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\line{\bf 2. Introduction\hfil}
\medskip
Since 1990, a series of seismic and borehole geophysical 
experiments were conducted across the Sudbury Structure in Ontario.
These experiments have indicated 
the potential for using
high-frequency
reflection seismic profiling as an exploration tool. While 2-D seismic 
lines delineate the main structural
features of the area, only true dip lines will permit accurate positioning
of seismic reflections in the subsurface. In the presence of steep dip, 
lines at significant angles to the true dip will contain
large errors in the lateral position of recorded events. 3-D surface seismic 
profiling solves this problem by permitting the dip to be measured across 
the survey area. 3-D seismic profiling will also record much more of 
the scattered energy from small bodies such as ore deposits, permitting 
location size and dip to be acurately resolved. 
\medskip
In this report we summarize some of the key results from previous studies
and address the major objectives of the current
IPP research project:
\smallskip
\item{(1)} To probe the cause of seismic reflections by
    establishing a direct link between lithology and physical rock
    properties.
\smallskip
\item{(2)} To integrate surface seismic images with available lithological, borehole
    geophysical, vertical seismic profiling and physical rock property
    data.
\smallskip
\item{(3)} To evaluate the effect of complex 3-D subsurface structures, such as embayments
in the footwall complex, on arbitrarily located 2-D seismic profiles.
\smallskip
\item{(4)} To study the 3-D response of massive sulphides such as the Creighton
402 body and the Gertrude deposit.
\medskip
In order to calibrate the existing seismic data sets and
to further study and optimize acquisition and processing 
parameters for a 3-D seismic experiment, we conducted a series of borehole geophysical
surveys and the first realistic 3-D forward modeling study of crustal structures.
In chapters 3, 5 and 6, we summarize this year's field program
and present geophysical logs through massive sulphides at Blezard, McCreedy East and Gertrude.
In chapter 4, we discuss the physical properties of massive sulphides as
measured on samples from the Sudbury Structures.
In chapter 8, we present the 3-D reflection response of the Creighton/Gertrude area
utilizing a detailed 
geological model and an extended physical rock property data base.  
This data base is  utilized for a detailed interpretation of Lithoprobe line 43
(chapter 10). 
During processing and interpretation of seismic data from the Sudbury  camp,
it became necessary to develop specialized seismic data processing and interpretation
software suitable for the crystalline environment.
New software had to be written to analyze and display densely sampled geophysical logs
(chapter 5).
The Born approximation modeling software package (chapter 7) was modified 
to support arbitrary 2-D line locations.
A new processing algorithm was developed to extract the seismic scattering response 
from high fold, multi-channel surface seismic data.
Results of the scattering analysis for the identification of 
high impedance bodies in the crystalline crust are presented in chapter 9.
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\medskip\noindent
\line{\bf 2.1 Location \hfil}
\smallskip
The Origin of the Sudbury Structure and associated ore deposits has long 
been a subject of controversy.
Understanding the geometry of the Sudbury Structure is important as it is 
relevant to the long term
exploration of its vast mineral resources. Recently,
several reconnaissance geophysical studies have been conducted 
across the Structure as part of the 
Lithoprobe program.  Spearheaded by high resolution reflection seismic profiling and
aided by physical rock property
	and borehole geophysical surveys,
results of the Lithoprobe project presented the first picture 
	of the highly asymmetric deep geometry of the Sudbury Structure.
In particular, a major consequence of the
	newly defined deformation zones is that some 
important ore-bearing horizons on the margin of the 
	Igneous Complex may be repeated at depth. 
Preliminary physical rock property studies indicate that in a geological setting such as the 
Sudbury Igneous Complex (SIC), massive sulphide bodies should cause 
seismic reflections many times more intensive than those
expected in a typical crystalline environment. Thus ore bodies could be 
detected and delineated
as localized "bright spots" in a 3-D data volume, 
in much the same way that "bright spot" technology is 
used to prospect for natural gas in the hydrocarbon industry.

Figure 2.1 shows
the general study area across the Sudbury Structure.
To meet the research objectives, 
Inco Exploration and Technical Services Inc. (IETS) 
and the GSC agreed on a comprehensive 6-month research
project that included (i)
a field program with VSP profiling, density and sonic logging
    surveys in boreholes BH85527 (McCreedy east), BH60100 (Blezard) and
BH855970 (Gertrude);
(ii) laboratory measurements of physical rock properties on representative
    samples of massive sulphides from the Sudbury Structure;
and (iii) building a detailed digital 3-D subsurface model of the footwall contact
and Gertrude/Creighton ore bodies based on drilling data provided by IETS.
\medskip\noindent
{\bf 2.2 Seismic Profiles 43 and 44}
\smallskip
New Lithoprobe
high resolution vibroseis lines 43 and 44 were acquired in the fall of 1993.  
Together with Lithoprobe line 41 and the IETS line 
across the Murray Mine, profiles 43 and 44 provide good control 
of major crustal structures in the Sudbury South Range (Fig. 2.1)
A common denominator of all seismic profiles 
across the Sudbury Structure is the highly reflective footwall beneath the 
SIC (i.e., high-grade rocks of the Levack Gneiss Complex beneath the North Range
and granite/greenstones beneath the South Range). To date,
our results indicate that the reflective footwall complex represents
an important regional marker horizon for seismic exploration 
of the Sudbury Structure.
