PACIFIC D1-1 Assessment of active seismic workflows

PACIFIC

Passive seismic techniques for environmentally friendly and cost efficient mineral exploration

D1.1– Assessment of successful active seismic

processing workflows

Grant agreement number 776622 Due date of Deliverable 30/09/2018

Start date of the project 01/06/2018 Actual submission date 30/10/2018

Duration 36 months

Lead Beneficiary UGA Contributors SISP, DIAS

Description

Report listing and evaluating all currently active seismic processing workflows.

Dissemination Level

PU Public X

CO Confidential, only for members of the consortium (including the Commission Services)

PACIFIC_D1.1_Assessment_of_active_seismic_workflows_vf

D1.1– ASSESSMENT OF SUCCESSFUL ACTIVE SEISMIC PROCESSING WORKFLOWS

Public © PACIFIC consortium / 14

Table of content

Table of content ...................................................................................................................................... 2

List of figures ........................................................................................................................................... 3

List of tables ............................................................................................................................................ 4

Executive Summary ................................................................................................................................. 5

1 Introduction .................................................................................................................................... 6

2 Active (controlled source) seismic method .................................................................................... 7

2.1 Principles of active seismic as applied to mineral exploration ..................................................... 8

2.2 Mineral survey processing sequence (workflows) ........................................................................ 8

3 Conclusion ..................................................................................................................................... 13

Bibliography .......................................................................................................................................... 14




List of figures

Figure 1. Sketch of seismic survey in a simple layer model (lower diagram) and resulting seismogram with appropriate seismic signals (upper diagram) adopted from Wiederhold (2007). Green shows direct travelling wave, blue shows refracted or head wave, and red shows reflected waves. ......................................................... 7 Figure 2: Comparison between a) Specific Mineral Survey Processing Sequence and b) common processing sequence. More details and higher frequency image is obtained from the specific MSPS (from Malehmir and Bellefleur, 2010) .................................................................................................................................................... 11 Figure 3. Seismic reflection image of the lithology and structures of the units that host the McLeay Ni-sulfide deposit in Western Australia. Clearly distinguished are the ultramafic host unit, a large fault, and the ore deposit itself (from http://www.hiseis.com). ....................................................................................................... 12




List of tables

Table 1. Standard Mineral Survey Processing Sequence (Salisbury and Snyder, 2007). ......................................... 9 Table 2 Specific Mineral Survey Processing Sequence (Malehmir et al, 2006; Malehmir and Bellefleu, 2010). ... 10




Executive Summary

Across the globe, the mineral industry is seeking new technologies to replace or complement existing

geological, geochemical and geophysical methods to improve exploration efficiency at depth and to

help design safer and more productive mines. These industries are increasingly using seismic methods

for a wide range of commodities including base metals, uranium, diamonds, and precious metals.

Seismic methods usually can be used for direct targeting of mineral deposits but particular care must

be taken during acquisition and processing of the data. To achieve the best results, different

processing sequences based on the target of the project are applied. Here we compare and discuss

how such workflows are used when treating active seismic data in order to provide a basis for their

use in the development of the passive seismic methods that form the basis of the PACIFIC project.




1 Introduction

The mining industry has traditionally used geologic field mapping, electromagnetic and potential field

techniques, and drilling to explore for new mineral deposits, but with new discoveries of large near-

surface deposits becoming increasingly rare, it is clear that new deep exploration techniques are

required to meet the future needs of industry and society (Salisbury and Snyder, 2007).

With other geophysical methods unable to resolve targets beyond about 500 m, high-resolution

controlled source seismic techniques similar to those used by the petroleum industry, but modified

for the hardrock environment, have the greatest potential for extending exploration to depths of 3

km, the current maximum depth of mining (Salisbury and Snyder, 2007). Petroleum companies have

used such methods for many decades during their exploration for oil and gas reservoirs in sedimentary

settings. The technology has evolved considerably leading to current fully 3D surveys that are capable

of determining the structure of sedimentary sequences to depths of many kilometres. Active seismic

methods provide high-resolution images of geologic structures hosting mineral deposits and, in a few

cases, can be used for direct targeting of ore bodies, but a complete survey of this type may cost

thousands to millions of euros.

