Geologian Tutkimuskeskus, Tutkimusraportti – Geological Survey of Finland, Report of Investigation 144

GEOLOGIAN TUTKIMUSKESKUS GEOLOGICAL SURVEY OF FINLAND

Tutkimusraportti 144 Report of Investigation 144

Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

JOINT CALIBRATION OF AIRBORNE GEOPHYSICAL INSTRUMENTS IN TEST AREAS IN FINLAND AND RUSSIA

North West Regional Geologic Center Petersburg Geophysical Expedition State Enterprise

Geologian tutkimuskeskus Espoo 1998 Chepick, A., Baranov, V. , Kurimo, M. & Multala, J. 1998. Joint calibration of airborne geophysical instruments in test areas in Finland and Russia. Geologian tutkimuskeskus, Tutkimusraportti – Geological Survey of Finland, Report of Investigation 144. 19 pages, 6 figures, 17 tables and one appendix. The North West Regional Geological Center, Petersburg Geophysical Expedition, State Enterprise (PGE)* and the Geological Survey of Finland (GSF) together made calibration measurements with their airborne geophysical systems on test areas in Russia and Finland. The study covers comparison of the calibration of airborne gamma-ray spectrometers and compensation of magnetometers. PGE uses both an MI-8 helicopter and a fixed wing Antonov-2 aircraft for airborne surveys. The RSDN-3 navigation system, based on stationary radio beacons, is also used for flight path recovery. From 1993 onwards a TNL-1000 GPS receiver has been utilized. In airborne geophysical surveys GSF has always used fixed wing aircrafts. Since 1980 the geophysical instruments have been installed in a Canadian Twin Otter turbo prop aircraft. During the survey flights the navigation was based on photo mosaic maps together with a in-house built navigation system based on the doppler counter. From 1992 onwards both the navigation and flight line recovery are based on Aschtec GPS receivers. The gamma-ray measurement system used by PGE is part of the multi- functional airborne geophysical 1990STK unit and consists of 12 NaI crystals, a total volume of 37.7 l, a 128 channel spectrometer and digital and analog recorders. The real-time energy stabilization of the crystals is based on Co-60 source. The integration time of the spectrometer can be 0.25, 0.5, 1.0, 2.0, 20 or 40 seconds. GSF used a McPhar spectrometer operating in 256 channel mode and the energy range 0.2 Me–3.0 MeV was registered once per second with 120 channels. The total volume of the six NaI crystals is 25 l. Although the calibration equations recommended by the IAEA and the equations actually used in Russia differs from each other the corrected results obtained with these two systems on test areas in Russia and in Finland are in good agreement. The pitch, roll and jaw movements of the aircraft cause during flight some additional effects on the airborne magnetic data. Different kinds of compensation systems have been developed in order to eliminate these distur- bances. The compensation system which is used by PGE in their STK- magnetometer is based on electromagnetic coils. The proton magnetometers used in the Finnish installation until 1992 were compensated for mainly with asspect to the flight direction during the data processing. From 1993 onwards GSF has been using Cs-magnetometers with real-time software compensation. The usefulness of the compensation can be seen from the data obtained from the test areas in Russia and Finland.

* PGE is incorporated (Autumn 1998) into the North-West Region Geological Centre, 24/1 Odoevsky Street, St. Petersburg 199155, Russia, Phone:+7 812 352 3014, fax:+7 812 352 2417

Key words (GeoRef Thesaurus, AGI): Geophysical surveys, airborne methods, gamma-ray spectra, calibration, magnetometers, instruments, testing, Finland, Russian Federation

Maija Kurimo Geological Survey of Finland, P.O.Box 96, FIN-02151 ESPOO, FINLAND E-mail: [email protected]

Jukka Multala Geological Survey of Finland, P.O.Box 96, FIN-02151 ESPOO, FINLAND E-mail: [email protected]

ISBN 951-690-723-7 ISSN 0781-4240 Chepick, A., Baranov, V., Kurimo, M. & Multala, J. 1998. Joint calibration of airborne geophysical instruments in test areas in Finland and Russia. Geologian tutkimuskeskus, Tutkimusraportti – Geological Survey of Fin- land, Report of Investigation 144. 19 sivua, 6 kuvaa, 17 taulukkoa ja liite. North West Regional Geological Center, Petersburg Geophysical Expedition, State Enterprise (PGE)* ja Geologian tutkimuskeskus (GTK) suorittivat vuosina 1990–1991 aerogeofysikaalisia kalibrointimittauksia venäläisillä ja suomalaisilla mittalaitteilla sekä Suomessa että Venäjällä, Laatokan pohjoispuolella sijaitsevilla testialueilla. Tutkimus keskittyi sekä gammasäteilyn mittauslaitteistojen kalibrointien että magnetometrien kom- pensointien vertailuun. PGE käyttää lentomittauksiinsa sekä MI-8 helikopteria että Antonov-2 lentokonetta. Navigoinnin apuvälineenä on ollut RSDN-3, kiinteisiin radiolähettimiin perustuva systeemi. Vuodesta 1993 alkaen on paikannuk- sessa käytetty TNL-1000 GPS vastaanotinta. GTK on käyttänyt lentomit- tauksissaan aina kiinteäsiipistä lentokonetta. Vuodesta 1980 lähtien mittaus- laitteet ovat olleet asennettuina kanadalaiseen Twin Otter potkuriturbiini- koneeseen. Navigointi on perustunut ilmakuvakarttoihin sekä itse kehitettyyn navigointiyksikköön, jonka perustana on doppler-laskin. Vuodesta 1992 alkaen navigointi ja lennon jälkeinen paikantaminen on tapahtunut GPS- vastaanottimien avulla. PGE:n käyttämä gammasäteilyn mittaussysteemi 1990STK koostuu 12 NaI kiteestä, yhteistilavuudeltaan 37,7 litraa, 128 kanavaisesta spektromet- ristä, digitaalisesta rekisteröintiyksiköstä sekä analogipiirturista, joka on kalibroitu näyttämään suoraan kaliumin, uraanin ja toriumin pitoisuuksia. Kiteiden reaaliaikainen energiastabilointi tapahtuu Co-60 avulla. Spektro- metrin summausaikana voidaan käyttää 0,25, 0,5, 1,0, 2,0, 20 tai 40 sekun- tia. Aerogammaspektrometrin kalibrointiyhtälöiden käyttöä havainnollistetaan esimerkkien avulla. Vertaamalla Venäjällä ja Suomessa tehtyjä mittauksia huomataan, että tulosten erot ovat pieniä. GTK käyttää 256 kanavaista McPhar spektrometriä, jonka 120 kanavaa (0,2 MeV–3,0 MeV) rekisteröidään kerran sekunnissa. Mittalaitteen antureina ovat 6 NaI kidettä, yhteistilavuudeltaan 25 litraa. International Atomic Energy Agencyn (IAEA) suositusten mukaiset korjausyhtälöt ja niiden sisältämien kalibrointivakioiden ratkaisemiseksi tarvittavat mittaukset esitellään. Suomessa ja Venäjällä suoritetut mittaukset osoittavat, että kalibroiduilla mittalaitteilla saadaan luotettavia tuloksia. Lentokoneen liikkeet aiheuttavat häiriöitä koneeseen kiinnitettyyn mag- netometriin. Näiden häiriöiden eliminoimiseksi on kehitetty erilaisia kom- pensointimenetelmiä. PGE:n käyttämän STK-magnetometrin kompensointi tapahtuu sähkömagneettisten kelojen avulla. GTK:n vuoteen 1992 asti käyttämät protonimagnetometrien mittausarvot kompensoitiin lähinnä mit- tauslinjojen lentosuunnan perusteella. Vuonna 1993 käyttöönotettujen Cs- magnetometrien mittausarvot kompensoidaan lennon aikana reaaliajassa matemaattisesti. Kompensoinnin tarpeellisuutta havainnollistetaan PGE:n ja GTK:n Venäjällä ja Suomessa suoritettujen mittaustulosten avulla.

