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11th SAGA Biennial Technical Meeting and ExhibitionSwaziland, 16
- 18 September 2009, pages 319 - 324
2D Seismic Reflection Surveys and Structural Mapping:
Faults,Dips and Domes
Geoff Campbell
1
1. SAGA, SEG, GAP Geophysics and Quik_Log Geophysics, South
Africa, [email protected]
ABSTRACT
In the local mineral exploration environment the high resolution
imaging capabilities of 2D seismic reflectionsurveys continue to
provide rapid, cost-effective mapping solutions over areas of
structural mapping concern nototherwise speedily available from
borehole data. This is particularly so where time-lines are short.
2D seismicsections sourced from the Wits Gold, Karoo coal, Canadian
diamond and Bushveld platinum exploration sectors
demonstrate in particular; technique versatility in mapping
orebodies or marker horizons having a tabular sheet geometry, and
satisfying
minimum thickness plus physical property requirements a large
spectrum of structural environments ranging from the benign to the
complex.This spectrum ranges from flat dips with minor rolls and
grabens through large scale wrench and listric faulting, tomoderate
dips with structural terraces or large grabens above Floor
doming.
1. INTRODUCTION
2D seismic survey contributions to RSA prospecting
activity over the last decade and a half have been
largelyovershadowed by the volume and detail of mapping
results from 3D seismic surveys over the establishedmines of the
Witwatersrand GoldFields, and establishedto newly developing
platinum mines of the BushveldComplex. Mining loss-of-ground
information ofunprecedently high spatial resolution is now
routinelyavailable for mine-planning purposes from such 3D
surveys , whose deployment currently appears ( at leaston paper
) to be de rigeur over all except the leastprospective of
prospects. This in part reflects the by-now highly mature nature of
Witwatersrand mines and afast-approaching maturity for many
Bushveld Mines
within their lease-area life-cycle.
Meanwhile, low levels of 2D seismic survey activityhave been
maintained over brownfield sectors, indirectexploration plays
mainly driven by the juniors overhitherto poorly mapped sectors of
the Witwatersrandand Bushveld Complex. These 2D surveys, while
lacking the fine mine-planning resolution of their
3Dcounterparts, are relatively inexpensive and require only
short implementation lead-times while providing earlymapping
solutions to litho-structural problems ofimmediate concern in
ore-body modeling e.g. where tooptimally place boreholes in a deep
drilling project. In
short, 2D data, in connecting the dots constitutingboreholes,
provides a reliable measure of orebody
continuity and advance mapping of faults with throws inexcess
of, say, 30m.
This paper focuses on 2D seismic mapping resultsacquired by GAP
Geophysics or its predecessor overRSA prospects which required the
early establishmentof major structures through a judicious
combination ofseismic and borehole data. The actual
litho-seismic
mapping target may be the orebody itself (VCR,PlatReef, coals)
or marker horizons in the hanging or
footwall (most gold reefs, Merensky Reef). Faultfeatures are
generally not directly mapped but inferredfrom correlatable
reflection event terminations andvertical displacements.
2. DATA ACQUISITION & PROCESSING
Data was acquired over the period 1983 to 2007 using
either Vibroseis (20Hz to 96Hz) or BOLT Land AirGunseismic
energy sources and a 96 channel seismograph.
Symmetrical, split-spread arrays with geophone stationspacings
of 20m or 50m were utilized in conjunctionwith similarly spaced
shotpoints, to generate 48 folddata. Data processing was standard,
included refractionand residual statics plus time migration.
3. CASE STUDIES
3.1 Wits Gold: Wrench and Listric Faulting
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2D Seismic Reflection Surveys and Structural Mapping
The N-S striking De Bron Fault is a steeply dippingwrench fault
which throws down to the west by up to1000m, and constitutes the
eastern boundary of theCentral Rand Group sediments of the OFS
Goldfields.
Early development of what is now the Joel Gold Mine
in the southern OFS GoldFields was hastened by resultsfrom
reconnaissance seismic and gravity surveys, whichuniquely located
the De Bron Fault on the JCI FarmLeeuwfontein, some 2km to the east
of its hitherto
assumed position (Figure 1). This significantlyincreased
combined VS5 Reef orebody tonnages overthis and the adjoining JCI
Farm (Leeuwbult). Prominentreflectors comprise the base-Karoo/top
Central RandGroup at TWT = 300msec (~550m) and the base-Central
Rand Group at TWT = 800msec (~2000m)which have flat apparent dips.
