Application of Seismic Reflection Surveys to Detect Massive Sulphide Deposits in Sediments-Hosted Environment

IOSR Journal of Applied Geology and Geophysics (IOSR-JAGG)
e-ISSN: 2321–0990, p-ISSN: 2321–0982.Volume 3, Issue 4 Ver. I (Jul - Aug. 2015), PP 46-51
www.iosrjournals.org
DOI: 10.9790/0990-03414651 www.iosrjournals.org 46 | Page
Application of Seismic Reflection Surveys to Detect Massive
Sulphide Deposits in Sediments-Hosted Environment
Okan Evans Onojasun
Department of Exploration Geophysics, Curtin University of Technology
Abstract: Seismic reflection techniques, the most widely used geophysical method for hydrocarbon exploration
has the capability to delineate and provide better images of regional structure for exploration of mineral
deposits in any geological settings. Previous tests on detection and imaging of massive sulphide ores using
seismic reflection techniques have been done mostly in crystalline environments. Application of seismic
reflection techniques for imaging sedimentary hosted massive sulphide is relatively new and the few experiments
carried out are at local scale (<500m). In this study, we analyze the feasibility of such regional exploration by
modelling three massive sulphide ore and norite lenses scenario using 2D seismic survey with relatively sparse
source-receiver geometry to image these deposits within 1.5km depth range. Results from the modelling
experiment demonstrate that 2-Dimensional seismic reflections survey can be used to detect massive sulphides
at any scale. The test further indicates that geologic setting and acquisition parameters are very important for
the detection of these ore bodies. Overall, the outcomes of the results support our started objective which is to
demonstrate that seismic reflection surveys can be used to detect the presence of sediment hosted massive
sulphides at regional scale.
Keywords: Deposit, Massive Sulphide, Regional, Seismic Reflection, Sediment-hosted
I. Introduction
Massive sulphide ores are generally characterized by high acoustic impedance (Z), which is the product
of compressional wave velocity (Vp) and density (P) [8, 7, 6, 2, 4].
Study into their physical rock property indicates
that massive sulphide bodies will make strong seismic reflectors in many geological environments. As
interesting as this revelation seems, most studies involving the application of seismic reflection techniques to
detect and delineate massive sulphide ores have only be restricted to crystalline environments. Application of
seismic reflection techniques for imaging sedimentary hosted massive sulphide is relatively new and the few
experiment carried out have are at local scale [3]
In this report, we extend the work of [3]
by applying seismic reflection surveys for detecting massive
sulphide deposits in sediment hosted environment at larger scale were the deposit may be located within 5km or
more. The overall objective is to demonstrate that these massive sulphide ores can be detected at any scale using
seismic reflection method which maintains resolution even at depth. Three sulphide deposits and a norite lens
were the reflective targets in the model. The acoustic impedance differences between all the lithologies are
greater than 2.5 x 105 g/cm2s, a minimum value for strong reflections [8]
. This study is particularly unique in
that it is the first time such regional scale experiment is applied to target sediment-hosted massive sulphide
deposits.
II. Materials and Method
2.1Survey Design
The geological model and synthetic survey design was intended to represent suitable field techniques
that are applicable to real-life seismic acquisition. For these reasons synthetic data was modelled with survey
parameters similar to what would have been used in real life situation. This involved a 5 km by 1.5km
geological model (Fig. 1), of which the primary zones of interest the (three sulphide deposit and norite lenses)
were situated within the centre 500 m. Two different survey parameters were used to collect the data; in the first
case, data was collected over 240 source shots, modelled at 20 metre intervals across the entire model, with a 35
Hz Ricker wavelet input as the dominant source frequency. Source positioning replicated rolling split-spread
acquisition, such that 1000 active receivers were split in the centre by the source at all shot points. Receivers
were spaced at 10 metre intervals, which provided 2400 metres of offset, each side of the source, at each shot
point.
In the second survey parameter, data was collected over 120 source shots, modelled at 40 across the
entire model maintaining same source positioning (rolling split-spread) and dominant source frequency (35 Hz)
Ricker wavelet. Receivers were spaced at 20. These sparse survey parameters were carefully adopted as we
anticipate that due to high acoustic impedance contrast; seismic reflection survey will be able to pin down the
likely structures. The petrophysical data used for this model is shown in table 1.