Salisbury and Snyder (2007) reported successful use of 2-D and 3-D surveys in detection of (1) large

massive sulphide deposits such as the magmatic and volcanic massive sulphide deposits, (2) massive

sedimentary exhalative deposits and (3) iron oxide copper gold deposits worldwide. In addition,

alteration haloes and the general geological setting can be used to explore for other types of deposits

such as lode gold and porphyry deposits, unconformity uranium deposits, and Mississippi Valley-type

deposits

The steadily increasing usage of reflection seismic methods demonstrates that they are finally

becoming recognized and established within the mining sector. However, because most economic

mineral deposits are found in "hard" (igneous or metamorphic) rocks, rather than sedimentary,

environments, and the impedance contrasts and reflection coefficients between most common

igneous and metamorphic rocks are smaller than those between sedimentary rocks. Because of this,

particular care must be taken during the acquisition and processing of seismic data.




2 Active (controlled source) seismic method

In controlled source seismic studies, receiver networks record seismic waves from an artificial source,

allowing detailed views of fine-scale structures (Figure 1). Betsy guns and weight drop systems,

dynamite in shallow boreholes, airguns in water-filled pits or fluid-filled pans attached to vehicles, and

vibroseis trucks are used as the seismic source, based on the requirements and the purpose of the

study.

Reflection and refraction seismology, both active seismic methods, are complementary imaging

techniques, that yield high-resolution images especially suitable for detecting and imaging seismic

interfaces showing a strong contrast in seismic velocity (Wagner et al, 2012). Reflection seismic

methods have been used worldwide to target mineral deposits. Important seismic studies that were

designed to guide exploration programs include Adam et al. (2003), Pretorius et al. (2003), Bellefleur

et al. (2004), Malehmir and Bellefleur (2009), Malehmir et al. (2006), Tryggvason et al. (2006),

Malehmir et al. (2010) and Dehghannejad et al. (2010).

Figure 1. Sketch of seismic survey in a simple layer model (lower diagram) and resulting seismogram with appropriate seismic signals (upper diagram) adopted from Wiederhold (2007). Green shows direct travelling wave, blue shows refracted or head wave, and red shows reflected waves.




2.1 Principles of active seismic as applied to mineral exploration

Two basic factors govern whether or not a potential reflector can be detected and imaged by seismic

reflection techniques: (1) the difference in acoustic impedance (velocity-density products) between

the deposit or horizon and its surroundings, and (2) its geometry, especially its size and depth of burial

(Salisbury and Snyder, 2007). Since ore-forming process changes the density, acoustic velocity or

strength of the rocks, a mineral deposit can be detected using seismic reflection methods.

At the regional scale, 2D transects can define major crustal boundaries associated with metalliferous

provinces, and can also image lithosphere-scale faults that act as conduits for ore-forming fluids. At

the camp scale (10s of km) seismic methods can image structures such as folds, unconformities and

faults that control ore deposition, and in some cases can distinguish lithologies such as intrusions or

favourable sedimentary units that host the mineralization. At the deposit scale (< 10km) 2D and 3D

seismic surveys can accurately define the geometry of ore surfaces or key marker reflectors. Offsets

and truncations of the ore due to faults and intrusions are also detectable. Large bodies of sulphide

ore constitute excellent seismic targets.

2.1.1 Special considerations for mineral exploration

When applying seismic reflection methods during mineral exploration, special care must be taken

during the fieldwork deployment and also afterward during data processing.

In "hard" (igneous or metamorphic) rocks, the impedance contrasts and reflection coefficients are

smaller than those found in sedimentary rocks, and the signal-to-noise (S/N) ratio in minerals surveys

will be low, making it more difficult to image structures. Particular care must thus be taken during

acquisition and processing to maximize the S/N ratio (Salisbury and Snyder, 2007). During field

acquisition, this is typically achieved by using explosives in shallow boreholes filled with water or

tamped with sand to ensure good source coupling to bedrock, and by using cemented or clamped

geophones whenever possible. Furthermore, during the data processing, in order to improve the

signal-to-noise ratio of the extracted reflected arrivals, the common mid-point gather technique is

used to facilitate data stacking and noise reduction. These reflections can be further migrated at

depth, using a refraction-based velocity model, in order to obtain a full surface distance-over-depth

reflectivity section of the studied area. When interfaces are steep, as is common in mineral resource

exploration, great care must be taken during this step of migration, which requires state-of-the-art

seismic modelling and inversion procedures.