The De Bron Fault in theeast has a westerly downthrow of 700m, with
CRG units
being absent over the far eastern horst. This
displacement is not well imaged in the seismic section(it was
after all JCIs first even reflection survey), butfault location is
unambiguously highlighted by theapparent change in strata dip in
the east, and by theKaroo hanging wall graben immediately to the
west of
the fault trace.
Aside from target location, seismic data played asignificant
role in Wits exploration with respect tomapping the large
loss-of-ground areas associatedwith fault zones, in advance of deep
drilling operations.
The example of Figure 2 shows a very large listricnormal fault
displacing Central and West Rand
lithologies in the Far West Rand, where the target is theVCR.
The exclusion zone is more than 1.5km atsurface. Along the complete
seismic section such faultsdefine an imbricate stack pattern.
3.2 Karoo Coal: Flat Dips and Rolls
Coal seams, characterized as they are by anomalouslylow
densities and sonic velocities, constitute excellent
reflectors (reflection coefficient, r > 30%) where
theysatisfy minimum thickness criteria. Minimumthicknesses are
generally regarded as being at least oneeighth to one twelfth of
the dominant wavelength of the
seismic signal (Widness, 1973). The seismic section ofFigure 3
is from the eastern Transvaal CoalFields and
shows a strong reflection event at TWT = 220msec(~400m depth),
mapping out an ~5m thick coal seam.That this seam appears as the
strongest event on thesection is in part due to thickness tuning
dependant onconstructive interference of top and bottom
seismicwavelets: the requirement here is that the target
thickness be around one quarter of the seismicwavelength
(Widness, 1973). On this basis significant
seismic energy is present at wavelengths of around 25m(following
a non-linear Vibroseis sweep of up to200Hz).
Only minor rolls detract from the otherwise flat dip of
the coal seam, and no major faulting (throw > 10m) ispresent.
A truly benign structural environment.
3.3 Kimberlite Dyke (Sill): Ultra-thin
Target and Shallow Dips
The Snap Lake Diamond Mine in Canada exploits a 2mto 3m thick
kimberlite sheet which dips at ~15 todepths in excess of 2000m.
Although tagged as a dykeit would locally be more appropriately
called a sill.
While acoustic impedance contrasts between thekimberlite and
host rocks point to reflection coefficientsin excess of 20%, it
would not normally be considered asuitable seismic target on
account of its limited width(Hammer et al 2004). However,
orientation surveysusing dynamite in deep shot-holes and Vibroseis
surface
sources generating return frequencies of up to 350Hz
(with a dominant frequency of 100Hz) allowed formapping of the
dyke (Figure xx). With seismicwavelengths of no less than 13m, this
implies mappingat the one sixth wavelength interval. On this basis
thedyke is not resolved, but detected: the reflection
package correlating with its locale comprises 3 to 5cycles
(~20msec or ~45m). This may reflect internalreverberations and/or
non-optimal data processing.
The dyke signature is confidently mapped from TWT =30msec (~60m)
to 425msec (1300m), at an average dipof around 15. No other
reflectors are mapped in this
granite environment. Borehole ties are good and
correlate well over sections where dip variations
areapparent.
3.4 Bushveld PlatReef: Moderate to Modest
Dips
The PlatReef horizon of the Northern Limb of theBushveld Complex
comprises a 100m to 300m thickpredominantly pyroxenitic package
overlain by MainZone gabbros, which at outcrop dips at ~40 to 45
tothe southwest. It carries PGM mineralization and
disseminated sulphides. Significant acoustic impedancecontrasts
at hanging (gabbros) and footwall (granite)
contacts predict average reflection coefficients at
theseelevations of +4% and -13% respectively. While suchvalues may
be expected to generate significantreflection events, gradational
changes at the top
PlatReef contact sometimes degrade seismic
mappingcapabilities.
Early drilling by Afri-Ore downdip from the Anglo-Platinum Open
Pit indicated an unexpected apparentshallowing of dip of the
PlatReef horizon which, if real(i.e. not due to faulting), could
considerably improvemining economics. An orientation seismic
survey
orthogonal to strike confirmed marked shallowing of
PlatReef dip at depths in excess of 500m (Figure 4).
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