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\line{\bf 10. Interpretation\hfil}
\smallskip
The following interpretation of the Lithoprobe seismic line 43
is consistent with geophysical logs,
physical rock property data and 3-D forward modeling results as well as 
geological mapping information and drill data made available to us.
The 3-D subsurface model of the Creighton/Gertrude area was modified to
accommodate (1) the mined-out portion of the Creighton 402 ore body
beneath 4500 ft depth (the northernmost portion of the body, located
east of line
43 in Fig 2.5), and (2) updated physical rock properties
based on the laboratory and {\sl in situ} logs.
Figure 10.1 shows the 2-D geological 
cross section beneath line Lithoprobe line 43. The section was extracted from
the 3-D subsurface model for the area. 
Note that the top of the Creighton 402 body is located west of the seismic profile
and the bottom is located east. 
The 3-D seismic 
model is based on a weak positive impedance contrast between norites of the SIC
and granite/greenstones of the footwall complex. Above 4500 ft, the 402 ore body
is characterized by a strong positive impedance contrast, and below 4500 ft by
an even stronger negative impedance contrast. Other parameters of the 3-D model such as the 
the Gertrude deposit
and the footwall contact remain unchanged. 
\smallskip
Figure 10.2 shows the zero-offset synthetic seismogram computed using the Born
Approximation. For comparision, the north-dipping footwall contact and ore body
are shown on the synthetic and observed seismic data.
The synthetic response is composed of (1) a large hyperbolic diffraction event, and (2) series
of strong diffraction-like events with prominent north-dipping amplitudes.
The synthetic response is in good agreement with the observed seismic data shown
in Fig. 10.3 and the synthetic data examples shown in Fig. 7.4. The horizontal interface 
at 4500 ft (caused by the mined-out lower portion of the ore body) will generate
the strongest event, a diffraction curve centered beneath station 400 at 4500 ft. 
The strongest reflection
amplitudes are observed close to the apex. The response of the "intact" ore is completely different.
As shown in figures 6.3 and 6.4, a steeply
dipping high impedance body will cause the characteristic high amplitudes
of the scattering response to be shifted towards larger offsets in the dip direction.
Figure 10.3 confirms that the high amplitude reflections caused by the Creighton 402 body can be observed
over a region which is
considerably larger than the actual size of the scattering body. The seismic energy is concentrated
primarily in the down-dip direction. Note that the strongest north-dipping reflections are
observed about 1500 m north of the scatterer.
\smallskip
The final composite cross section for Lithoprobe line 43 in the Creighton area is
shown in Fig. 10.4. This simple geological model produces a remarkable match between synthetic
and observed seismic data. The MUSIC stack revealed the presence of seismic 
scatterers beneath line 43 (or in the immediate vicinity of line 43).   
In our interpretation sattering events 1 and 2 are located west of line 43. This confirms that
line 43 is not an ideal strike line in the Creighton area. Event 4 was seen on the original
seismic section, but is caused by the mined-out portion of the Creighton 402 ore body.
Events 5-7 are located east of the seismic line and could be caused by ore bodies/cavities
of the Creighton mine. 
\bye