Application of Seismic Reflection Surveys to Detect Massive Sulphide Deposits in Sediments….
Figure 1: Synthetic 2D geology model showing the location of sulphides, norite and a sediment host rock.
Lithology Average Vp(m/s) Average Vs(m/s) Vp/Vs Ratio Density (kg/m3
)
metasediment 4235 2137 1.98 2.69
Sulphides 5612 3430 1.64 3.81
Barite 4602 2715 1.69 3.92
Table 1: Petrophysical data used as input for the 2D synthetic geology model
2.2 Modelling and Processing of Synthetic Data
All Synthetic seismograms were computed using stress-velocity formulation [9, 1]
implemented in
Tesseral-2D full Elastic modelling software The created shot records in SEG-Y format were processed more
thoroughly using RadexPro package with basic processing steps shown in (Fig. 2). Once SEG-Y data files were
imported to RadexPro software, geometry was assigned to the data sets after which it was sorted into Common
Depth Point (CDP) bins, which had been defined in the geometry process. The CDP bins were defined at 10 m
intervals based on the source-receiver midpoint locations. All traces from any shots which had source-receiver
midpoints that fell within the predefined CDP location bins were gathered into the same CDP bin along the
already defined geometry. We then applied first amplitude correction to the data in order to account for the
spherical divergence of the seismic energy as it propagated from the source and band pass filter to remove noise
outside the seismic sweep signal frequency band.
Figure 2. Basic processing flow used in this experiment

Stacking velocity analysis was carried on the synthetic data using velocity estimation and plotting
module that applied a series of normal move-out corrections to designated CDP gathers according to user
specified velocity functions. Velocity picks for reflections were made on the basis of maxima in coherency,
flattening across the CDP gather and the quality of narrow stack panels. Several passes were made, including
initial estimates of velocity.
Finally, post stack depth migration was applied in order to move data to its correct spatial location. The
depth migration methods were applied to the stacked data using a smoothed velocity model derived from the
stacking velocities using Stolt F-K migration algorithm. This enables dipping reflections visible on the stack to
move up dip and become steeper and shorter while diffractions visible on the stack collapse to a small region on
the migration. Further improvement in the processing of the data was achieved by applying F-K filters. This
greatly improved retention of amplitude information and overall reflectivity character of the final sections.
III. Results and Discussion
Results from this modelling experiment are presented below. Fig (3) is an example of synthetic shot
records for source no 112. Records are displaced from 0-2000 ms while actual reflectivity events are visible up
to 1500ms. The shots are displayed using true relative amplitude without correction for spherical divergence.
Shot depth for all gathers is 0m. Gathers are generated using Ricker wavelet source cantered at 35Hz. The
synthetic seismogram is composed of a large hyperbolic diffraction-like event (L).
Figure 3: Synthetic shot records for source number 112 from the geological model (1) is the direct arrival
signals, (R) is the reflections signals. The shot is displayed using true relative amplitude without correction for
spherical divergence. Shot depth for all gathers is 0 m.
Fig (4a and 5a) are the depth migrated sections for both survey parameters (20 m source vs 10 receivers
spacing and 40 m source vs 20 m receivers spacing respectively). Fig. 4b and 5b are the expanded sections of
both tested parameters. Results in (Fig.4) has higher resolution when compared to results in (Fig.5) due to the
densely acquisition parameters of 20 m source and 10 m receivers spacing used as opposed to the 40 m source
and 20 receivers spacing used for result in Fig. 5.
R
1

