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Depth-Conversion of Seismic Reflection Data in the Eromanga Basin, South Australia

Timothy A. Rady, 2006
Bachelor of Science (Petroleum Geology & Geophysics)
Australian School of Petroleum
The University of Adelaide


Depth-conversion of seismic reflection data in the Eromanga Basin is traditionally performed using
an average velocity model from datum to target horizon. Two-way time and depth data for the target
horizon at each well are used to calculate a pseudo-velocity, which is then gridded and contoured to
produce a velocity map. The average velocity map is then combined with an inte1preted two-way time
map to yield a depth map for the target horizon. The average velocity model is only controlled at each
well and relies on interpolation and extrapolation of values between wells to produce a velocity map,
hence any velocity variation between wells due to gross changes in lithology, pressure or burial depth
are not accounted for.
The top McKinlay Member and top Hutton Sandstone horizons were depth-converted using the
average velocity, interval velocity layer cake, constant velocity layer cake and processing velocityderived
techniques. Each technique was selected to attempt to improve on the currently widely-used
average velocity technique by accounting for variation of geological factors, such as lithology,
thickness and burial depth in their velocity models. Velocity survey data is not widely m'ailable for the
study area, hence analytic velocity functions such as V0-k and other sonic log-derived models for
depth-conversion were not considered. As a result, the depth-conversion techniques utilized in this
study rely on velocity models derived by pseudo-velocities. A second type of input velocity data -
seismic processing velocities - were used to construct a velocity model from datum to each target
horizon for use in the fourth depth-conversion technique.
The average velocity technique was used as a reference with which to compare the other three
techniques. A layer-cake technique comprising methodology similar to the average velocity technique
was undertaken to take variation in lithology and formation thickness into account. A second layer
cake technique using a constant velocity to derive an approximate depth from a twt thickness map was
undertaken to improve the confidence in depth prediction between wells. This technique also
considers the effect of lithology and thickness variation on velocity. For both layer cake techniques,
the sequence from datum to top Hutton Sandstone was split into five layers for the depth conversion:
mean sea level to top Oodnadatta Formation (layer 1), top Oodnadatta Formation to top Coorikiana
Sandstone (layer 2); top Coorikiana Sandstone to top Cadna-Owie Formation (layer 3); top CadnaOwie
Formation to top McKinlay Member (layer 4) and top McKinlay member to top Hutton
Sandstone (layer 5).

The average velocity method predicts depth to the top McKinlay Member with a mean error of 2.1m
and standard deviation of the error of 4.9111. The mean and standard deviation of the errors predicted
by each tested depth-conversion technique are similar to average velocity technique at both target
horizons. Mean errors for the four techniques range between 0. 7- 2.5 m at top McKinlay Member
and 2.2- 4.8m at top Hutton Sandstone. The standard deviation ofthe errors for the four techniques
ranges between 4- 6.6m for top McKinlay Member and 4.9- 8.5 m for top Hutton Sandstone. The
maximum error and range of error are more significant in determining the success of each depthconversion
technique as they give an insight into the potential error bar of depth prognoses in future
drilling. The lowest maximum error at top McKinlay Member is 9.9 m and lowest range of error (the
difference between minimum and maximum errors) is 14.9 m, both of which are recorded by the
average velocity depth-conversion technique. At top Hutton Sandstone, the lowest maximum error is
13.5 m and lowest range of error is 18m, also recorded by the average velocity depth-conversion
The layer cake techniques show no significant improvement over the currently widely-used average
velocity technique. This suggests that the identified geological factors have a negligible effect on
velocity variation or that the increased source of error in these multilayer velocity models oufll!eighs
their potential for improvement. Depth-conversion utilizing seismic velocities produces relatively
accurate depth prognoses, uses a densely-sampled and geologically-feasible velocity model and, as a
result, greatly increases the confidence in depth prognosis away from existing well control. In light of
these results, future depth prognoses in the study area and geologically-similar areas should be
calculated using the average velocity depth-conversion technique. Additionally, the careful use of
seismic velocities can be successful in deriving accurate depths from seismic reflection data in
regions of little well control or greater distances befll!een wells.

Australian School of Petroleum



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