Exploration Geophysics - Volume 25, Issue 2, 1994

Today’s 3-D surveys can be large or small, laid in straight lines across deserts or tundra or along winding roads through forest and jungles. Because 3-D acquisition requires careful planning it is essential to analyse the complexities which naturally arise. Computer programs are now available to assist the explorationist with this task.

The fundamentals of 3-D design start with the interpreter who must analyse the likely target geology and establish the desired fold (hence likely signal/noise) and bin size (spatial resolution and maximum non-aliased frequency on dipping events). Ray paths to formations will determine minimum and maximum acceptable shot-receiver offsets.

The basic equations governing all 3-D surveys are:

NS = F/(NC.b.b)

SLI = 1/(NS.b.2)

where NC = number of recording channels, b = bin size in metres, and NS is the number of shots per square metre needed to create the fold F, and SLI = shot line interval. Receiver line interval depends only on the minimum acceptable offset and is a critical part of determining the layout strategy – straight lines, bricks, zigzags, buttons to name a few. Increasing receiver line interval can, of course, reduce clearing costs. Each strategy has pros and cons from the processor’s and the field crew’s perspective and must be analysed. How will the data resulting from such a survey respond to velocity analysis, static corrections, DMO, muting, and migration? Will any noise in the data be cancelled by the stacking process (stack-array effect for linear noise, or enough offsets to attenuate multiples)? A comprehensive analysis of the fold and the offset and azimuth distribution in each CDP bin is essential.

Finally the field crew’s concerns must be addressed. Equipment is expensive to place and expensive to move. Different designs can save dollars – roll-on versus. roll-off for example. The computer program must allow easy movement of shots to undershoot lakes, rivers, pipelines, buildings or to fill in ‘holes’ created by shooting or recording along non straight lines.

Changes at each stage of the design process can be made and costed quickly leading to a successful, efficient 3-D seismic survey.

Integrating reflection seismic interpretation of the onshore eastern Otway Basin and offshore Torquay Embayment with published and in progress thermochronology has had an impact upon some of the recent models proposed for the Mesozoic to Recent history of the area.

Analysis of Early Cretaceous-age faulting with significant heaves in the Eastern Otway-Torquay offshore regions suggest that much of it strikes E-W, implying a general N-S extension at that time. To the east of an area centred around the Stoneyford Gravity High the strike of Early Cretaceous faults gradually changes from E-W to NNE-SSW over several tens of km. There is no evidence for an abrupt discrete transfer fault, controlling the change in fault orientation.

The sedimentary section around 143°E in the Port Campbell Embayment records a near orthogonal change in fault strike from WNW-ENE to SW-NE to the east. This change has been recognised for some time from surface data and published tectonic maps and has been used as corroborative evidence for locating a major structural boundary. The WNW-ESE fault trends seen on regional maps in the Port Campbell Embayment were initiated in the Cenomanian-Santonian; thus the orthogonal relationship of the fault trends seen around 143°E is a consequence of juxtaposing faults of different ages.

A large scale boundary in this general area is still supported by the significant change in Late Cretaceous subsidence patterns evident in the sedimentary record inferred from the seismic and local thermochronological interpretation. However that boundary, at least at shallow basement levels, is not manifested as a transfer zone and may lie further east, around the Stoneyford Gravity High, rather than on trend with Woorndoo Fault zone.

The removal of native vegetation for the development of land has caused the salinisation of surface soil and water resources in Australia. It is important to determine the existence of potential salinity hazards so that preventative measures may be taken at an early stage. The current hydrogeological conditions and the topography of a given catchment influence the movement of saline groundwater, and it is of particular importance to identify areas of high- and low-recharge to groundwater. On a catchment scale, the delineation of high-recharge areas by drilling can be prohibitively expensive, and more efficient mapping methods are required.

Broadband electromagnetic (EM) sounding methods offer the possibility of obtaining the vertical conductivity distribution in the regolith, to produce three-dimensional maps of the resistivity structure. These maps could be used to interpret the distribution of features related to hydrology, such as recharge and stratification of stored salt in catchments. For salinity-related problems, a regolith depth of about 50 m is relevant. It is therefore necessary to measure the high-frequency content of the EM response to differentiate shallow conductive layers.

For both ground-based and airborne EM methods, the response has been modelled in a frequency range of a few hundred Hz to 50 kHz. Salinity profile types have been identified to characterise different types of recharge. These profile types have been simulated with models consisting of horizontal layers of different resistivity. The theoretical modelling has determined that EM methods can be used to distinguish different types of salinity profiles. These results are supported by inversion of EM data collected in the field in a number of catchments.

The transient electromagnetic (TEM) method is used for ground-based broadband EM measurements, and modelling shows that both the depth and thickness of the salt accumulation can be resolved when its depth is as little as 6 m and its thickness 10 m. Additionally, a relatively resistive basement below the salt accumulation may be distinguished from a conductive one. Since the former case implies the presence of a high-recharge salinity profile and the latter case a low-recharge profile, this method may be used to differentiate the two types of profiles.

Down-hole conductivity logs in catchments near Collie and at East Yornaning, WA, confirm that different salinity profiles can be distinguished with EM methods. At Collie, the TEM method has detected the presence of salt accumulations and has determined the depth of their lower boundary. Inversion of these results shows the presence of a resistive electrical basement below the salt accumulations, and therefore indicates that relatively high recharge conditions are associated with these salinity profiles. Similarly, measurements along traverse lines in the East Yornaning catchment show a correlation between the inversions results and known geological features.