In addition, since some types of ores only have small impedance contrasts with many common host

rocks, it is often advisable to conduct laboratory measurements of the velocities and densities of the

ores and host rocks in a potential survey area to determine whether reflections are even possible and

the survey worth conducting (Salisbury and Snyder, 2007).

2.2 Mineral survey processing sequence (workflows)

Since even rich ore deposits are often fairly small (<1 km across) or have small impedance contrasts

with the host rocks, the processing of reflection data from mineral surveys plays an important role to

image the structures. Different studies have suggested different workflows for the processing of

reflection data (Table 1 and 2). In general, there is no unique way for processing the reflection data

and the approach to be adopted must be based on the geology, survey characteristics and the nature




of the study. Salisbury and Snyder (2007) suggested the Mineral Survey Processing Sequence as shown

in Table 1 for mineral surveys that involves two processing streams, one to image structures such as

folds or faults in country rock and another to locate, enhance, and trace diffractions to their sources

using unmigrated data. Malehmir et al. (2006) and more recently Malehmir and Bellefleur (2010)

suggested a more detailed processing procedure for reflection data in active seismic survey (Table 2)

and they applied this approach for several case studies.

The reflection data processing includes several major steps, as shown in Figure 2. After setting up the

field geometry and collecting the data, these data are converted to an appropriate format (usually

SEG-Y or SEG2); then after selecting the first pick, a series of trace statistics is applied on the trace. In

the next phase, different filters are applied to obtain a clearer signal and to prepare the data for

velocity analysis. After applying Normal-moveout and Dip-moveout corrections, all the traces

are stacked. At the end, by filtering the resultant data to remove the noise and do the

migration, the results are ready for interpretation (Figure 3).

Table 1. Standard Mineral Survey Processing Sequence (Salisbury and Snyder, 2007).

1. Pre-processing (Geometric spreading correction, Set up field geometry, Application of field statics)

• Demultiplex

• Set up field geometry

• Edit

• True amplitude recovery

2. Deconvolution

3. Band-pass filtering

4. Gain control

5. First break mute

6. Refraction statics corrections

7. Residual statics corrections

8a.Unmigrated Stack

• Dip moveout (DMO) corrections

• Velocity analysis

• NMO correction (Muting, Stacking)

• Stacking

• Band-pass filter

8b. Migrated Stack

• Prestack migration

• Velocity analysis

• Scaling

• Band-pass filter




Table 2 Specific Mineral Survey Processing Sequence (Malehmir et al, 2006; Malehmir and Bellefleu, 2010).

1. Read (window)-s data

2. Build geometry data: Extract and apply geometry (several tests to obtain optimum bin size)

3. Trace editing and polarity reversal

4. Pick first breaks: full offset range, automatic neural network algorithm but manually inspected and corrected

5. Refraction static and elevation static corrections

6. Geometric-spreading compensation

7. Band-pass filtering

8. Surface-consistent deconvolution

9. Top mute

10. Direct shear-wave attenuation (near-offset)

11. Air blast attenuation

12. Trace balance using data window

13. Velocity analysis (iterative)

14. Residual static corrections (iterative)

15. Normal moveout corrections (NMO)

16. Dip moveout (DMO) corrections (iteratively link to velocity analysis)

17. Stack

18. Fx-deconvolution (post-stack coherency filter)

19. Trace balance: 0–1500 ms

20. Phase-shift migration: using smoothed stacking velocities,

21. Time-to-depth conversion: constant 5500 m/s




Figure 2: Comparison between a) Specific Mineral Survey Processing Sequence and b) common processing sequence. More details and higher frequency image is obtained from the specific MSPS (from Malehmir and Bellefleur, 2010)




Figure 3. Seismic reflection image of the lithology and structures of the units that host the McLeay Ni-sulfide deposit in Western Australia. Clearly distinguished are the ultramafic host unit, a large fault, and the ore deposit itself (from http://www.hiseis.com).