Figure 4a) Depth migrated sections with 20m source and 10 m receivers spacing 4b) Expanded section of the
massive sulphide region in the model. Arrows in the expanded section indicate reflections from sulphides and
norite.
5km

Figure 5a) Depth migrated sections with 40m source and 20 m receivers spacing 4b) Expanded section of the
massive sulphide region in the model. Arrows in the expanded section indicate reflections from sulphides and
norite.
The observed migrated sections for both cases are in good agreement with the synthetic geological
model. The three sulphide deposits and the norite lenses generate very strong reflections amplitude as expected
due to its high impedance contrast associated with them. The high impedance contrast between the steeply
dipping norite and the sulphide ores will obviously cause the characteristics high amplitude of the reflection
responses observed in the migrated images. This high amplitude reflection caused by the massive sulphides can
be observed at a distance which is considerably larger than the actual size of the orebody.
IV. Conclusion
This modelling experiment has demonstrated that 2-Dimensional seismic reflections survey can be
used to detect massive sulphides at 1000m or more depth. The results also indicate that geologic setting and
acquisition parameters are very important for the detection of these ore bodies. The success of imaging the
target structures depends on the effective removal of the strong surface waves and this was aptly demonstrated
during processing as the presence of strong surface waves in sediments environments were removed via F-K
filtering. However, to image the true shape and location of these sulphide orebodies, a three dimensional seismic
survey design is required. With such 3D designs, optimum offset windows are used such that reflections from

the target can be seen with minimal interference from the surface and direct waves. Overall, the outcomes of the
results support our started objective which is to demonstrate that seismic reflection surveys can be used to detect
the presence of sediment hosted massive sulphides at regional scale.
References
[1]. Bohlen, T., (2002); “Parallel 3-D viscoelastic finite difference seismic modelling”, Computers and Geosciences 28, pp.887-899.
[2]. Eaton, D., Crick, D., Guest, S., Milkereit, B., and Schmitt, D., 1996, Seismic imaging of massive sulphide deposits, Part 111:
Imaging near-vertical structures: Economic Geology, v. 91. P. 835-840
[3]. Laura Q., and Emmanuel, B., 2012, On the road to 3D seismic imaging of massive Sulphide Deposits in a Sediment-Hosted
Permafrost Environment, AAPG GeoConvention 2012
[4]. Milkereit, B., Adam, E., Burnes, A., Beaudry, C., Pineault, R. and Cinq-Mars, A., 1992a, an application of reflection seismology to
mineral exploration in Managami area, Abitibi belt, Quebec: Canada Geological Survey Current Research 92-1C, p. 13-18
[5]. Milkereit, B., Eaton, D., Wu, J., Perron, G., Salisbury, M., Berrer, E.K., and Morrison, G., 1996, Seismic imaging of massive
sulphide deposits: Part II. Reflection seismic profiling: Economic Geology, v. 91, p. 829-834.
[6]. Reed, L. (1993), “Seismic Reflection Surveying for Mining Exploration Applications, A Review of Practice Past and Current with
an Outlook for the Future”, MITEC, 200p.
[7]. Salisbury, M.H., Harvey, C.W., Matthews, L. (2003), “ The Acoustic properties of Ores and Host Rocks in Hardrock Terranes” in
W. Eaton, B. Milkereit, and M. H. Salisbury, eds., Hardrock seismic exploration: SEG, pp.9-19.
[8]. Salisbury, M.H., Milkereit, B., Bleeker, W. (1996).”Seismic Imaging of Massive Sulphide Deposits: Part I. Rock Properties”,
Economic Geology 91 (5), pp.821-828.
[9]. Virieux J., 1986. P-SV wave propagation in heterogeneous media: velocity-stress finite-difference method, Geophysics, 51, 889–
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Application of Seismic Reflection Surveys to Detect Massive Sulphide Deposits in Sediments-Hosted Environment

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Application of Seismic Reflection Surveys to Detect Massive Sulphide Deposits in Sediments-Hosted Environment