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Solid-earth geophysics in Czechoslovakia
Solid-E rth Geophysics in Czechoslovakia A SHORT HISTORY OF GEOPHYSICS IN CZECHOSLOVAKIA G e o p h y s i c a l r e s e a r c h on t h e t e r r i t o r y of p r e s e n t - d a y C z e c h o s l o v a k i a h a s a t r a d i t i o n of m o r e t h a n a h u n d r e d y e a r s s t a n d i n g . L e t it be m e n t i o n e d , e . g . , t h a t a s e a r l y a s 1839 t h e d i r e c t o r of t h e P r a g u e A s t r o n o m i c a l O b s e r v a t o r y , K. K r e i l , e s t a b l i s h e d one of t h e f i r s t g e o m a g n e t i c obs e r v a t o r i e s in t h e K l e m e n t i n u m . He a l s o c a r r i e d out t h e f i r s t g e o m a g n e t i c s u r v e y of C z e c h o s l o v a k t e r r i t o r y in t h e y e a r s 1 8 4 3 - 1 8 5 8 . A s e c o n d m a g n e t i c s u r v e y w a s m a d e at t h e e n d of t h e l a s t c e n t u r y by a n o u t s t a n d i n g C z e c h g e o m a g n e t i c i a n , J . L i z n a r ( 1 8 6 2 - 1 9 4 3 ) . T h e o l d e s t r e l a t i v e p e n d u l u m m e a s u r e m e n t s on C z e c h o s l o v a k t e r r i t o r y w e r e c a r r i e d out in t h e m i n e s of B ~ e z o v ~ H o r y in 1882 a n d 1883 by a n a t i v e of P r a g u e , T . S t e r n e c k , w i t h t h e a i m of e s t a b l i s h i n g t h e law of v a r i a t i o n of g r a v i t y w i t h d e p t h u n d e r t h e e a r t h ' s s u r f a c e . B e t w e e n 1889 a n d 1895 he a l s o c a r r i e d out t h e f i r s t g r a v i m e t r i c s u r v e y of C z e c h o s l o v a k t e r r i t o r y . In S l o v a k i a , o b s e r v a t o r y g e o p h y s i c s d a t e b a c k to 1893 in S t a r ~ b a l a , t o d a y ' s H u r b a n o v o . In 1908 t h e f i r s t s e i s m i c s t a tion w a s f o u n d e d in Chub. One of t h e f i r s t e a r t h tide m e a s u r e m e n t s in t h e w o r l d w a s c a r r i e d out by F . K ~ h l e r in 1911 at a d e p t h of 1,100 m in m i n e s if t h e Bi~ezov~ H o r y . C o n s i s t e n t a n d p u r p o s e f u l d e v e l o p m e n t of C z e c h o s l o v a k g e o p h y s i c s , h o w e v e r , w a s b e g u n in 1920. It w a s t h e n t h a t V. L ~ s k a ( 1 8 6 2 - 1 9 4 3 ) f o u n d e d t h e S t a t e G e o p h y s i c a l I n s t i t u t e in P r a g u e a n d b e c a m e i t s f i r s t d i r e c t o r . C o n s i d e r a b l e c r e d i t i s due to L ~ s k a f o r c r e a t i n g c o n d i t i o n s f o r t h e d e v e l o p m e n t of m o d e r n g e o p h y s i c a l r e s e a r c h in C z e c h o s l o v a k i a , a l t h o u g h at t h e b e g i n n i n g t h e I n s t i t u t e took a long t i m e to o v e r c o m e t h e s h o r t a g e of i n s t r u m e n t s , s p a c e , a n d t r a i n e d , a s w e l l a s a u x i l i a r y , p e r s o n n e l . T h e s t a f f s h o r t a g e w a s s o l v e d by L ~ s k a by i n v i t i n g e x t e r n a l c o o p e r a t i o n by s c i e n t i s t s w h o u n d e r t o o k t h e s o l u t i o n of s p e c i a l g e o p h y s i c a l p r o b l e m s ( e . g . , F. C e c h u r a on t h e o b s e r v a t i o n of e a r t h t i d e s , B. K l a d i v o on r e l a t i v e g r a v i t y p e n d u l u m m e a s u r e m e n l : s , a n d V. ~ p a ~ e k on t h e d e v i a t i o n of t h e v e r t i c a l , etc.). A f t e r V. L,~ska r e t i r e d in 1933, he w a s s u c c e e d e d a s d i r e c t o r of t h e r n s t i t u t e by B. S a l a m o u n . D u r i n g h i s e r a , p r e p a r a t i o n s w e r e b e g u n f