{ "currentVersion": 10.91, "id": 164, "name": "gtk_vlf_surveys", "type": "Feature Layer", "description": "VLF- ja VLF-R-menetelmät kuuluvat ns. sähkömagneettisiin tasoaaltomenetelmiin ja ne perustuvat kaukaisten radioasemien lähetinkenttien hyväksikäyttöön. Radioaallot ovat matalataajuisia 10-30 kHz, mistä johtuu nimi VLF (Very Low Frequency). Radioasemat on tarkoitettu sotilaallisten tietoliikenne- ja navigointijärjestelmien ylläpitoon. Geofysikaaliset sovellukset saivat alkunsa 1960-luvun alussa, jolloin kanadansuomalainen V. Ronka kehitti ensimmäiset maastomittauksiin soveltuvat laitteet. Nykyään laitteita on useilla laitevalmistajilla, mutta vanhat Rongan ideoimat ja Geonics Ltd:n kehittämät laitteet ovat edelleen GTK:ssa pääasiallisimpina mittausvälineinä. \nVLF- ja VLF-R-menetelmiä käytetään maankamaran sähkönjohtavuusvaihtelujen kartoittamiseen. Käytännön sovellukset liittyvät pääasiassa malminetsintään, geologiseen kartoitukseen ja kallioperän heikkousvyöhykkeiden paikantamiseen. Maaperään liittyvissä tutkimuksissa menetelmää on käytetty saastuneiden alueiden kartoitukseen.\nVLF- ja VLF-R -mittauksia on tehty hajaprofiilimittauksina ja systemaattisina mittauksina. Hajaprofiilimittaukset ovat yksittäisiä lähinnä tunnusteluluontoisia mittauksia. Systemaattisissa mittauksissa on yleisesti käytetty 50-100 metrin linjaväliä ja 10-20 metrin pisteväliä ja mittauslinjojen suunta on valittu paikallisen geologian mukaan. Mittauksia on tehty eri puolilla Suomea lähinnä malminetsinnän tarpeisiin.\nVLF-menetelmässä mitataan magneettikentän polarisaatioellipsin kallistuskulma ja elliptisyys, jotka muunnetaan reaali- ja imaginäärikomponenteiksi. VLF-R-menetelmässä mitataan magneettikentän lisäksi myös sähkökenttä. Niiden suhteesta lasketaan näennäinen ominaisvastus ja kenttien välinen vaihe-ero eli vaihekulma. Näennäinen ominaisvastus vastaa johtavan puoliavaruuden ominaisvastusta.\n/\nThe VLF and VLF-R methods are electromagnetic plane-wave methods based on the utilisation of the transmitter fields of remote radio stations. The radio waves used by this method are low frequency waves, 10-30 kHz, hence the name VLF. The radio stations are used to maintain military telecommunications and navigation systems. Geophysical applications were invented in the early 1960s, when a Finnish-Canadian V. Ronka developed the first devices suitable for field measurements. Several brands commercial devices are now available, but GTK still uses devices based on Ronka\u2019s ideas and manufactured by Geonics Ltd. \nThe VLF and VLF-R methods are used to map the variations in the electrical conductivity of the ground. Practical applications are mostly related to ore exploration, geological surveys and locating weakness zones in the bedrock. In soil surveys, the methods have been used to locate polluted areas.\nVLF and VLF-R measurements have been conducted as individual profile measurements and systematic surveys. Individual profile measurements are used for reconnaissance purposes. Systematic measurements commonly used a line interval of 50-100 metres and a point interval of 10-20 metres. The direction of measurement lines have been based local geology. The measurements have been conducted in various locations in Finland mostly for ore exploration purposes.\nThe VLF method measures the inclination angle and ellipticity of the polarisation ellipse of the magnetic field and converts these to real and imaginary components. The VLF-R method also measures the electric field in addition to the magnetic field. Their ratio is used to calculate the apparent resistivity and the phase difference of the fields, i.e. phase angle. 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