* PGE liitettiin (syksyllä 1998) North-West Region Geological Centre, 24/1 Odoevsky Street, St. Petersburg 199155, Russia, Phone:+7 812 352 3014, fax:+7 812 352 2417

Avainsanat (Fingeo-sanasto, GTK): Geofysikaaliset tutkimukset, lento- mittaukset, gammasädespektri, kalibrointi, magnetometrit, laitteet, kokeet, Suomi, Venäjä

Maija Kurimo, Geologian tutkimuskeskus, PL 96, 02151 ESPOO Sähköposti: [email protected]

Jukka Multala, Geologian tutkimuskeskus, PL 96, 02151 ESPOO Sähköposti: [email protected] CONTENTS

Introduction ...... 5 Aircraft and navigation systems ...... 5 Russian systems ...... 5 Finnish systems ...... 5 Calibration of airborne gamma-ray instruments ...... 6 Calibration procedure ...... 6 Russian instrumentation ...... 6 Finnish instrumentation ...... 8 Calibration facilities ...... 8 Measurements in Russia ...... 9 Measurements in Finland ...... 10 Calibration coefficients for Russian installation ...... 11 Calibration coefficients for Finnish installation ...... 12 Conclusions ...... 13 Comparison of compensation of airborne magnetometers ...... 14 Russian instrumentation ...... 14 Compensation procedure for the STK station magnetometer...... 14 Finnish instrumentation ...... 15 Compensation procedure for Finnish magnetometers ...... 15 Compensation procedure of Finnish Cs magnetometers ...... 17 Test measurements with the Finnish proton magnetometer ...... 17 Measurements in Finland with Russian instruments ...... 18 Conclusions ...... 18 Technical advice and recommendations ...... 19 Literature ...... 19 Appendix 1. Review map of calibratin test flights over the test ranges in Finland and West Karelia. Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia

INTRODUCTION

During 1990–1991 specialists from the Geolog- netic measurement, followed by directions as how ical Survey of Finland (GSF) and Petersburg Geo- to optimize standardization procedures for airborne physical Expedition (PGE) of Sevzapgeologia State instrument output used by PGE and GSF in geo- Geological Enterprise, acting within the frame- physical projects. work of topic 4.2 "Airborne Geophysics" of Finn- This calibration project forms the basis for an- ish–Russian scientific and technological coopera- other PGE and GSF joint project in which aerogeo- tion in geology, carried out calibration test flights physical maps will be compiled for the region over the test areas in Finland and West Karelia, for straddling the Finnish Russian border, using and which purpose the airborne geophysical measuring combining aerogeophysical data acquired by both units of both parties were employed. PGE and GSF. The present report outlines the progress made in The authors Anatoli Chepick and Vladimir Bara- implementing the joint program of instrument cal- nov from PGE as well as Maija Kurimo and Jukka ibration and comprises separate considerations of Multala from GSF would like to thank all those fellow airborne gammaspectrometry and airborne mag- workers who made this project and report possible.

AIRCRAFT AND NAVIGATION SYSTEMS

Russian systems

The gamma-ray spectrometer has been installed path recovery. An Antonov-2 aircraft is also used in a Russian MI-8 helicopter. The nominal cruising for geophysical mapping with a cruising speed of speed is about 160 km/h and maximum flight time about 160 km/h and maximum flight time of 7 4 hours without refueling. Navigation is with the hours. From 1993 onwards a TNL-1000 GPS-re- RSDN-3 radionavigation system (indicator Balti- ceiver has been in use. ca). Black and white film camera is used for flight

Finnish systems

The Geological Survey of Finland commenced several modifications were made to its electrical the second phase of its national aerogeophysical systems in order to reduce the electrical noice level. mapping project in 1972 using a 200 metres line The flight time navigation has been based upon spacing and a nominal terrain clearence of 30 visual navigation with photomosaic maps and a metres. Since 1980 these measurements have been custom-built left-right indicator based on a Decca done using a twin engine DHC-6/300 Twin Otter doppler unit. For the flight path recovery a modified (OH-KOG) owned by the Karair company. During Panasonic 6200 VHS video recorder has been used. the survey flights the cruising speed is about 50 m/s The system also includes a radar altimeter (Collins), (180 km/h). The Twin Otter was selected for the barometric altimeter, cabin and outside thermome- measuring platform as the most suitable fixed wing ters, accelerometer and spherics monitor. During the aircraft for low level STOL operations. The aircraft survey flight data is recorded on a PC hard disk offers several major advantages in terms of utility which is afterwards copied to a 150 Mb cassette. and cost, including excellent performance reserves, In 1992 two GPS-receivers were acquired. One low-speed handling characteristics and operational is installed in the aircraft and the other is located flexibility to operate from unpaved airstrips with- within the survey area. This so called Differential out any ground equipment. GPS-system (DGPS) provides accuracy of better During the manufacturing of the Twin Otter than 10 metres on every single measurement.