o r c o n s t r u c t i o n of a n e w g e o m a g n e t i c o b s e r v a t o r y in P r f i h o n i c e n e a r P r a g u e , a n d s e i s m i c r e s e a r c h a s w e l l a s e x p l o r a t i o n g e o p h y s i c s b e g a n to e x p a n d . M o s t c r e d i t in t h e s e b r a n c h e s i s due to two new s c i e n t i s t s of t h e I n s t i t u t e : A. Z ~ t o p e k and H. B ~ h o u n e k . In 1942 t h e S t a t e G e o p h y s i c a l I n s t i t u t e w a s a b o l i s h e d by t h e N a z i o c c u p a t i o n f o r c e s . A f t e r t h e l i b e r a t i o n of C z e c h o s l o v a k i a in 1945, t h e I n s t i t u t e w a s r e v i v e d a g a i n u n d e r t h e d i r e c t o r s h i p of B. S a l a m o u n . T h i s w a s t h e b e g i n n i n g of a n e r a of i n t e n s i v e w o r k d e v o t e d to t h e r e n e w a l of t h e I n s t i t u t e a n d f i l l i n g in t h e g a p s c a u s e d by t h e o c c u p a t i o n . At t h e s a m e t i m e , n e w g e o p h y s i c a l i n s t i t u t i o n s of a n a p p l i e d n a t u r e w e r e b e i n g e s t a b l i s h e d ( e . g . , oil a s well a s o r e a n d coal s u r veys, etc.). In 1950 t h e S t a t e G e o p h y s i c a l I n s t i t u t e w a s t e m p o r a r i l y d i s b a n d e d a n d a t t a c h e d t o t h e C e n t r a l I n s t i t u t e of P h y s i c s a s a D e p a r t m e n t of G e o p h y s i c s . I m m e d i a t e l y a f t e r t h e c r e a t i o n of t h e C z e c h o s l o v a k A c a d e m y of S c i e n c e s (~.S.A .V.), t h e G e o p h y s i c a l I n s t i t u t e w a s e s t a b l i s h e d a g a i n on 1 J a n u a r y 1953 a s one of i t s p r i n c i p a l i n s t i t u t e s . T h e r e n e w e d I n s t i t u t e i m m e d i a t e l y c a r r i e d on in t h e t r a d i t i o n s of t h e f o r m e r S t a t e G e o p h y s i c a l I n s t i t u t e , a n d a n i n t e n s i v e d e v e l o p m e n t of t h e G e o p h y s i c a l I n s t i t u t e of t h e ~.S.A.V., a s well a s of o t h e r g e o p h y s i c a l e s t a b l i s h m e n t s , e n s u e d . T h i s w a s b a c k e d by t h e e d u c a t i o n of a n e w s c i e n t i f i c s t a f f a t t h e C h a r l e s U n i v e r s i t y , w h e r e t h e D e p a r t m e n t of G e o p h y s i c s h a d b e e n e s t a b l i s h e d w i t h i n t h e F a c u l t y of M a t h e m a t i c s a n d P h y s i c s a n d t h e D e p a r t m e n t of A p p l i e d G e o p h y s i c s w i t h i n t h e F a c u l t y of S c i e n c e in 1952. T h e p r i n c i p a l r e s e a r c h t r e n d in t h e G e o p h y s i c a l I n s t i t u t e of t h e ~ . S . A . V . i s t h e s o l u t i o n a n d s t u d y of p r o b l e m s c o n c e r n i n g t h e e n t i r e e a r t h . I t s r e s e a r c h p r o g r a m i s c o m p i l e d in s u c h a w a y a s to r e n d e r [he r e s e a r c h p r o b l e m s c o n c o r d a n t with n a t i o n a l r e q u i r e m e n t s a s w e l l a s w i t h r e q u i r e m e n t s f c r m u l a t e d by i n t e r n a t i o n a l s c i e n t i f i c o r g a n i z a t i o n s ( e . g . , p a r t i c i p a t i o n in t h e I.G .Y., I . O . S . Y . , U . M . P . , etc.). T h e e n s u i n g t a s k s a r e c o n d u c t e d by s i x s c i e n t i f i c d e p a r t m e n t s of t h e I n s t i t u t e ( D e p a r t m e n t s of G r a v i m e t r y , S e i s m o l o g y , G e o m a g n e t i s m , G e o e l e c t r o c i t y , R a p i d V a r i a t i o n s of t h e E l e c t r o m a g n e t i c F i e l d of t h e E a r t h a n d of t h e I o n o s p h