Seismic reflection profiling for massive sulfide exploration in Finland

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http://urn.fi/URN:ISBN:978-952-10-5074-9
Title: Seismic reflection profiling for massive sulfide exploration in Finland
Author: Heinonen, Suvi
Contributor: University of Helsinki, Faculty of Science, Department of Physics, Division of Geophysics and Astronomy
Institute of Seismology, Department of Geosciences and Geography
Publisher: Helsingin yliopisto
Date: 2013-10-18
URI: http://urn.fi/URN:ISBN:978-952-10-5074-9
http://hdl.handle.net/10138/40786
Thesis level: Doctoral dissertation (article-based)
Abstract: Depletion of the near surface mineral deposits has created a need to develop new, innovative deep exploration tools. Seismic reflection soundings have been successfully used for hydrocarbon exploration over a century, but method s applications in the hardrock environment have been limited until recently. Strongly 3-dimensional and discontinuous geological structures are typical for the crystalline bedrock and especially for mining camps. This causes challenges to the seismic data acquisition and processing as well as to the interpretation. In this thesis, seismic reflection profiling is applied to the exploration of massive sulfide deposits in the Pyhäsalmi, Vihanti and Outokumpu area. Study areas belong to the Raahe-Ladoga belt containing 90% of the known sulfide deposits of Finland. In each of the study sites a network of seismic reflection profiles was acquired during the HIRE (HIgh REsolution reflection seismics for ore exploration 2007-2010) project by the Geological Survey of Finland. The multiphase deformation history of the study areas is demonstrated by folding, faulting and shearing that cause the complex subsurface reflectivity patters observed in the seismic data. These data enable thorough discussion on the applicability of the reflection seismic profiling for massive sulfide exploration in a geological environment that is highly deformed and metamorphosed. Careful static corrections and velocity analysis are the most important steps in the processing of the seismic reflection data acquired in hardrock terrains. Steeply dipping structures are not uncommon in the study sites and proper stacking of these structures require use of unrealistically high NMO velocities. Steep and horizontal features are best interpreted from separate stacks using 3D-visualization and modeling software. Results of velocity analysis can also be utilized in the seismic interpretation. For example in Vihanti low NMO velocities correlated with the known wide fracture zone. Difference in the acoustic impedance of the rocks defines the strength of a reflection originating from the rock contact. Geophysical drill hole logging shows that in addition to the massive sulfides, the hosting rock sequences are also strongly reflective in Vihanti, Pyhäsalmi and Outokumpu. Reflecting hosting lithology enables the determination of favorable exploration environments. In Pyhäsalmi, the known massive sulfide deposit could not be reliably identified from the seismic section because of the crooked acquisition lines, noise caused by the functioning mine and lithologically heterogeneous and strongly 3-dimensional geological background of the deposit. Based on the drill hole logging data, most reflections originate from contacts between mafic volcanic and other rocks in the Pyhäsalmi area. Mineralizations typically occur at the transition from the felsic to mafic volcanism. Reflectivity in the Pyhäsalmi area indicates continuation of the ore-critical bimodal volcanic sequence underneath wide, but shallow, granite intrusions, which has not been explored previously. Moreover, amplitude enhancement of this interpreted contact was proved to be caused by the uneconomic zinc mineralization. Amplitude enhancement inside of the reflective units interpreted as ore hosting lithology was also used as an exploration criteria in the Outokumpu area. Because sulfide ores are small compared to the typical seismic wavelength, they cause diffractions instead of reflections. Diffraction analysis was used for seismic deep exploration in Outokumpu and Vihanti. Good seismic interpretation employs the structural geology of the study area. In Pyhäsalmi, only hinges of the subvertical folds were imaged while steep limbs could not be directly imaged by seismic profiling. Gentle open folding is typical for Vihanti region and was shown as undulating reflectivity in the seismic profiles. Reverse faults interpreted in all study regions are most prominent in the Vihanti area where strong reflectors are cut by faults. Knowledge of the fault locations is important for exploration because they might have acted as pathways for fluids carrying sulfide minerals during deformation. Seismic reflection data provides an insight to the deep continuation of the fault zones. Research presented in this thesis shows that seismic reflection profiling is a useful tool for deep ore exploration. The depth resolution of the method is excellent and interpretation of the crossing seismic profiles enables the 3D geological modeling of the subsurface structures and lithological contacts. Successful modeling requires the use of other available geophysical and geological data. A geological 3D-model combines the knowledge acquired by different methods making strategic planning of exploration possible, in particular by facilitating the decisions of expensive deep drill holes. Seismic reflection profiles create the framework on which the geological model is built on.Tunnettujen, lähellä maanpintaa sijaitsevien mineraalivarojen ehtymisen takia on tarpeen kehittää uusia, innovatiivisia syvämalminetsintämenetelmiä. Seismistä heijastusluotausta on käytetty öljynetsinnässä menestyksellisesti jo lähes vuosisadan ajan, mutta menetelmää on toistaiseksi sovellettu kiteisen kallioperän malminetsintään vain rajoitetusti. Pyhäsalmen, Vihannin ja Outokummun kaivosalueet kuuluvat Raahe-Laatokka vyöhykkeeseen, jossa sijaitsee 90% Suomen tunnetuista sulfidimalmivaroista. Alueilla uskotaan edelleen olevan ennestään tuntemattomia malmeja erityisesti yli 500 metrin syvyydessä, mihin perinteisten geofysikaalisten malminetsintämenetelmien syvyysulottuvuus ei riitä. Seismistä heijastusluotausprofilointia testattiin syvällä sijaitsevien malmien etsintään. Sulfidimalmit ovat pieniä verrattuna seismisiin aallonpituuksiin malminetsintäsyvyyksillä, joten niiden suora havaitseminen seismisissä profiileissa on epätodennäköistä. Tämän takia seismisessä malminetsinnässä kannattaa keskittyä tunnistamaan malmien isäntäkiviä sekä malmiesintyymiä kontrolloivia geologisia rakenteita. Pyhäsalmella, Vihannissa ja Outokummussa sulfidimalmit liittyvät tyypillisesti tiettyihin kivilajisekvensseihin, joiden todettiin kairarei issä tehtyjen geofysiikan mittausten perusteella olevan heijastavia kaikilla tutkimusalueilla. Geologiset prosessit ovat poimuttaneet, siirrostaneet ja hiertäneet tutkimusalueiden alunperin vaaka-asentoiset kerrostumat monimutkaisiksi, kolmiulotteisiksi rakenteiksi. Pyhäsalmella kairareiässä haivattu malmin isäntäkivisekvenssi pystyttiin korreloimaan seismiseen heijastajaan, jota seurattiin seismiseltä heijastusluotauslinjalta toiselle aiemmin malminetsinnän ulkopuolelle jätetyn graniitti-intruusion alle. Tämä on suurentanut alueen malmipotentiaalia merkittävästi. Yhdistämällä seismiset tulkinnat risteävillä 2D-linjoilla pystytään luomaan 3D-malli maanpinnanalaisista geologisista rakenteista useiden kilometrien syvyyteen. Geologinen 3D-malli yhdistää eri menetelmillä saadun tiedon kallioperän rakenteista ja mahdollistaa paremman malminetsintästrategian, sekä helpottaa erityisesti syväkairareikien suunnittelua.
Subject: geofysiikka
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