5 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

CALIBRATION OF AIRBORNE GAMMA-RAY INSTRUMENTS

Calibration procedure

The calibrations of the two systems were essen- Table 1. Energy ranges of the four windows to be used in airborne gammaray spectrometry measurements according to tially carried out according to the International the IAEA recommendations. (International Atomic Energy Atomic Energy Agency’s (IAEA) recommenda- Agency 1991, p 13). tions. The physical background to the calibration Window name Energy range Radio nuclide Major Peak procedure is therefore not repeated here in detail as MeV MeV it can be found in (International Atomic Energy Thorium 2.41–2.81 Tl-208 2.614 Agency 1991). Only the equations for different Uranium 1.66–1.86 Bi-214 1.765 corrections are given below. Potassium 1.37–1.57 K-40 1.46 In order to calibrate airborne gamma-ray spec- Total count 0.41–2.81 trometer output to ground concentrations of potassium, uranium and thorium it is recommended that the windows shown in Table 1 be used. (Standard Temperature 0oC and Pressure 1013.25 The first correction that must be applied to the mbar) is determined using Equation 4, in which Ht gamma-ray spectrometer data is the background is the observed radar altitude, T is the measured air correction, which takes account of the influence of temperature in oC, P is the barometric pressure in both atmospheric radiation and any radiation ema- mbar, and H is the effective or equivalent height at nating from the aircraft. The background values of STP. the four windows are usually measured during flights over lakes or the sea. Some airborne systems 273.15 P have special upward oriented crystals for monitor- H = Ht ———— * ——–— (4) ing background radiation. The detailed procedure T+273.15 1013.25 for calibrating upward directed crystals can be µ found in IAEA 1991. Calibration in this project Ni = Nc*e i(H-Ho) (5) was done however without upward directed crystals. Expression 5 is used to correct the background After the background correction has been per- and stripping corrected count rates Nc to the nom- formed the so called stripping correction should be inal survey altitude Ho. H is the effective (air µ applied to the data. This correction eliminates the pressure and temperature corrected) altitude and i influence of Compton scattering and is done ac- the linear absorption coefficient of the air for win- µ cording formulae 1–3. dow i. The constants i are usually determined by flying at different altitudes over a test area. Thc = (Thb - a*Ub)/(1-a*a) (1) The corrected count rates are finally converted to percentages for potassium, ppm (parts per mil- Uc = (Ub - a*Thb)/(1-a*a) (2) lion) for eU and eTh (equivalent uranium and thorium) using equations 6, 7 and 8. kK, kU and Kc = Kb - ß*Thc - g*Uc (3) kTh are the three sensitivity coefficients, while K, U, and Th are the fully corrected count rates of The a, α, ß and γ are the four stripping potassium, uranium and thorium windows. coefficients to be determined with four calibration blocks or pads of known concentrations of K, U and K% = kK * K (6) Th. Thb, Ub and Kb are the background corrected count rates of the thorium, uranium and potassium eUppm = kU * U (7) windows. The equivalent height of the aircraft at STP eThppm = kTh * Th (8)

Russian instrumentation

During the flights over the test ranges in Russia ma radiation of rocks and their respective concen- and Finland records were taken with the multifunc- trations of radioelements; geomagnetic field inten- tional airborne geophysical STK unit which can, sity; horizontal component of VLF-station EM specially, measure the following parameters: gam- field; absolute and relative flight altitudes; terrain

6 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia topography along a flight line; time at each way- rates (accumulation time 0.25 s) and provides a point of flight route; photograph landmarks and digital/analog output of these results, transmitting radionavigation parameters. the digital data into the magnetic logger. In addi- The station control unit provides registration of tion the spectrometer facilitates the introduction of the measuring channels at time intervals of 0.25, various corrections into the measurement results 0.5, 1.0, 2.0, 20 and 40 seconds, and digital data presented in analog form, allows calculation of U, transmission into a magnetic logger. Th and K concentrations (U and Th in ppm, K in %), The model 1990 STK unit consists of six gam- and records the measurement results on analog ma-ray detector units with a total volume of 37.7 record. U-Th-K concentrations are calculated in liters. Each unit incorporates: two Na(Tl) scintilla- analog form by using an airborne computing device tion detectors (dimensions of single crystals are that performs addition with specific weighted coef- 200 x 100 mm), a 128 channel amplitude analyzer ficients of voltages from the output of the differen- and a digital system for the automatic stabilization tial channels, and also makes altimetric correc- of the energy range scale with reference to Co-60 tions, residual background subtractions and auto- beta-gamma coincidence. The detector unit output matic reduction of the measurements to ground comprises the pulses corresponding to 7 energy level. Adjustment of the airborne computing de- intervals which represent, respectively, 1 radio- vice is defined by the manufacturer that defines metric channel, 4 differential channels in the range dimensionless calibration coefficients Bji with ref- of 1.12, 1.76 and 2.62 MeV photopeak values, a erence to the spectrometric performances of the cosmic radiation channel with low threshold at 3 airborne gamma-ray spectrometer (j=Th, U, K; i= MeV, and a channel to provide a stabilizing effect I, Σ, III). at the Co-60 photopeak value of 1.33 MeV. Measuring ranges for basic parameters of the Measuring energy ranges for the STK differen- STK airborne gamma-ray spectrometer are listed in tial and radiometric channels are as follows: DK-I table 2. (Th-window) 2.37–2.95 MeV; DK-Σ (two U-win- The calibration survey flown in 1991 utilized dows: DK-II and DK-IV) 1.64–1.95 MeV and 1.06– 1:50 000 scale topographic base maps and photo 1.21 MeV; DK-III (K-window) 1.34–1.60 MeV; maps. Accuracy in positioning the MI-8 helicopter total count rate channel 0.3–3.0 MeV. Figure 1 was checked by taking photos of the landmarks. illustrates the location of DK-windows in respect Since 1991, for the integrated airborne geo- of gamma spectra. physical surveys, PGE has been performing posi- The gamma-ray spectrometer records the count tion fixing of the aircraft by using the Russian

Fig. 1. The location of the DK-I, -II, -III and -IV windows of the STK-spectrometer in respect of gamma spectra.