e r e ) , t o g e t h e r w i t h five observatories (Pl~ibram - earth tides; Prfihonice - geomagnetism, ionosphere, seismology, p a l a e o m a g u e t i s m ; P ~ n s k a V e s - i o n o s p h e r e ; K a ~ p e r s k ~ H o r y - s e i s m o l o g y ; Budkov - g e o electricity, geomagnetism). T o c o v e r t h e r e q u i r e m e n t s of g e o p h y s i c a l e x p l o r a t i o n on C z e c h o s l o v a k t e r r i t o r y , t h e I n s t i t u t e of A p p l i e d G e o p h y s i c s w a s c r e a t e d f r o m t h e I n s t i t u t e for Oil R e s e a r c h . I t s c e n t r e i s in B r n o w i t h a s u b o r d i n a t e d e p a r t m e n t in P r a g u e . It i s s u p p o r t e d by a t t a c h i n g s e v e r a l s p e c i a l i s t s f r o m o t h e r e s t a b l i s h m e n t s . T h e m a i n t r e n d in t h i s I n s t i t u t e l i e s in t h e u t i l i z a t i o n of m o d e r n g e o p h y s -
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ical methods of exploration for the requirementsof the national economy and for practical purposes. The details in the history of Czechoslovak geophysics would do justice to a much more extensive paper. The facts presented here are merely a rough outline and by no means take into account all the material which is available. J . PfCHA GEOMAGNETISM IN CZECHOSLOVAKIA Basic research ingeomagnetism can be divided into twomain groups: internal and the external electromagnetic fields. From among the investigated problems relevant to the study of the solid earth, the most important are the following: (i) Palaeomagnetic research of rocks with the aim of obtaining data on the earth's magnetic field in the past and on the trajectory of polewandering in the course of the individual geological epochs. Another goal is contributing tothe elucidation of continental drift andthe investigation of some problems of tectonic geology. (2) Archaeomagnetic research onbaked objects enables the investigation of the magnitude and the secular variation of the geomagnetic field during the past few thousand years which, together with the determination of the magnitude of the field during the Quaternary, form a basis for improving the theory of the creation and regeneration of the geomagnetic field. (3) Research on the magnetic properties of rocks and minerals and investigations of the effect of various physical and chemical factors on the generationand stability of remanent magnetization yields datafor judging the suitability of the individual types of rocks for palaeomagnetic studies and elucidates processes concerningthe creation and formation of magnetic minerals. At the same time it contributes to the determination of the kinds of ferromagnetic minerals contained in the rock. (4) Distribution of the geomagnetic field on the territory of Czechoslovakia, investigated by ground-level as well as aeromagnetic surveys, has become the basis for deriving the normal field and regional isoanomaliesfrom which the subsurfacestructurescan be investigateddown to depths of several kilometers, and the concentrations of rocks with higher magnetic content determined. (5) Deep electromagnetic sounding of the earth, using the variations of the natural magnetic field of the earth, enabled the existence of conductive interfaces at depths of about 30 and 150 to 200 km in South Slovakia to be proven on the basis of magnetotelluric curves, and enabled material for the complex interpretation of this anomalous region to be obtained. (6) Research on the temperature and pressure dependence of electric conductivity in rock samples