Exploration Geophysics - Volume 38, Issue 2, 2007

Despite continued improvements in acoustic logging technology, logs often remain influenced by formation damage and mud-filtrate invasion. Seismic data, another type of measurement that is not affected by drilling, can be integrated in the understanding of incorrect log readings resulting from damage in the immediate vicinity of the wellbore. An iterative workflow of log conditioning, petrophysical interpretation, rock physics modelling, and synthetic-to-seismic matching is applied to ensure the P-sonic and density logs represent the true in situ properties of the rocks. Key to this integration is the development of consistent petrophysical interpretations and rock physics models.

In this paper, we discuss how we arrived at a seismically constrained petrophysical interpretation in the Scarborough gas field in the Exmouth Sub Basin, offshore North West Australia. The logs of these relatively old wells were badly affected by deep invasion of the drilling fluids into the gas sand reservoirs. Conditioning of the well logs was essential to replace the poor quality data. The conditioned logs were used to create an initial rock physics model. Synthetic seismograms were then created using the conditioned logs from the rock physics modelling. Through iterating between petrophysics, rock physics, and synthetic-to-seismic matching we arrived at the final interpretation that is consistent with all available subsurface data. The final synthetic-to-seismic matches for the wells were significantly improved. The integration of different subsurface data types through rock physics modelling significantly reduced the uncertainty in the reservoir properties of the Scarborough gas field.

The Campaspe Deep Lead, which underlies the modern-day Campaspe River Valley in north-eastern Victoria, is an important local source of ground water. The exact location of the lead near Goornong is disputed, and it is consequently of interest to see whether a geophysical method can add information to the existing hydrological and geological data.

Two microtremor methods have been applied in this area. The horizontal to vertical spectral ratio method (HVSR) was used as a reconnaissance tool to gain information about the change in resonant frequency across the field area. Modelling of the HVSR data qualitatively estimated a basement depression below and east of the river. The passive seismic spatial autocorrelation (SPAC) method was applied at four locations in the vicinity of the aquifer to gain information about depth to bedrock and layer intervals as well as shear wave velocities. Unconstrained inversion of SPAC data resulted in similar basement topography, but overestimated by tens of metres. Constraining the basement depth to that found in nearby boreholes gives a relatively unchanged data misfit, and an approximate location of the vertical boundaries of the deep lead. Use of these methods in groundwater exploration would primarily be for cost-effective mapping of regolith and basement structure between boreholes, especially in saline overburden, land or urban environments.

In the absence of attitude and altitude sensors directly attached to the bird, helicopter airborne electromagnetic (AEM) data are typically interpreted assuming that the sensor bird maintains a fixed attitude as well as fixed vertical and horizontal offsets relative to the helicopter during survey. Laser altimeters fitted to the bird can be used for measuring bird-height over land, but these altimeters do not necessarily function over seawater, and in this case a fixed vertical offset is subtracted from the helicopter altimetry to estimate the height of the bird above sea level. With current navigation technology, these assumptions could be overcome by incorporating suitable altimetry and navigation sensors into AEM systems. We constructed an airborne testing rig to represent an AEM bird and fitted GPS, inertial navigation, and altimetry sensors to accurately measure bird attitude and height above seawater (as required for bathymetric mapping) during typical and atypical AEM-survey flight conditions. Bird height above sea level was measured with radar and laser altimeters, and was also estimated from the GPS receiver height. Bird attitude was obtained from the inertial navigation unit (INU) data and was compared with attitude data derived from a triangular configuration of three GPS antennas. Each antenna was linked to a pair of GPS receivers to allow comparison between dual-frequency, high-fidelity and single-frequency, low-fidelity measurements.

Bird attitude and altimetry measurements were recorded during surveys flown offshore Tickera Bay (Spencer Gulf, SA) and within Broken Bay (Sydney, NSW). These surveys were flown at a maximum altitude of 180 m, with bird roll angles up to about ± 40°. Using dual-frequency GPS receivers, the agreement between heights derived from GPS and laser (radar) altimeter data is typically ~0.3 m (0.6 m). GPS antenna separations computed during flight from measured GPS positions gave agreement to within 0.4% (typically 0.2%) of measured values. The agreement between pitch and roll angles computed from GPS antenna positions and INU measurements was within ~1° and 2° respectively, neglecting the effects of any offsets in the alignment of coordinate axes between the two systems. A comparison of pitch and roll angles obtained from single and dual-frequency GPS receivers showed that the accuracy of pitch and roll angles obtained from single-frequency GPS receivers is generally about ± 2°. The discrepancy in results from the different GPS receivers increases at various points along the flight path. This increase was attributed to a decrease in accuracy in results from the single frequency GPS receivers. Roll and pitch angle profiles show oscillations consistent with harmonic (pendulum) motion of the bird at the end of the tow cable connecting the bird to the helicopter. We recommend the use of inertial navigation interfaced to a single dual-frequency GPS receiver for accurate attitude and position measurements, combined with laser and radar altimetry sensors. The future implications of this study are that we expect to accurately measure attitude and altitude of a towed bird over seawater and that consequently these altimetry and attitude sensors will be implemented on a new helicopter TEM system (SeaTEM) specifically designed for bathymetric mapping.

The potential of obtaining water depths derived from AEM data that are accurate to ~0.5 m cannot be fully realised if the variable geometry between transmitter and receiver is neglected. The rigorous approach of monitoring the bird offset in three dimensions is currently too costly. However, neglecting the bird offset variations from the assumed fixed nominal offset is problematic because the measured decay shape is sensitive to the transmitter–receiver geometry. An alternative procedure is to invert both layered earth parameters and geometric parameters using a non-linear least-squares method. This inversion procedure has been tested on 25 Hz GEOTEM data from a survey flown in the Torres Strait. The accuracy of the inverted water depths was appraised using data from laser airborne depth sounding surveys. Good agreement, to within ~1 m was obtained between the different methods.