7 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

Table 2. Measuring ranges for basic parameters of the STK airborne gamma-ray spectrometer. STK parameter performances Tolerance limits Actual tolerance limits for the instrument utilized in joint projects Range of recorded gamma photon energy 0.25–3.0 MeV 0.25–3.0 MeV Energy resolution in Cs-137 line with 0.662 MeV energy <= 16% From 13.5 to 15.0% depending on a scintillation detector units’ resolution number Integral nonlinearity of the energy scale =< 3% 2.5% Additional error in gamma-radiation energy measurements =< 1% < 1%

RSDN-3 radiotechnical system (receiver indica- GPS NAVSTAR system operating a Trimble Nav- tors A-723, A-720 and the Baltica model) and a igation receiver of TNL-1000 type.

Finnish instrumentation

The modified McPhar gamma-ray spectrometer drift of the photomultipliers. Registration occurs at a consists of six NaI crystals, with total volume of 25 frequency of one per second (50 m). The system was liters and a spectrometer which operates on a 256 updated in 1991 with 2 NaI upward directed crystals channel mode, although the energy range 0.2–3.0 (total volume 8.3 liters) for monitoring background MeV is registered on only 120 channels, each 24 keV. radiation. At least once a year the resolution of each The two thermally isolated crystal boxes have auto- crystal is measured with a Cs-137 source. The resolu- matic temperature control in order to minimize the tion of the crystals varies from 7.2% to 9.2%.

Calibration facilities

In the Askola area, about 40 km east from Hel- located on the north coast of Lake Ladoga and has sinki (see Appendix 1), a test profile is used to dimensions of approximately 1.5 km by 2.5 km. determine the attenuation coefficients of the air. The Pusunsaari area, including its "northern" and The profile commences on the coast of the Gulf of "southern" segments has been chosen as a test Finland, thus permitting easy background meas- range for calibrating the airborne gamma-spec- urements over sea. The topography of this calibra- trometers because of its flat topography and high tion line is reasonably flat and it consists of do- concentrations of radioelements, the factor of gam- mains having different ratios of potassium, urani- ma-radiation intensity variation not exceeding 15%. um and thorium. The flight line over the island is altogether about 2– The stripping coefficients are determined with 3 km long. The mean concentrations of radioele- small calibration blocks of size 1 meter by 1 meter ments within the test area have been calculated by 30 cm thick, and weighing about 675 kg. These from the results of laboratory analyses and are blocks are manufactured in Canada by Gammabob given in Table 4. U(Ra) and Th concentrations are Co (Grasty, Holman, Blanchard 1991). The radioe- derived from radiochemical analysis data checked lement concentrations of these blocks as given by against radiometric measurements. Potassium con- the manufacturer are presented in Table 3. centrations are defined by flame photometric anal- The main runway of the Helsinki-Vantaa airport ysis. The Pusunsaari calibration line was also used is used to convert the fully corrected count rates to for double determining the attenuation coefficients, equivalent concentrations of thorium (eTh ppm) as well as for determining the sensitivities and and uranium (eU ppm) and % of potassium. The comparing the calibration accuracy of the instru- mean concentrations in 1991 were about 4.2% K, ments of PGE and GSF. 6.5 ppm eU and 14.5 ppm eTh. The locations of each of the calibration sites used The island of Pusunsaari ("Island of Kisses") is in this study are shown on the map in Appendix 1.

Table 3. Concentrations of transportable calibration pads in Table 4. Radioelement concentrations of the Pusunsaari test Finland. range. Potassium % U ppm Th ppm Area/Concentrations Th ppm U ppm K % B Pad 1.34 +/- 0.01 0.98 +/- 0.02 2.28 +/- 0.07 Pusunsaari/North 25 3 4.5 K Pad 7.98 +/- 0.18 0.46 +/- 0.03 1.82 +/- 0.06 Pusunsaari/South 29 4.8 4.7 U Pad 1.25 +/- 0.01 53.33+/- 0.39 3.20 +/- 0.09 T Pad 1.34 +/- 0.01 2.31 +/- 0.04 110.0 +/- 1.42

8 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia

Measurements in Russia

The Finnish Twin Otter was flown to St Peters- over the Pusunsaari test range. The STK unit took burg (then Leningrad) on August 20 in 1990. On radioelement concentration measurements at an August 21 a single flight was made along the average altitude of 55 m, recording them on a paper calibration profile over Pusunsaari in the Sortavala tape, and also took readings of the counts from area (West Karelia, Russia) with GSF instruments. differential channels, recording them in a digital The test profile was flown at four nominal altitudes logger. Based on the results of six repeated flights and the background radiation was measured above a graphic record scale range was defined for the Lake Ladoka during ascent from one altitude to the information recorded in a visual logger. In addition next one. mean count rates for corresponding differential tz On May 5, 1991 calibration flights were made channels (Ni ) and scaling factors Mj (Table 5) with a MI-8 helicopter along the central profile were derived.

tz Table 5. Mean values of count rates Ni , less residual background, and graphic record scales (results of Pusunsaari flights). Area Channels Differential, counts/s Concentrations DK-I DK-Σ DK-III Th*10-4 % U*10-4 %K % Pusunsaari/ north 168 285 495 For the second scale: ratio at scale 1:2:4 3.99 1.53 2.18 Pusunsaari/ south 202 346 548

zb µ Scaling factors for digital recording are derived Ni = (ni - ñi ) * exp( ihi) (12) from the expression

tz Σ tz where: Mj=qj /( Bji*Ni ) (9) n is countrate in the i-differential channel where: i DK (counts/s); tz qj is the concentration of Th, U and K within zb the test ranges, according to the analytical ñi is the average of values recorded by the data; digital logger in i-differential channel DK, as a result of measuring a constant compo-

Bji are dimensionless calibration factors de- nent of the residual background level; pending on spectrometric performances of the spectrometer (j=Th, U, K and i=DK- h is flight altitude measured synchronously Σ i I, DK- , DK-III). with "ni"; During surveys in Russia the airborne gamma- µ is air attenuation factor for the gamma-ray spectrometer calibration procedure utilizes mathe- i photons recorded at the survey altitude. matical solutions that are somewhat different from This factor is derived from the results of those in other European countries; these solutions specific measurements over the test area are therefore given here. with a uniform distribution of radioele- Concentrations of natural radioelements, based ments and at a survey altitude above sea on airborne gamma-ray survey data recorded in the level — absolute altitude of H=0 at STP digital logger, are calculated from the formula: (15 degrees of Centigrade, 760 mm) under specific air temperature (t in oC) and pres- q = M * N (10) j j sure (p in mm Hg)

where N = ΣB * N (11) j ji i µ o i(t C,p)= (13) The count rate N in any differential channel is i µ (15oC,760 mm)*(p/760)*[288/(273+to)]∆ found from the expression: i