with the aim to determine the regions of various physical mechanisms. (7) Research on the sources of geomagnetic activity, their variation and the structure of the ionosphere contributes to the establishment of a suitable model, which would contain and explain all the observed phenomena of the external electromagnetic field. The above-mentioned problems are being investigated in the Geophysical Institute of the ~.S.A.V., the Geophysical Institute of the S.A.S. (Slovak Academy of Sciences), the Institute of Geophysics at the Charles University in Prague and in the Institute of Applied Geophysics in Brno which, besides aeromagnetic survey and the study of the magnetic properties of rocks, has also undertaken research into a number of regional projects based on magneticfield measurements.This Institute has also undertaken the development of new geomagnetic instruments. The Geomagnetic Observatories in Prhhonice, Budkov, and Hurbanovo, which come under the Geophysical Institute of the ~.S.A.V. and the Geophysical Institute of the S.A.S., take care of basic observatory services. The results of the continuous records of the variations of the geomagnetic field and the absolute measurements are regularly published in reports: Results of Geomagnetic and Telluric Measurements, a n d a l s o in t h e f o r m of m o n t h l y r e p o r t s to t h e W e a t h e r D a t a C e n t e r s a n d t h e P e r m a n e n t S e r v i c e . At t h e s a m e t i m e t h e y f o r m a b a s i s f o r r e s e a r c h on s h o r t a n d l o n g - p e r i o d v a r i a t i o n s of t h e g e o m a g n e t i c f i e l d . T h e m o s t i m p o r t a n t r e s u l t s w e r e a c h i e v e d by i n v e s t i g a t i n g n e w d y n a m i c r e g u l a r i t i e s of t h e g e o m a g n e t i c f i e l d in t h e p e r i o d s of t h e i n d i v i d u a l p h a s e s of d e v e l o p m e n t of t h e e a r t h , a n d in e s t a b l i s h i n g t h e e x i s t e n c e of t h e c o n d u c t i v e i n t e r f a c e s in t h e d e e p e r p a r t s of t h e e a r t h ' s c r u s t a n d m a n t l e . J u s t a s s i g n i f i c a n t a r e t h e r e s u l t s p e r t a i n i n g to t h e m a g n e t i c p h e n o m e n a o b s e r v e d on t h e w h o l e t e r r i t o r y b y m e a n s of a e r o m a g n e t i c a n d g r o u n d - l e v e l s u r v e y s a n d t h e m e a n s of i n t e r preting the results. W i t h i n t h e f r a m e w o r k of i n t e r n a t i o n a l c o o p e r a t i o n , u n i f i e d m a g n e t i c m a p s of c e n t r a l a n d southeastern Europe are being assembled, common palaeomagnetic and archaeomagnetic inv e s t i g a t i o n s of s a m p l e s f r o m i d e n t i c a l g e o l o g i c a l p e r i o d s a r e b e i n g m a d e , a n d c o n c l u s i o n s on the t r a j e c t o r i e s of the p a l a e o m a g n e t i c p o l e s a n d on c o n t i n e n t a l d r i f t h a v e a l s o b e e n m a d e . A 56
V. BUCHA
GRAVIMETRY IN CZECHOSLOVAKIA G r a v i m e t r i c r e s e a r c h in Czechoslovakia is being followed at institutes of the Academy of S c i e n c e s (Geophysical Institute of the ~.S.A.V. in Prague and the Geophysical Institute of the S.A.S. in Bratislava), at u n i v e r s i t i e s (Technical University in Prague, C h a r l e s University in Prague, Technical College in Ko~ice), in institutes of applied geophysics (Institute of Applied Geophysics in Bruo and other e s t a b l i s h m e n t s of l e s s e r significance), and in the geodetic s e r v ice ( R e s e a r c h Institute of Geodesy and Cartography and the