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\medskip\noindent
{\bf 5. Density and Velocity Logging}
\smallskip
{\sl In situ} physical property measurements are necessary for the conversion of reflection
times to depth and to fully
understand the cause of seismic reflectivity observed on Lithoprobe seismic profiles
across the Sudbury Structure.
In particular, this study will confirm whether massive sulphide
bodies can generate strong reflections. The
experiments have been conducted in three separate boreholes: BH60100 (Blezard),
BH85527 (McCreedy East)
and BH855970 (Gertrude).  Due to technical difficulties (induction effects ?), several
logging runs were required to obtain complete seismic
velocity information from the massive sulphide zones at Blezard and McCreedy East.
\smallskip
The compressional wave (P-wave) was estimated by interactively picking
the first arrivals on the near and far receivers. The P-wave velocity is
calculated by dividing the distance that separates the two receivers by the
delay between first arrival times. Velocity readings were obtained every
0.1 m. 
\medskip\noindent
{\bf 5.1 Logging results from borehole BH60100}
\smallskip
At Blezzard, density and full waveform sonic data were collected to a depth of 
640 m (Fig. 5.1.1).  Velocities above 6 km/s were recorded in most of the rock units. 
Amphibolites in the footwall complex exhibit high velocities in the range of 6.8 to 7.2 km/s while
granites (6.0 - 6.1) and massive sulphides (5.5 - 6.0) have relatively low velocities.
Felsic Norites have a narrow range of
velocities, varying from ~6.1 km/s to around ~6.4 km/s. Note evidence for fractures in the upper
150 m. Measured densities follow the same trend as the P-wave velocities: 
low densities (2.6) are associated with granites, the densities of other
footwall rocks are scattered between 2.9 and 3.15, and massive sulphides have the highest
densities. Note the smooth density log for felsic norites (2.85).
\smallskip
Important for the interpretation of the seismic data (and its time to depth conversion)
is the observation that the {\sl in situ} velocities in the felsic norite exceed 6000 m/s.
This is in good agreement with earlier physical rock property data (see chapter 4).
\smallskip
For the analysis of seismic reflectivity, acoustic impedance (the density
- velocity product) becomes the most important parameter.  Acoustic impedance contrasts
control the strength of the seismic reflection response (i.e,
contacts between lithological units of
different acoustic impedances are likely to generate reflections). 
Impedances calculated from {\sl in situ} densities and P-wave velocities are
presented in figure 5.1.1.  Again, low impedances are found for granites and felsic norites
and
high impedances are associated with amphibolites, sublayer and massive sulphides.
At the bottom of the SIC, prominent
reflections are likely caused when felsic norites are juxtaposed against massive sulphides,
sublayer or amphibolites.
\smallskip
All {\sl in situ} 
physical rock property measurements from BH 60100 are
displayed on Figure 5.1.2.  P-wave velocity is plotted as a function of
density for the different lithological units intersected. More
than 6000 readings are plotted.
In orange, the granites form a distinct group, as do the rocks of the footwall
complex (mostly amphibolites; green stars) and norites (pink triangles).  
The sublayer rocks overlap the fields of amphibolites and norites.  The
dashed lines indicate constant impedance (Z) values.  Most of the lithologies
have impedance values ranging from 14 to 20.  Data from the 
 massive sulphides in borehole BH60100  are plotted as
red triangles.  Note that most of the massive sulphides have impedances between 20 and 26.
These {\sl in situ} data measurements are in good agreement with the results 
obtained from high-pressure laboratory studies.
\medskip\noindent
{\bf 5.2 Logging results from borehole BH85527}
\smallskip
At McCreedy East,
 density and full waveform sonic data were collected to a depth of 
1680 m (Fig. 5.2.1).  Velocities above 6 km/s were recorded in most of the rock units. 
Metagabbros in the footwall complex exhibit high velocities in the range of 6.8 to 7.2 km/s while
norites (6.1 - 6.4) and massive sulphides (5.0 - 6.0) have relatively low
velocities.
Measured densities follow the same trend as the P-wave velocities: 
low densities (2.8) are associated with norites, the densities of the
footwall rocks are scattered between 2.8 and 3.15, and massive sulphides have the highest
densities. Note the smooth density log for norites to about 600 m depth.
\smallskip
Important for the interpretation of the seismic data from the North Range
is the observation that the {\sl in situ} velocities in the SIC and the Levack Gneiss Complex
exceed 6000 m/s.
This is in good agreement with earlier logging results (i.e., Milkereit et al, 1994).