9 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

In order to calculate the factors Bji, it is necessary Values of spectrum coefficients di(j), as defined to define the values of spectrum coefficients "di(j)" from the results of measurements over the artificial using Th, U, K and backround pads. In this way Bji- ore models, with reference to the corresponding factors are derived for the expression: increments over the B-pad while a differential channel DK operates in countrate mode, are tabu- Σ Nj = iBji * Ni (14) lated in Table 6. Dimensionless scaling factors Bji for STK station are also given. Σ ∆ BThl = [1-d3(U )*dΣ(K)]/ (15) Based on the results of surveys over the Pusun- saari test area, scaling factors Mj in equation (9) Σ ∆ BThS = dl(U )/ (16) were found to be equal to: Σ ∆ BTh3 = dl(U )*dΣ(K)/ (17) MTh = 0.0921 MRa = 0.0231 MK = 0.0086 ∆ BUI = [d3(Th)*dΣ(K)-dΣ(Th)]/ (18) Results of digital data, based on computer processing, display good reproductibility and pre- ∆ BUΣ = 1/ (19) cision of radioelement concentration measurements throughout the test range: B = -d /∆ (20) U3 Σ(K) Th = 24.6 and 29.6 ppm B = [d *d -d (Th)]/∆ (21) Kl Σ(Th) 3(UΣ) 3 U = 3.2 and 4.4 ppm B = [d *d -d (UΣ)]/∆ (22) KS 1(UΣ) 3(Th) 3 K = 4.6 and 4.7% for north and south respec- ∆ tively. BK3 = [1-dΣ(Th)*d1(UΣ)]/ (23)

B = [1-d*∆ -1d] (24)

∆= (25) 1+d (UΣ)*d *d -d *d -d *d 1 Σ(K) 3(TH) 3(UΣ) Σ(K) 1(UΣ) Σ(Th) Table 6. Values of spectrum coefficients di(j), as defined from the results of measurements over the artificial ore models, with reference to the corresponding increments over the B-pad while The concentrations are determined as follows: a differential channel DK operates in countrate mode. Element d B q = M *(B *N +B *N +B *N ) (26) t(j) ji Th Th Thl Th ThΣ Σ Th3 3 (j) DK-I DK-Σ DK-III DK-I DK-Σ DK-III q = M *(B *N +B *N +B *N ) (27) Thorium 1.0 0.6608 0.5074 1.0304 -0.6558 -0.1357 U U Ul Th UΣ Σ U3 3 Uranium 0.0472 1.0 0.5902 -0.0546 1.1561 -0.6547 Potassium 0 0.1842 1.0 0.0101 -0.2129 1.1200

qK = MK*(BKl*NTh+BKΣ*NΣ+BK3*N3) (28)

Measurements in Finland

On July 4–5, 1991, flights were carried out over plot (Fig. 2) were drawn up. the test area in the Askola region using a Russian The Finnish transportable calibration pads were MI-8 survey helicopter equipped with the PGE also measured with the Russian instruments for STK-08 airborne geophysical station. The test range about 10 minutes per pad. Each pad was in turn was flown at the altitudes of 30, 50, 75, 100 and 120 placed along the central axis line of the scintillation metres and the background radiation was measured detector assembly beneath the helicopter fuselage. for about 10 minutes over the sea. Before landing at Clearance between the upper planes of the pads and Helsinki-Vantaa airport the main runway was meas- the bottom plane of the central crystals in the ured from a nominal altitude of 30 meters. As scintillation detector assembly was about 30 cm. results of the Askola region calibration flights, Corresponding flights and measurements were graphs of radioelements and gamma-field-pattern also made using the Finnish system.

10 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia

Fig. 2. Results of the Askola region calibration flights, graphs of radioelements and gamma-field-pattern plot.

Calibration coefficients for Russian installation

Average count rates in each differential channel Table 7. Count rates over the calibration pads. over each calibration pad are given in Table 7. Element Counts/second (pad) DK-I DK-Σ DK-III Stripping factors are: Th-pad 706 1336 1136 U-pad 445 1567 1167 a = 0.0046, K-pad 430 1305 1413 α = 0.333, B-pad 430 1244 1075 ß = 0.2210 and γ = 0.2848 Mean values of the count rates measured over the in Table 8. GSF test profile at five different altitudes are given The corresponding background values are given

Table 8. Mean values of measured count rates over the Askola test area. Measurement Channel Counts/s, altitude (H) in metres H=31 H=51 H=70 H=90 H=110 Along test strip Total 9719 8793 8011 7369 6794 DK-I 147 134 125 115 108 DK-Σ 468 440 417 396 378 DK-III 738 688 648 612 581 Over water surface Total 2703 2703 2728 2777 2826 DK-I 54 54 54 54 54 DK-Σ 253 253 253 253 253 DK-III 399 399 399 399 399

11 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

Table 9. Background corrected count rates for the 5 altitudes. Table 11. The values for linear absorption coefficients ui, calculated using Equation 5. Altitude 31 51 70 90 110 Window Attenuation Correlation Total-window 7016 6090 5283 4592 3968 coefficient u coefficient Th-window 93 80 71 61 53 i U-window 215 187 164 143 125 Total count rate 0.0072 0.98 K-window 339 289 249 213 182 Thorium 0.0078 0.99 Uranium 0.0068 0.98 Potassium 0.0070 0.98