Geodetic and Cartographic Institute in Prague). The Institutes of the Academy of S c i e n c e s and the institutes of the u n i v e r s i t i e s , as well as institutes of the geodetic s e r v i c e , undertake the b a s i c r e s e a r c h in g r a v i m e t r y , w h e r e a s the o t h e r e s t a b l i s h m e n t s deal in applied g r a v i m e t r y and in the consistent mapping of Czechoslovakia. Basic r e s e a r c h in g r a v i m e t r y may be divided as follows: (1) D e t e r m i n a t i o n of the shape of the e a r t h which involved, f i r s t of all the problem of s u b s t i tuting a m a t h e m a t i c a l model for the actual earth. The question of the m o s t suitable shape and size of this model was studied as well as its position with r e s p e c t to the earth. Mathematical f o r m u l a e w e r e d e r i v e d for t r a n s i t i o n f r o m one plane of r e f e r e n c e to another u n d e r absolutely g e n e r a l conditions of t r a n s f o r m a t i o n . The effects of various quantities onthe magnitude of s o called absolute deviations of the v e r t i c a l w e r e studied. The shape of the geoid for the t e r r i t o r y of Czechoslovakia was d e t e r m i n e d in g r e a t detail for the t e s t i n g a r e a s in Czechoslovakia and in the W e s t e r n Alps. (2) F o r investigating t h e o r e t i c a l c o n s i d e r a t i o n s , two g r a v i m e t r i c testing a r e a s w e r e surveyed in high mountain regions. The f i r s t is designed for testing the methods of d e t e r m i n i n g the shape of the e a r t h by g r a v i m e t r i c methods, the second for verifying the methods for d e t e r m i n i n g the shape of the e a r t h by a combination of gravitational m e a s u r e m e n t s and t r i g o n o m e t r i c levelling. (3) The e a r t h tides a r e r e c o r d e d by a s y s t e m of horizontal pendulums. The r e c o r d s are conc u r r e n t l y subjected to h a r m o n i c analysis and a r e regularly published. M e a s u r e m e n t s of the v e r t i c a l cornponentusing the Gs. 11 g r a v i m e t e r a r e in an e x p e r i m e n t a l stage. The i n s t r u m e n t s have been placed in the d e e p e s t tidal station of the world in B@ezov4 Hory n e a r P g i b r a m , at a depth of 1,300 m. (4) Methods of interpretation. F o r p u r p o s e s of applied g r a v i m e t r y , solution of the d i r e c t and i n v e r s e p r o b l e m by using models, substituting the r e a l e a r t h gravitational f i e l d b y a s y s t e m of m a t e r i a l points, etc., a r e being investigated. Methods of an analytical extension of the function, the use of higher d e r i v a t i v e s of gravity potential, especially with a view to d i r e c t application in oil r e s e a r c h , have also been elaborated. (5) Concerning the determination of gravity topographic c o r r e c t i o n s , special c a r e was given to the calculation of gravity topographic c o r r e c t i o n s with a v i e w to the c o n s i d e r a b l e height det a i l s in Czechoslovakia. Maps of gravity c o r r e c t i o n s w e r e constructed for the entire state t e r r i t o r y , including the influence of r e g i o n s f r o m 5 to 166.7 km with an accuracy of m o r e than 0.5 regal. F c r calculating the influence of the i n t e r n a l region, s e v e r a l methods w e r e suggested using models, and the a p p r o p r i a t e aids w e r e derived. (6) For the g r a v i m e t r i c survey of Czechoslovakia, a basic net of the f i r s t and second o r d e r was s u r v e y e d using N o e r g a r d and Gs. 11 