3 Conclusion

The steadily increasing usage of reflection seismic methods demonstrates that they are finally

becoming recognized and established within the mining sector. Due to the small size of most deposits,

the structural complexity of hard rock terranes and their low signal-to-noise ratios, the best results

are obtained from carefully designed surveys using high frequency sources and customized processing

sequences. To have a better and clearer image for the interpretation in reflection surveys, based on

what has been shown in Table 1 and 2 and Figure 3, the processing sequence requires that a series of

major steps are applied on the collected data. PACIFIC will use this information for the collection of

data from the reflection passive seismic survey in Marathon.

Using different filters to increase the S/N ratio, applying the NMO and DMO corrections and finally

stacking and migrating the data to have a clear image are the most important steps in seismic data

processing. PACIFIC will use this information for the collection and treatment of data from the

reflection passive seismic survey in Marathon.




Bibliography

Adam, E., G Perron, G Arnold, L Matthews, B Milkereit. 2003. 3-D seismic imaging for VMS deposit exploration,

Matagami, Quebec, in Eaton, D., Milkereit, B., and Salisbury, M., eds., Hardrock Seismic Exploration: Society

of Exploration Geophysicists. Geophysical Development Series, v. 10, 229-246.

Bellefleur, G., C Muller, D Snyder, L Matthews. 2004. Downhole seismic imaging of a massive sulphide ore body

with mode-converted waves, Halfmile Lake, New Brunswick. Geophysics, v. 69, 318-329.

Dehghannejad, M., C Juhlin, A Malehmir, P Skyttä, P Weihed. 2010. Reflection seismic imaging of the upper crust

in the Kristineberg mining area, northern Sweden.

Malehmir, A., A Tryggvason, C Juhlin, J Rodriguez-Tablante, P Weihed. 2006. Seismic imaging and potential field

modelling to delineate structures hosting VHMS deposits in the Skellefte Ore District, northern Sweden.

Tectonophysics 426 (3-4), 319-334.

Malehmir, A., G Bellefleur. 2009. 3D seismic reflection imaging of volcanic-hosted massive sulfide deposits:

Insights from reprocessing Halfmile Lake data, New Brunswick, Canada. Geophysics 74 (6), B209-B219.

Malehmir, A., G Bellefleur. 2010. Reflection seismic imaging and physical properties of base-metal and

associated iron deposits in the Bathurst Mining Camp, New Brunswick, Canada. Ore Geology Reviews, No. 38

(4), 319-333.

Pretorious, C.C., M.R Muller, M Larroque, C Wilkens. 2003. A review of 16 years of hardrock seismics on the

Kaapvaal craton, in Eaton, D.W., Milkereit, B., and Salisbury, M., eds., Hardrock Seismic Exploration. Society

of Exploration Geophysicists, Geophysical Development Series, v. 10, 247-268.

Salisbury, M., D Snyder. 2007. Application of seismic methods to mineral exploration, in Goodfellow, W.D., ed.,

Mineral Deposits of Canada: A Synthesis of Major Deposit-Types, District Metallogeny, the Evolution of

Geological Provinces, and Exploration Methods: Geological Association of Canada, Mineral Deposits Division.

Special Publication No. 5, 971-982.

Tryggvason, A., A Malehmir, J Rodriguez-Tablante, C Juhlin, P Weihed. 2006. Reflection seismic investigations in

the western part of the paleoproterozoic VHMS-bearing Skellefte district, northern Sweden. Economic

Geology 101 (5), 1039-1054.

Weiderhold, H. 2007. Seismic methods.

Wagner, M., E Kissling, S Husen. 2012. Combining controlled-source seismology and local earthquake

tomography to derive a 3-D crustal model of the western Alpine region. Geophys. J. Int. (2012) 191, 789–

802.

PACIFIC D1-1 Assessment of active seismic workflows

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