cutting the region.

The principal stresses are assumed to be the vertical stress (S ), and a maximum and minimum horizontal stress, SH and Sh respectively. Borehole breakouts and drilling-induced tensile fractures (DITFs) interpreted from image logs indicate SH has an average orientation of 107°N and Sh is orientated 017°N. Leak-off test data compiled from well completion reports reveal the magnitude of Sh increases with depth at a rate of 17.1 MPa/km. Density log data show Sv can be approximated by a power law function. An upper bound to SH is calculated using the frictional limit to stress beyond which faulting occurs when using a frictional coefficient of 0.6. Better constraints on the magnitude of SH are gained using rock mechanical data, knowledge of Sh and S, mud weights, and the occurrence of borehole breakouts and DITFs. Stress magnitudes show that the tectonic regime is strike-slip (shvH).

The likelihood of fault reactivation in Mutineer-Exeter is expressed as the increase in pore pressure required for fault slip. Results show that faults are non-optimally orientated for reactivation by the stress field. The likelihood of brittle seal failure due to fault reactivation is low, primarily because of non-optimally orientated faults. The creation of new faults requires greater increases in pore pressure than reactivation and is thus seen as being more unlikely. The results have implications for seal integrity, well bore stability, and the safe and successful production of the fields.