PACIFIC CONCEPT AND APPROACH
Seismology has been used for over a century to explore the structure, composition and dynamics of the earth’s interior.
The first observations of the Earth employed a form of passive seismology in which the source of seismic signals were remote earthquakes that occurred in an irregular and unpredictable manner.
Active seismic exploration emerged in the 1920’s when it was first used in the petroleum industry. The technique requires the employment of an active source of seismic waves, either explosions or an air gun or a seismic vibrator.
Although the basic principles of the passive seismic technique have been known for almost 50 years, processing and storage of the large quantities of data generated by the technique had to await the development of more powerful computers. The codes that are now used to treat the data were developed only a decade ago. An additional technological advance was the development of autonomous seismometers (nodes) that contained batteries sufficient to record data for periods up to one month. This eliminated the need to deploy clumsy and costly arrays of cables.
Design of the 1200-node array in Marathon, Canada.
PACIFIC innovative approach: the passive seismic technique
The passive seismic technique relies on the deployment of nodes to produce a tomographic image of the sub-surface based on the detection of ambient seismic noise.
In typical passive seismic surveys, many hundreds (or even thousands) of nodes are deployed.
Passive seismic arrays are typically deployed for periods of weeks to several months and record body and surface seismic waves.
A major problem with the traditional approach is the rapid attenuation of surface waves with depth. PACIFIC intends to develop two new passive seismic techniques:
passive seismic technIques
PASSIVE REFLECTION SEISMICS
The challenge that PACIFIC faces is to develop new acquisition and processing procedures that will be capable of extracting body-waves from ambient seismic noise.
Once body waves have been extracted, the data can be processed to acquire reflection seismic sections, following industry standard methods as commonly applied in the hydrocarbons sector.
THE MULTI-ARRAY APPROACH – USING SEVERAL VERTICAL ARRAYS COMBINED WITH A SURFACE ARRAY
The traditional passive technique uses surface waves that have poor resolution and weak amplitude at depth. PACIFIC will develop a complementary technique – the new multi-array approach.
In this method, a surface array is used in conjunction with vertical arrays deployed in drill holes. This makes the extraction and use of body-waves much easier.
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.
RISK PERCEPTION, COMMUNITY ENGAGEMENT AND ACCEPTANCE OF MINING RELATED ACTIVITIES
Opposition to the activities of the minerals industry is increasing throughout Europe.
This may be due at least in part to how the industry informs local communities and how those communities perceive potential risks. Research has shown that how information is presented to the public significantly influences their decision-making.
It is essential that the development of new techniques in the mining sector is accompanied by high quality, robust experimental data from the social sciences.
Further research seems necessary to determine governance arrangements to help establish social acceptance in the mining sector in particular – though the statements could apply to any industry with environment or societal impact.
DEVELOPMENT OF SPECIFIC CODES AND PROCEDURES TO OBTAIN HIGH-RESOLUTION DATA
Noise-based, passive seismic imaging requires the processing of very large volumes of data.
The development of a new method as passive reflection seismic requires the development of efficient codes and procedures for high-performance computing of the subsequent data.
PACIFIC will develop optimised codes for seismic array processing, necessary to separate body- from surface-waves and perform high-resolution imaging.
CONDUCTION OF PILOT TESTS
Pilot project 1: Marathon deposit, Canada. Test of the passive reflection seismic method.
Lake Superior shore, Marathon site, Canada.
Cliff at Marathon site, Canada.
This site was chosen for several reasons:
- The geology, the nature of the ore bodies and controls on mineralisation are well known.
- The region has been well explored using geological, geochemical and geophysical methods.
- Advanced exploration has continued in the region over the past 50 years. Relationships between Stillwater and the local communities are good and this provides an excellent platform for evaluating the perception and reaction of the local authorities, landowners, First Nation peoples and the general public to the passive seismic technique.
- Tests of the surface-wave passive seismic method, carried out in 2017, provided valuable background data.
Results from Phase II of the project at Marathon in Canada. The image, a slice at 250m depth, shows relative seismic velocities (blue – fast, yellow – slow). The red lines show the border of the intrusion that contains Cu-PGE mineralisation and the seismic data trace the lithological boundaries.
Pilot project 2: Kallak Iron Ore Deposit, Sweden. Test of the multi-array technique.
The Kallak resource is located in northern Sweden.
The prospect has been explored for the past five years and represents an ideal target for the passive seismic technique.
The site provides a straightforward test of the method, principally the multi-array technique; further drilling is planned in coming years and these drill holes will provide the sites for deployment of vertical seismic arrays.
Figure: Sketch of possible deployment of a surface array and down-hole vertical arrays, as proposed for the Kallak site.
Drill rig at Kallak site, Sweden.
CONTRIBUTION TO SUSTAINABLE MINING: SAFETY, ENVIRONMENTAL AND SOCIETAL ASPECTS
All mining and mineral exploration has an impact on the environment and entails safety risks.
The activities of PACIFIC focus on exploration, and these have less negative consequences than mining itself. A major advantage of the passive seismic technique is the elimination of active sources, which have major impact on the environment and introduce major safety risks.
PACIFIC will carry out several actions for assessing and mitigating risks including establishing an Environmental and Safety Management Committee (ESMC).
FOSTERING INDUSTRY-ACADEMIA INTERACTION AND TRAINING OF NEW GENERATIONS OF PROFESSIONALS
An important PACIFIC goal is to foster closer interaction between the public and private sectors.
The project will require considerable fundamental research and development which will be conducted in the universities; the practical application of this R&D will be the responsibility of the industrial partners.
Another goal is to support programmes to educate and train future professionals in the minerals industry. In addition, the consortium will develop courses on the passive seismic method.
COLLABORATION WITH OTHER RESEARCH INITIATIVES IN THE SAME AREA
PACIFIC partners will collaborate with ongoing research initiatives in the mineral exploration area.
These collaboration will take place both within the EIP Raw materials and within H2020, so as to create synergies and optimise project results and impact.