and Gs. 12 Askania g r a v i m e t e r s . Connection to the international s y s t e m of P o t s d a m was established. The whole of Czechoslovakia was g r a v i m e t r i c a l l y s u r v e y e d with a gravity point density of at l e a s t one point p e r 5 km 2, and maps of Bourguer anomalies with and without topographic c o r r e c t i o n s to ~ s c a l e of 1:200,000 w e r e constructed. A unified p u n c h - c a r d pool of all gravity m e a s u r e m e n t s in Czechoslovakia was established. (7) Concerning i s o s t a s y r e s e a r c h , m a p s of i s o s t a t i c anomalies for the t e r r i t o r y of Czechoslovakia have been derived. M. PICK SEISMOLOGY IN CZECHOSLOVAKIA In Czecho~,~lovakia t h e r e a r e six main institutions working in the field of seismology: the Geophysical Institute of the Czechc31ovak Academy of Sciences in P r a g u e , t h e Geophysical Institute of the Slovak Academy of Sciences in B r a t i s l a v a , the Institute of Geophysics of the C h a r l e s University in Prague, the Institute of Applied Geophysics in Brno, the Mining R e s e a r c h Institute in Prague and the I.G.H.P. ( D e p a r t m e n t of Engineering Geology) in Prague. The f i r s t t h r e e institutions deal with b a s i c r e s e a r c h and o p e r a t e s e i s m o l o g i c a l stations, the last t h r e e institutes a r e active in geophysical p r o s p e c t i n g and in the application of s e i s m o l o g i c a l methods in mining and engineering. Seismological stations in Prague, Prfihonice ( c e n t r a l station), Ka~persk~ Hory, Bratislava, ~rob~irov~i HL,rbanov5 and Skalnat~ P l e s o f o r m the Czechoslovak s e i s m o l o g i c a l network and a r e
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run by the f i r s t t h r e e i n s t i t u t e s . T e n - y e a r , annual and monthly station r e p o r t s containing s e i s m o g r a m a n a l y s e s and m a c r o s e i s m i c o b s e r v a t i o n s a r e d i s t r i b u t e d r e g u l a r l y to s t a t i o n s and c e n t r e s all o v e r the world. B a s i c r e s e a r c h in s e i s m o l o g y is c o n c e n t r a t e d on the following p r o b l e m s : (1) The t h e o r y of s e i s m i c w a v e s in l a y e r e d media, with s p e c i a l e m p h a s i s on d y n a m i c p r o p e r t i e s of r e f l e c t e d , r e f r a c t e d head w a v e s and s u r f a c e w a v e s of the SH type. (2) S e i s m i c m o d e l s of the c r u s t and u p p e r m a n t l e with p o s i t i v e and n e g a t i v e velocity g r a dients; a s p e c i a l m o d e l l i n g technique ( m u l t i c o m p o n e n t g e l s , u l t r a s o n i c a p p a r a t u s , t r a n s d u c e r s , m o d e l l i n g bench) h a s been developed. (3) S t r u c t u r e of the c r u s t and u p p e r m a n t l e u s i n g k i n e m a t i c a s well a s d y n a m i c p r o p e r t i e s of s e i s m i c w a v e s r e c o r d e d up to 3,000 k m f r o m the focus. The e a r t h ' s c r u s t is studied u s i n g explosion r e c o r d s along the m a i n profile c r o s s i n g C z e c h o s l o v a k i a f r o m the Ore Mountains (Kru~n~ Hory) to South Slovakia. P r e l i m i n a r y r e s u l t s indicate the depth of Moho v a r i e s between 28-36 km. A s c h e m e of a p o s s i b l e u p p e r m a n t l e s t r u c t u r e in s o u t h e a s t e r n E u r o p e h a s been d e r i v e d u s i n g an a n a l y s i s of the o s c i l l a t o r y c h a r a c t e r of amplitude c u r v e s of body waves. The m a i n c h a r a c t e r i s t i c s of the s t r u c t u r e a r e