Table 10. Mean count rates (counts/s) after stripping corrections. Altitude 31 51 70 90 110 spectra. Therefore corrections need to be intro- DK-K 267.1 226.6 194.1 165.3 140.4 duced. This is not the case in Finland, where the DK-U 186.9 162.8 142.5 124.6 109.0 DK-Th 84.4 72.5 64.5 55.2 47.9 gamma-ray energy spectra of the calibration pads Total count rate 7016 6090 5283 4592 3968 are much closer to those of geologic targets, which eventually results in different values of the spec-

trum coefficients in terms of lessening dDK-Σ(Th), dDK-III(Th), dDK-III(U), dDK-II(K). By applying appropriate calibration factors of equations (15) to (24) we in table 9. attain in fact, 95% reproductibility of radioelement Table 10 shows mean count rates (counts/s) after concentration values for the Pusunsaari test range stripping corrections. in Russia. Based on the values in Table 11 the linear absorp- Applying these calibration coefficients to the tion coefficients ui were calculated using Equation 5. data measured above the main runway at Helsinki– In order to calibrate airborne gamma spectrom- Vantaa airport, the following mean concentrations eters in Russia, small-size ore models are used were obtained: 14.7 ppm Th, 6.3 ppm U and 4.2% K. whose gamma-radiation equivalents do not corre- These concentrations are practically the same as the spond to those of natural targets (test ranges in values of 14.5 ppm eTh, 6.5 ppm eU and 4.2% K, particular), since they have different gamma-ray given on page 8.

Calibration coefficients for Finnish installation

One transportable calibration pad at a time was Table 12. Measured count rates of the Finnish system above placed immediately below the detectors and measure- transportable pads. Average count-time 10 minutes. ments of approximately 10 minutes duration were Potassium Uranium Thorium Window Window Window carried out. The average counts for each window and counts/s counts/s counts/s pad are given in Table 12. An example of the measured mean spectra for the U-pad is shown in Figure 3. B-pad 571 126 97 The manufacturer of the transportable pads also K-pad 763 128 95 U-pad 648 244 103 provides a computer program PADWIN, originally Th-pad 613 164 247 developed by L. Lovborg (Grasty et al. 1991) for a rapid computation of the stripping factors. Using Table 13. Mean values of measured counts from Askola test the data in Table 12 as input for the program results range and corresponding background values, counts/s. in the following values for the stripping factors: Window Counts/s, altitude in meters a = 0.0068, Altitude 44 55 73 85 105

α = 0.2336, Along Total 2014 1859 1734 1545 1358 test Potassium 194 189 165 155 136 ß = 0.2668 and range Uranium 42 41 37 35 31 Thorium 47 47 41 39 35

γ = 0.6733 Back- Total 223 223 224 224 236 ground Potassium 28 28 28 28 28 The mean values for the measurements along the Uranium 6 6 6 6 6 Askola test range at five different altitudes are Thorium 5 5 5 6 5 given in Table 13. The background values are mean values of 10 minute measurements over the sea. The background corrected data on effective alti- The next step was to apply the stripping correc- tudes are given in Table 14. tion to every one-second measurement value. The

12 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia

Fig. 3. Mean spectra for U-pad measured with finnish installation.

Table 14. Background corrected mean values for the Askola Table 15. Count rates for the four windows after stripping calibration line (counts/s) and corresponding effective alti- corrections and corresponding effective altitude. tudes. Altitude 42 59 77 86 109 Altitude 42 59 77 86 109 Total 1791 1636 1510 1321 1122 Total 1791 1636 1510 1321 1122 Potassium 128.5 124.4 104.7 96.9 81.9 Potassium 166 161 137 127 108 Uranium 24.2 23.2 20.9 19.7 16.6 Uranium 36 35 31 29 25 Thorium 40.1 40.2 34.4 31.5 28.7 Thorium 42 42 36 33 30

Table 16. Height attenuation coefficients for the Finnish sys- mean count rates of the 4 windows on each altitude tem. are given in Table 15. Window µ correlation coef. The attenuation coefficients were calculated from Total 0.007068 0.9850 the data matrix given in Table 15 using Equation 5. Potassium 0.007129 0.9867 The coefficients are given in Table 16, together Uranium 0.005716 0.9836 with the respective correlation coefficients. Thorium 0.005586 0.9646 Finally, the sensitivity coefficients were calculated using the data measured 30 meters above the main runway at the Helsinki–Vantaa airport. The the Pusunsaari calibration line was converted to coefficients as referred to in Equations 6–8 are: ppm of eU, eTh and % K by applying all these kK= 0.01146 ppm/cps, kU=0.1008 ppm/cps and corrections and coefficients. The mean concentra- kTh=0.1747 ppm/cps. tions were: 4.48% K, 6.05 ppm eU and 25.0 ppm In order to be able to compare the precision of the eTh. These values, except for the uranium concen- calibration line of Pusunsaari to the main run-way trations, are in good agreement with the values of Helsinki–Vantaa airport the data measured along given in Table 4.

Conclusions

Cooperation in the calibration of the STK and ematic procedures and software, all require atten- GSF-80 airborne gamma-ray spectrometers in the tion. test areas in Finland and Russia have shown that the The mathematical approach used in Russia for problems of instrument calibration, together with calculating radioelement concentrations from the designing the relevant technical facilities and math- airborne gamma spectrometry data is somewhat

13 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala different from that in Finland, but the final results along the Finnish-Russian state border would prove of measurements are, after all, compatible, which is to be extremely useful for the purpose of tying the therefore seen as justifying the utilization of such calibration results. techniques for airborne gamma surveys. This joint calibration project provides a valuable Regular flights with Finnish and Russian instru- basis for future cooperation with the common goal ments over a jointly arranged test range with uni- to produce joint aerogeophysical maps across the formly distributed concentrations of radioelements Finnish-Russian border.

COMPARISON OF COMPENSATION OF AIRBORNE MAGNETOMETERS

In airborne magnetic surveys a magnetometer lines at different headings. sensor is subject to the influence of both the terres- Random error — makes the registered field trial magnetic field and that generated by instru- fluctuate with a random noise component naviga- ment carrier (fixed wing aircraft or helicopter), tion while flying en route. which can obviosly significantly distort the meas- The first two errors are easily assessed by mak- urement results. Magnetic noise components result ing appropriate corrections to the observed values in deviation errors that can vary in origin as fol- during data processing. However, unless the noise lows: components due to random errors are compensat- Systematic level shift — one which causes a ed, rms error in measurements during operating shift in the observed ∆T-field over flight lines, a conditions increases significantly the possibility of shift which is in no way influenced by the heading instrument error. but is dependent instead on a target area (magnetic Intensive scrutiny and concern to reduce devia- field intensity and inclination). tion errors as much as possible has clearly been one Systematic error — another type of effect which of the common goals of Finnish and Russian geo- makes the observed ∆T-field change over the flight physicists (within the framework of topic 4.2).

