Imaging Dinosaur Fossils by Seismic Tomography and GPR

Imaging Dinosaur Fossils by Seismic Tomography and GPR
Travis Crosby*, Min Zhou, Scott Sampson and Gerard T. Schuster, University of Utah
Summary
Seismic and radar experiments were conducted nest to
a dinosaur fossil quarry in southern Utah in the Fall of
2001. The objective was to determine if radar and seismic imaging methods could detect the location of fossil
deposits to a depth of several meters. More than 46,000
thousand first arrivals were picked from the seismic data
and inverted for the P-wave velocity distribution. In addition, ground penetrating radar (GPR) images were used
to help evaluate the velocity anomalies in the tomograms.
Preliminary results indicate a localized low-velocity zone
at a depth of 1-3 m that could be a fossil deposit. However, we do not know if deposits are indicated by lowor high-velocity anomalies. Excavation of this site in the
near future should answer this question.
Seismic and Radar Experiments over a Dinosaur Quarry
A 2-D grid of 216 40-Hz geophones was sited next to a
known dinosaur quarry in southern Utah. A hand-held
hammer was used to excite seismic energy at each geophone location and 0.5 m offline from each geophone. In
addition, small-offset GPR data were collected along the
seismic lines, with a GPR source point at almost every
geophone location. The objective was to collect both
seismic and GPR data and determine the feasibility of detecting dinosaur fossils next to an adjacent fossil quarry.
The depth of the fossils was expected to be no more than
several meters.
The geophone grid encompassed an 8 m by 11.5 m
patch of ground, with 540 shot gathers and each shot
gather consisting of 216 traces. The inline geophone spacing was 0.5 m and the crossline spacing was 1.0 m. First
arrival traveltimes were picked from each trace to give a
total of 116,640 traveltimes, of which less than a half were
used for this report. Figure 1 depicts the field site.
Numerical Results
As a test of the tomography algorithm, traveltimes were
generated by a finite-difference solution to the 3-D eikonal
equation for the source-receiver geometry associated with
the S. Utah field experiment. The top part of Figure 2
shows the model at different depths and Figure 3 shows
the associated tomograms. These velocity anomalies were
chosen to simulate hypothetical deposits of dinosaur fossils as high-velocity anomalies, although it is not known
whether actual fossils represent slow- or fast-velocity
anomalies. Nevertheless, there is acceptable agreement
between the locations and geometries of the actual velocity anomalies and those depicted in the tomograms.
For the S. Utah field data, around 46,000 first-arrival
traveltimes were picked from the shot gathers where the
shot points visited each geophone location. These data
were inverted for the velocity model using a preconditioned steepest descent method with multiscale regularization (Morey and Schuster, 1999). Twenty-one iterations were required to bring the RMS traveltime residual to below 1.62 ms, and the effective cell size was no
smaller than a 1 m by 1 m by 0.5 m cube. Slices of the
reconstructed tomograms at different depths are shown
in Figure 4. There is a prominent low-velocity feature in
the central part of the panel between the depths of 1.5
m and 3.0 m. This feature may or may not be indicative of a fossil deposit, but the GPR data should help
validate its existence. We will soon integrate both the
tomogram and the GPR data to indicate the liklihood of
a fossil deposit, and will report our results at the SEG
meeting. It is not known whether fossils are indicated by
high or low-velocity anomalies, but this site should soon
be excavated to determine the tomographic signature of
a dinosaur fossil.
Discussion
The results presented in this abstract are considered preliminary, but indicate the possible presence of a lowvelocity zone between the depths of 1.5 m and 3.0 m. If
fossil deposits in this quarry are characterized by low velocities then this could possibly indicate a potentially rich
site of duck-bill dinosaur fossils. We will soon evaluate the
GPR data to see if they indicate amplitude anomalies at
the low-velocity zones.
However, possible sources of tomogram error should
be evaluated. For example, tomograms will be checked
against their consistency with common-offset gathers; offset gathers with intermediate offsets should clearly show
time delays in the first arrivals that visit the alleged lowvelocity zone. We will also check for anisotropy effects
by displaying constant offset gathers as a function of
azimuthal angle; periodic undulations in the traveltime
residuals can be indicative of anisotropic rocks. Finally,
the GPR data will be compared to the tomograms and determine if GPR amplitude anomalies are consistent with
the anomalous velocity zones. The success of this method
in detecting fossil locations can greatly increase the efficiency of paleontologists in finding new deposits of dinosaur fossils. Greater efficiency can mean accelerated
Imaging Dinosaur Fossils by Seismic Tomography and GPR
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progress in the science of paleontology.
References
Morey, D., and Schuster, G.T., 1999, Paleoseismicity of
the Oquirrh fault, Utah from shallow seismic tomography:
Geophysical J. Int., 138/1, 25-35.
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Figure 1: (Top) Array of 40-Hz geophones deployed
in S. Utah in Fall of 2001. Geophones are small orange devices connected to black cables, and dinosaur
quarry is to the right side of this picture. (Bottom)
Duck-bill dinosaur fossilized bone in above quarry
with lens cap as scale.
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Figure 2: Horizontal slices of 3-D test velocity model
at different depths. Source and receiver geometry is
almost identical to actual field experiment in S. Utah,
and velocity colorbar is in units of m/s.
Imaging Dinosaur Fossils by Seismic Tomography and GPR
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Figure 3: Depth slices of tomogram reconstructed
from synthetic traveltimes. There is mostly acceptable agreement between the above reconstructed velocity anomalies and the actual anomalies in Figure 2.
This indicates the capability of resolving fast anomalies with spatial dimensions of about 1 meter or more.
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Figure 4: Depth slices of tomograms obtained by inverting over 46,000 traveltimes from the field data.
The RMS residual was approximately 1.6 ms and the
observed first arrivals contained first arrivals with frequencies greater than 500 Hz. Irregular boundary
between the dark blue and lighter colors at shallow
depths is indicative of the dipping topography at the
field site.