the low-velocity channel at a depth of about 100 k m and the d i s continuity at a depth of about 400 km. Between the bottom of the c r u s t and the low-velocity channel t h e r e a r e s e v e r a l d i s c o n t i n u i t i e s of the f i r s t and s e c o n d o r d e r s . Below the low-velocity channel, two d i s c o n t i n u i t i e s m a y e x i s t at a depth of about 150-200 km. (4) E a r t h q u a k e activity and t e c t o n o p h y s i c s . The s e i s m o c i t y of Europe and the M e d i t e r r a n e a n a r e a i s studied u s i n g e p i c e n t r e s and i n t e n s i t y m a p s , s t r a i n r e l e a s e c u r v e s , m a g n i t u d e - f r e quency r e l a t i o n s and total e n e r g y r e l e a s e . A m i g r a t i o n of e p i c e n t r e s of s t r o n g e s t s h o c k s w a s o b s e r v e d along the North Anatolian fault s y s t e m (1930-1953) and in s o u t h e r n Italy (1783-1785, 1908-1920). A detailed s e i s m o t e c t o n i c i n v e s t i g a t i o n of s o m e r e c e n t e a r t h q u a k e s w a s p e r f o r m e d , e.g., Skopje (26 J u l y 1963), Varto (19 A u g u s t 1966), e a s t e r n Alps (1963-1964), etc. (5) M i c r o s e i s m s in the p e r i o d r a n g e T= 0.1-9 s e c a r e investigated. Short p e r i o d noise h a s a p r o n o u n c e d m a x i m u m n e a r T = 0.5 sec. The s t o r m m i c r o s e i s m s m o s t l y display f r e q u e n c i e s of T = 5 sec. The origin and d e v e l o p m e n t of s t o r m m i c r o s e i s m s w a s found to be c l o s e l y connected with a rapidly moving p r e s s u r e s y s t e m in the North Atlantic. (6) T h e o r y and c o n s t r u c t i o n of i n s t r u m e n t s . The t h e o r y of e l e c t r o m a g n e t i c s e i s m o g r a p h s w a s developed in the field of calculation of the coupling coefficient and the p a r t i a l c o n s t a n t s . F o r r e j e c t i o n of d i s t u r b i n g s e i s m i c n o i s e , a g a l v a n o m e t r i c a m p l i f i e r with a r b i t r a r y t r a n s m i s s i o n c h a r a c t e r i s t i c s and a s y s t e m of r e j e c t i o n f i l t e r s w a s designed. Using the p r i n c i p l e s of the p i e z o p h o t o g r a p h i c method, a s i m p l e cell f o r r e c o r d i n g e x p l o s i o n - g e n e r a t e d p r e s s u r e s n e a r the s o u r c e (7.5-22,000 k g / c m 2 ) w a s c o n s t r u c t e d . A p r e c i s e u l t r a s o n i c a p p a r a t u s f o r s e i s m i c m o d elling and l a b o r a t o r y m e a s u r e m e n t s on rock s a m p l e s w a s developed ( r e s o l u t i o n power 0.01 s e c , two t i m e c h a n n e l s and t h r e e s i g n a l channels). (7) E l a s t i c p r o p e r t i e s of r o c k s . Velocity and attenuation of u l t r a s o n i c w a v e s in the Mc r a n g e u n d e r high p r e s s u r e s w e r e i n v e s t i g a t e d . Recently, the e l a s t i c a n i s o t r o p y of i g n e o u s and m e t a m o r p h i c r o c k s h a s been studied. S e i s m o l o g i c a l m e t h o d s play an i m p o r t a n t role in the r e s e a r c h of r o c k p r e s s u r e and rock b u r s t s in deep m i n e s . T h e r e f o r e , s e i s m i c p i c k - u p s have been i n s t a l l e d in the coal m i n e s n e a r Kladno and in the o r e m i n e s n e a r Pf~fbram. Some r e s e a r c h w a s done in the g e n e r a t i o n of s e i s m i c waves. Simple t y p e s of s o u r c e s w e r e s i m u l a t e d in the l a b o r a t o r y and the p r o p a g a t i o n of p r e s s u r e w a v e s in s o i l s w a s studied in situ. V.