Russian instrumentation

The STK station magnetometer has a specially duces the influence of errors on the measurement designed built-in deviation compensator which re- results.

Table 17. Basic parameters of PGE’s STK magnetometer system Parameter Presented by Actual performances manufacturer of the instrument engaged in joint project Measuring range, nT 20 000–100 000 20 000–100 000 Rms-deviation +/-3.0 +/-1.4 (From interval to interval), nT Systematic error, nT +/-2.5 +/-2.1 =accuracy Deviations in terms of plane orientation: in heading, nT +/-3.0 +/-1.5 in roll and pitch, nT/degree +/-0.2 +/-0.18

Compensation procedure for the STK station magnetometer

The compensating device is designed to operate current from a constant source current source, thus on a "physical compensation" principle and con- ensuring cancellation of any field caused by mag- sists of the compensators of constant and induced netic masses of in or due to helicopter (or airplane). noises. To produce a field that compensates the The other section employs an unsteady current constant noise parameters, a system of 3 concentric which varies with the fluctuations of a board circuit coils normal to each other is used. Each coil is made voltage. up of 2 sections, one which is fed by controlled To reduce the nonuniformity of the compensating

14 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia field, an extra system of coils is installed between As a result of compensating the helicopter mag- the primary compensating device and a sensor. The netic noise over the "Kobona" test area, the initial current in the coils of the auxiliary compensator is in range of heading deviation was reduced from +/- proportion to that of the coils fed by the constant 57 nT down to +/-1.5 nT, roll deviation from 2.6 nT/ o source, whereas the field is inverse in direction. down to 0.18 nT/o, and total magnetic field error Adjusting a proportionality factor for the currents of from 163 nT to virtually zero (considering a normal the extra and stabilized channels, facilitates com- field vertical gradien). Subsequent checking of pensation of the gradient of the stabilized channel residual deviation involved in making several short field at the center of the magnetometer sensor. flights over one and the same point at each 45o In order to eliminate the noise’s inductive com- azimuth sector (producing a star-like pattern of ponent, rods of magnetic soft material are used and flight routes) and specific routes with headings 0, these are arrayed in a specific manner around the 90, 180 and 270o with regular roll and pitch manou- sensor. vres (+/-15o amplitude and approximately 10 sec- Magnetic noise compensation and reduction of onds period). residual deviation were effected on May 30, 1991 The achieved residual deviation values of at the "Kobona" test range (on the southern coast of +/-1.5 nT for heading deviation and 0.18 nT/o for Lake Ladoga). This work was carried out by the roll deviation enable the accuracy of measurements PGE specialists and the airborne prospecting divi- to be much higher than in the case of uncompensat- sion of NPO "Rudgeofyzika" following precisely ed deviations and therefore justifies application of the standard procedure adopted by PGE. this rather laborious "physical" compensation tech- Prior to the flights over the test area adjustment nique. Further research into the problem of devia- and ground calibration of the compensating unit tion errors associated with magnetic noise of the was made, the adjustment being made on the heli- aircraft should involve an automatic compensating copter following headings of 60, 120, 240, 270, or system that will enable: 300o (+/- 10o). Calibration was done on a flat area * normalization of the requirements of the chosen especially for the navigation (gradient in residual heading/roll deviation range, topography no more than 3o). After that the heading and roll deviation param- * substantial reduction of flight time and eters were measured during the flight. Measure- enhancement of measurement reliability. ments were taken over an easily distinguishable landmark at headings of 0, 180, 90, 270, 0, 45, 225, The Petersburg Geophysical Expedition has al- 135, 315, 0o respectively. Repeated measurements ready been using the above described compensa- in one heading (0o) enable corrections for varia- tors for several years in high precission prospect- tions to be made rapidly. ing for oil and gas in promising target areas.

Finnish instrumentation

During the years 1972–1992 the magnetic field ters were replaced by two Scintrex Cs-magnetom- was registered with either one, two or three proton eters and by an automatic compensating unit (MAC- magnetometers. Since the year 1989 two proton 3 with CS-2 sensors and MEP-2111 processor mod- magnetometers at wing tips have been in use. The ule). The expected sensitivity is 0.001 nT and while sensor distance of this configuration, a transverse the registration rate can be as much as 10 samples/ horizontal gradiometer, was 21.36 metres. Regis- sec, the rate of 4 samples/sec is preferred. The tration has been performed at 4 times per second, magnetic reference stations are equipped with Ge- and with a flight speed of 50 m/s the measurement ometrics G803 magnetometers registrating every is approximately 12.5 m. The magnetometers used fifth second. In some small survey areas a portable were Geometrix G-811/813, which have a sensitiv- datalogger with a portable proton magnetometer is ity 1 nT or better. In 1993 the proton magnetome- being utilized instead of the station.

Compensation procedure for Finnish magnetometers

The measurements are corrected for variations ence Field 1965.0 at every measured point is in the geomagnetic field and for the influence of the achieved by subtracting the field of the aircraft and magnetic field of the aircraft. The magnetic field at both transient and secular variations from the sur- the level of the International Geomagnetic Refer- vey data.

15 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

The effect of aircraft magnetism depends upon to the IGRF 1965.0 level is made using annual data flight direction. Test flights in eight principal di- from observatories in Finland, Sweden and Russia. rections are performed two or three times every In order to correct the data for transient varia- year above a magnetically flat test area in which tions, there are always two mobile, local base detailed ground measurements have been carried stations that measure and digitally record the out. The direction corrections for every magnetom- magnetic field once in five seconds. The base eter are achieved from the test flight measurements stations are installed in two trailers. The first base (Fig. 4). The change in the remanent field of the station is at the local airfield. There, the flight aircraft is not linear throughout the year and there- operator can monitor the magnetic variations before the linear direction correction does not correct fore the flights. The airfield is often too far from for this drift precisely. To decrease the level differ- the survey area and hence, the second base station ence between the magnetometers, a statistical cor- is needed within the survey area. In southern rection is made by calculating the mode of the Finland the distance between the reference station difference of the two magnetometers. and the farthest point of the survey area should The secular variation of the geomagnetic field always be less than 100 km. In northern Finland, changes considerably in different parts of Finland. near the Auroral Zone, the reference station should There are two geophysical observatories in Fin- be inside the survey area. All the station sites land: Nurmijärvi in the south and Sodankylä in the should be in a geophysically quiet area. There are north and the difference in secular variation from large, electrically conductive zones in Finland, present to 1965.07 between northern and southern where special attention must be paid to the siting Finland varies between 700 and 750 nT. Reduction of the stations.