In the Levack Gneiss Complex, prominent
reflections are likely caused when mafic rocks are juxtaposed against felsic
rocks. The lower portion of the 
SIC will appear transparent,
due to a lack of impedance contrasts in the norites.
\smallskip
Impedances calculated from {\sl in situ} densities and P-wave velocities are
shown in figure 5.2.1.  Again, intermediate impedances are found for norites and most footwall
rocks; high impedances are associated with metagabbros and massive sulphides.
\smallskip
All {\sl in situ} 
physical rock property measurements from BH85527 are
displayed on Figure 5.2.2.  P-wave velocity is plotted as a function of
density for the different lithological units intersected. More than 14000 readings are plotted.
In orange, the metagabbros form a distinct group, as do the other rocks of the footwall
complex (in green) and norites (pink triangles).  
The sublayer rocks overlap the fields of footwall rocks and norites.  The
dashed lines indicate constant impedance (Z) values.  Most of the lithologies
have impedance values ranging from 14 to 20.  Data from the 
 massive sulphides in borehole BH85527  are plotted as
red triangles.  Note that most of the massive sulphides have impedances between 20 and 29.
\medskip\noindent
{\bf 5.3 Logging result from borehole BH855970}
\smallskip
At Gertrude,
 density and full waveform sonic data were collected to a depth of 
290 m (Fig. 5.3.1).  Velocities above 6 km/s were recorded in most of the rock units. 
Metagabbros in the footwall complex exhibit high velocities in the range of 6.5 to 7.0 km/s while
felsic norites (6.1 - 6.3) and massive sulphides (5.2 - 5.8) have the lowest.  
Measured densities follow the same trend as the P-wave velocities: 
low densities (2.85) are associated with norites, the densities of the
footwall rocks are scattered between 3.0 and 3.15, and massive sulphides have the highest
densities. Note the smooth density and velocity logs for felsic norites to about 190 m depth.
\smallskip
Important for the interpretation (i.e, time-to-depth conversion) of the seismic data from the South Range 
is the observation that the {\sl in situ} velocities in the SIC and the footwall complex
exceed 6000 m/s. At Gertrude, prominent
reflections are likely caused when norites are juxtaposed against massive sulphides, sublayer
or metagabbros.
Again, the lower portion of the 
SIC a will appear transparent,
due to a lack of impedance contrasts in the norites.
\smallskip
Impedances calculated from {\sl in situ} densities and P-wave velocities are
shown in figure 5.3.1.  Low impedances are found for norites; high impedances are 
associated with metagabbros and massive sulphides.
All {\sl in situ} 
physical rock property measurements from BH855970 are
displayed on Figure 5.3.2.  P-wave velocity is plotted as a function of
density for the different lithological units intersected. More 
than 2500 readings are plotted.
In orange, the metagabbros form a distinct group, as do the 
norites (pink triangles).  
The sublayer rocks overlap the fields of footwall rocks and norites.  The
dashed lines indicate constant impedance (Z) values.  Most of the lithologies
have impedance values ranging from 14 to 20.  Data from the 
 massive sulphides in borehole BH855970  are plotted as
red triangles.  Note that most of the massive sulphides have impedances between 20 and 27.
\medskip\noindent
{\bf 5.4. Discussion}
\smallskip
{\sl In situ} studies in the three boreholes confirm that significant impedance contrasts
exists at the contacts between
major lithological units of the SIC and the footwall complex. 
Figures 5.1.2, 5.2.2 and 5.3.2 show that the 
the density-velocity
values for granites, norites, sublayer and footwall rocks
follow the Nafe-Drake curve,
which is an empirical relationship 
defining the average density-velocity behaviour for most crustal
rocks (i.e., silicate rocks). Important for the direct seismic mapping of ore bodies 
is a knowledge of 
impedance values for disseminated and massive sulphides. In chapter 4 we 
demonstrated that the ores occupy a density-velocity field which is distinct from that
of common silicate rocks and governed by simple mixing rules between the 
properties of host rocks and end-member sulphides. Our {\sl in situ} velocity and density
measurements confirm
that ore samples from Sudbury occupy distinct density-velocity field away from
the Nafe-Drake curve. Po-rich ores are characterized by high impedance values.
However, metagabbros, amphibolites and sublayer can show similar high impedance values.
In chapter 11 we will discuss how we can distinguish between po-rich massive sulphides and
other
high impedance rocks in the footwall complex. 
In Sudbury, even po-rich massive sulphides in contact with granites or norites, will produce 
impedance contrasts which are much
greater
than those expected at other lithological contacts 
in a typical crystalline environment.
These {\sl in situ} data measurements are in good agreement with the results 
obtained from high-pressure laboratory studies (chapter 4).

\bye