Line 43 was acquired along highway 144 (Chelmsford bypass). 
High-frequency vibroseis data acquisition
started
at the junction of highway 144 with the TransCanada Highway and ended
20 km to the north in Chelsmford. 
Fig. 2.2 shows the migrated seismic image of line 43 in the vicinity of Inco's
Creighton mine.
The north-dipping footwall complex can be traced to about 1 s two-way reflection time
(approx. 10,000 ft depth) where the south-dipping South Range Shear Zone intersects
the contact. Norites of the SIC are free of reflections (=transparent) with the exception
of an area 3.4 km north of the footwall contact (marked "?" in Fig. 2.2).
The origin of these reflections is unknown. Borehole geophysical logs 
were obtained from Inco's Gertrude exploration area (located about 600 m west of 
highway 144). A detailed interpretation
of Lithoprobe line 43 is given in chapter 10.
\smallskip
Line 44 follows highway 69 from Sudbury to Val Caron
(Fig. 2.1). The total length of line 44 is 8.0 km. 
Fig. 2.3 shows the migrated seismic image in the vicinity of Inco's Blezard
exploration area. The highly reflective footwall of the SIC dips to the north
and is "intersected" by the south-dipping South Range Shear Zone at about 3500 ft.
Local high amplitude reflections observed along line 44
are the target of borehole geophysical logs and 
vertical seismic profiling study. The origin of prominent north-dipping reflections
in the footwall complex (but above the deformation zone) is unkown
(marked "?" in Fig. 2.3).
\medskip\noindent
{\bf 2.3 Seismic Data Interpretation}
\smallskip
Figures 2.2 and 2.3 show the migrated sections of lines 43 and 44. 
The location
of Blezard VSP experiment is indicated. Important for the overall interpretation
of the seismic sections are (i) the origin of the prominent north-dipping reflection 
package
imaged between surface and 1000 ms, tentatively correlated with the footwall
complex of the SIC (ii) the close spatial relationship between local amplitude
anomalies (on line 44) or
bright diffraction-shaped events (on line 43)
and massive sulphide deposits. {\sl In situ} 
velocities are required to calibrate the seismic interpretation and to establish
a link between reflection time and target depth. 
In order to use surface seismic surveys to identify potential massive sulphide
ore deposits we need to know more about seismic reflections from sulphide
bodies. In particular, we have to address the question {\sl "can large massive 
sulphide ore bodies produce seismic reflections?"}. 
A series of borehole 
geophysical surveys were scheduled for three holes at McCreedy East,
Blezard and Gertrude.
The objective of vertical seismic profiling (VSP) 
is to identify unambiguously the precise locations from which seismic reflections originate.
In a VSP, shots fired at the surface are recorded with
a 3-component geophone positioned at different depths along
the borehole. When the geophone is located adjacent to a fault, lithological contact, or 
ore body, we know the reflection we record originates from the particular 
geological setting.
The VSP surveys were supported by density and full waveform sonic logging.
Because the geophones are positioned below the surface, it is often possible
to follow reflections to their point of origin,
removing a source of ambiguity in the interpretation of surface seismograms.
Shallow boreholes or water-filled pits for small seismic charges were
located about 100 - 200 m away from the borehole. 
The general VSP data acquistion geometry is shown in Fig. 2.4.
\medskip\noindent
{\bf 2.4 Vibroseis or Dynamite ?}
\smallskip
Vibroseis data have to be acquired along existing roads or trails. Thus,
seismic profiles are often winding, in places extremely crooked, and may
not traverse the exploration areas as a true strike or dip line, making
detailed integration of surface and borehole geolocial 
data very challenging. As discussed in chapters 3 and 6, the vertical seismic 
profiling experiments indicate that utilizing small explosive charges in shallow
drill holes
could provide the necessary seismic energy (i.e,
bandwidth and dynamic range)
for off-road 2-D and 3-D 
seismic exploration programs.
\medskip\noindent
{\bf 2.5 Location of 3-D Modeling Study}
\smallskip
While results from 2-D reconnaissance seismic surveys acquired across key 
geological structures provide important information on 
the regional geological setting, there are problems integrating results 
from this new mapping technique 
into normal exploration procedures. Major drawbacks of 2-D profiling technique are:
\smallskip\noindent
\item{(1)} As mentioned above, seismic
profiles often follow winding roads, in places extremely crooked, and
may
not traverse the exploration areas as a true strike or dip line, making
detailed integration of surface and drill hole
data very difficult.
\smallskip\noindent
\item{(2)} 2-D seismic data are often contaminated by the reflection/scattering 
response of heterogenous
crustal structures. The level of 3-D contamination is often difficult to ascertain.
\medskip
Earlier research 
has demonstrated that subsets of the high frequency reflection seismic data across
the Sudbury Structure, treated as 
partial 3-D data sets, can overcome some of the problems
associated with conventional profiling data (Wu et al., 1995).
A detailed 3-D modeling study was conducted between the Creighton and Gertrude
deposits. Figure 2.5 shows the location of the steeply notheast-dipping 
Creighton 402 ore body (located between 3000 and 5000 ft depth), Lithoprobe
line 43 (as indicated by station numbers) and the densely spaced grid
of the 3-D seismic modeling study. The main purpose of the 3-D modeling study
is to evaluate the effects that realistic 3-D geological structures may have on 
2-D seismic seismic imaging.
\bye