Fig. 4. The actual flight route of the aircraft to determine the effect of the aircraft magnetisms dependence upon flight direction.

16 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia

Survey flights are not carried out during magnet- the magnitude of the variations changes inside the ic storms. Tolerable limits of magnetic variations survey area or when a substantial part of the vari- have been defined because difficulties arise when ations is derived from magnetic rocks.

Compensation procedure of Finnish Cs magnetometers

The compensation procedure of Cs magnetome- main directions. The test flight is made in conjunc- ters is based on an automatic compensation system tion with every new flight area. Both compensated for the aircraft movements. The compensation func- and uncompensated magnetic measurements are tion is calculated during a test flight, and yaw, pitch recorded. and roll variations are introduced when flying to all

Test measurements with the Finnish proton magnetometer

The magnitude of the magnetic noise of the Twin pitch and roll tests were made. The differences Otter aircraft was measured over the magnetometer between wing tip magnetometers are of the same test range in the Lake Pyhäjärvi area (Russian Kare- magnitude as those recorded at Emäsalo. On nor- lia) in 22.8.1990. Variations in the magnetic field mal test lines the yaw differs less than one degree during the flight were registered at Helylä airdfield from the nominal flight direction, the pitch varies near Sortavala with a proton magnetometer (Scin- less than 4 degrees and the roll less than 5 degrees. trex) and the data were stored with a portable data The magnetic noise is 1–2 nTs when varying the logger. The Emäsalo compensation flight in Finland pitch between +/10 degrees. When varying the roll was carried out 6 days earlier. The calculated direc- +/15 degrees (yaw varies less than 3 degrees) the tion corrections are shown in Figure 5. magnetometer values changed by up to 20 nTs. At Pyhäjärvi the four cardinal directions were During normal flights roll variation is the most flown at a flight altitude of 100 ft. In addition some detrimental to magnetic measurements.

Fig. 5. Finnish Twin Otter installation: direction corrections calculated from the Emäsalo aeromagnetic test flight of 16.8.1990. The true magnetic field of the area is subtracted from the measured values.

17 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Anatoli Chepick, Vladimir Baranov, Maija Kurimo and Jukka Multala

Fig. 6. Results of measuring primary (1) residual (2) deviation.

Measurements in Finland with Russian instruments

According to measurements made over the pensating device of the STK-08 station magnetom- Emäsalo test area in Finland, the residual deviation eter. This magnitude is comparable to or lesser than did not exceed +/-2.0 nT (Fig. 6), when measured other sources of random noise. during the functioning of the magnetic noise com-

Conclusions

The +/-1.5 nT magnitude of residual deviation The compensation procedure which was used for obtained for STK-08 permits acquisition of air- the Finnish proton magnetometer system only com- borne magnetic survey data of relatively high accu- pensates the deviation for the heading. Although racy compared to measurements made with uncom- the amplitudes of magnetic-field anomalies are pensated magnetometer installations. most commonly large and the field gradients steep

18 Geologian tutkimuskeskus, Tutkimusraportti — Geological Survey of Finland, Report of Investigation 144, 1998 Joint calibration of airborne geophysical instruments in test areas in Finland and Russia during low altitude flights in Finland, roll varia- Cs magnetometers results in an extremely low tions may generate significant noise. The new au- noise level for Finnish geological surroundings. tomatic compensation system with high accuracy

TECHNICAL ADVICE AND RECOMMENDATIONS

To ensure compatibility and highest possible ments, it is advisable to specify common formats quality of aerogeophysical maps for the border for airborne geophysical data recording as well for areas between Russia and Finland it is recommend- the programs for entry, processing and interpreta- ed that parallel flights over test ranges be carried tion. out at least once every 3–5 years. It is further It is recommended that the magnetic noise com- recommended that a new test profile line be estab- pensation technique be mastered by means of adopt- lished traversing the Finnish-Russian border. This ing the automatic compensation system. line could be accessed by both parties from their It is desirable to develop further the contacts estab- own territories. lished between PGE and GSF in improving airborne To provide a high accuracy of field measure- geophysical survey techniques and engineering.

LITERATURE

Grasty, R. L., Holman, P. B. & Blanchard, Y. B. 1991. Belgium, June 3–6, 1986, p. 81. Transportable Calibration Pads for Ground and Airborne Multala, J. 1981. The Construction of Gamma-ray Spectro- Gamma-ray Spectrometrs, Geological Survey of Canada meter Calibration Pads. Geoexploration, 19 (1981), 33–46. Paper 90–23, 25 p. Vacuro, A. E. & Tcirel, V. S. 1983. Notes on Magnetic Field International Atomic Energy Agency 1991. Airborne Gam- Compensation Procedure Applied to the Airborne ma Ray Spectrometer Surveying, Technical Reports Series Geophysical Instruments Carriers. Internal publication of No 323, Vienna. the NPO "Rudgeofyzika" scientific and production asso- Krasnov, A. I. & Novikov, U. B. 1987. Medium-scale airborne ciation, Leningrad, USSR, 107 p. gamma-ray spectrometer surveying. Internal publication of Vavilin, L. N., Matveev, A. V. et al. 1977. A Guide to the NPO "Rudgeophysica", Leningrad, USSR, 119 p. Specifications and Requirements of Airborne Gamma-ray Kurimo-Salminen, M., Oksama, M. & Valli, T. 1985. Spectrometer Surveying. USSR Ministry of Geology, Matalalentomagneettisten karttojen valmistus. Geologian Moscow, USSR, 188 p. tutkimuskeskus, geofysiikan osasto (in Finnish), 34 p. Vavilin, L. N., Vorobijov, V. P. et al. 1982. Airborne gamma Kurimo, M., Oksama M. & Valli, T. 1986. Airborne spectrometry in geology. "Nedra" Publishing House, Le- horizontal magnetic gradiometer system of the Geological ningrad, USSR, 271 p. Survey of Finland. Abstract. 48th EAEG meeting, Ostende,

19 Appendix 1. Review map of calibratin test flights over the test ranges in Finland and West Karelia.