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K,~RN~K
'-
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ical methods of exploration for the requirementsof the national economy and for practical purposes. The details in the history of Czechoslovak geophysics would do justice to a much more extensive paper. The facts presented here are merely a rough outline and by no means take into account all the material which is available. J . PfCHA GEOMAGNETISM IN CZECHOSLOVAKIA Basic research ingeomagnetism can be divided into twomain groups: internal and the external electromagnetic fields. From among the investigated problems relevant to the study of the solid earth, the most important are the following: (i) Palaeomagnetic research of rocks with the aim of obtaining data on the earth's magnetic field in the past and on the trajectory of polewandering in the course of the individual geological epochs. Another goal is contributing tothe elucidation of continental drift andthe investigation of some problems of tectonic geology. (2) Archaeomagnetic research onbaked objects enables the investigation of the magnitude and the secular variation of the geomagnetic field during the past few thousand years which, together with the determination of the magnitude of the field during the Quaternary, form a basis for improving the theory of the creation and regeneration of the geomagnetic field. (3) Research on the magnetic properties of rocks and minerals and investigations of the effect of various physical and chemical factors on the generationand stability of remanent magnetization yields datafor judging the suitability of the individual types of rocks for palaeomagnetic studies and elucidates processes concerningthe creation and formation of magnetic minerals. At the same time it contributes to the determination of the kinds of ferromagnetic minerals contained in the rock. (4) Distribution of the geomagnetic field on the territory of Czechoslovakia, investigated by ground-level as well as aeromagnetic surveys, has become the basis for deriving the normal field and regional isoanomaliesfrom which the subsurfacestructurescan be investigateddown to depths of several kilometers, and the concentrations of rocks with higher magnetic content determined. (5) Deep electromagnetic sounding of the earth, using the variations of the natural magnetic field of the earth, enabled the existence of conductive interfaces at depths of about 30 and 150 to 200 km in South Slovakia to be proven on the basis of magnetotelluric curves, and enabled material for the complex interpretation of this anomalous region to be obtained. (6) Research on the temperature and pressure dependence of electric conductivity in rock samples with the aim to determine the regions of various physical mechanisms. (7) Research on the sources of geomagnetic activity, their variation and the structure of the ionosphere contributes to the establishment of a suitable model, which would contain and explain all the observed phenomena of the external electromagnetic field. The above-mentioned problems are being investigated in the Geophysical Institute of the ~.S.A.V., the Geophysical Institute of the S.A.S. (Slovak Academy of Sciences), the Institute of Geophysics at the Charles University in Prague and in the Institute of Applied Geophysics in Brno which, besides aeromagnetic survey and the study of the magnetic properties of rocks, has also undertaken research into a number of regional projects based on magneticfield measurements.This Institute has also undertaken the development of new geomagnetic instruments. The Geomagnetic Observatories in Prhhonice, Budkov, and Hurbanovo, which come under the Geophysical Institute of the ~.S.A.V. and the Geophysical Institute of the S.A.S., take care of basic observatory services. The results of the continuous records of the variations of the geomagnetic field and the absolute measurements are regularly published in reports: Results of Geomagnetic and Telluric Measurements, a n d a l s o in t h e f o r m of m o n t h l y r e p o r t s to t h e W e a t h e r D a t a C e n t e r s a n d t h e P e r m a n e n t S e r v i c e . At t h e s a m e t i m e t h e y f o r m a b a s i s f o r r e s e a r c h on s h o r t a n d l o n g - p e r i o d v a r i a t i o n s of t h e g e o m a g n e t i c f i e l d . T h e m o s t i m p o r t a n t r e s u l t s w e r e a c h i e v e d by i n v e s t i g a t i n g n e w d y n a m i c r e g u l a r i t i e s of t h e g e o m a g n e t i c f i e l d in t h e p e r i o d s of t h e i n d i v i d u a l p h a s e s of d e v e l o p m e n t of t h e e a r t h , a n d in e s t a b l i s h i n g t h e e x i s t e n c e of t h e c o n d u c t i v e i n t e r f a c e s in t h e d e e p e r p a r t s of t h e e a r t h ' s c r u s t a n d m a n t l e . J u s t a s s i g n i f i c a n t a r e t h e r e s u l t s p e r t a i n i n g to t h e m a g n e t i c p h e n o m e n a o b s e r v e d on t h e w h o l e t e r r i t o r y b y m e a n s of a e r o m a g n e t i c a n d g r o u n d - l e v e l s u r v e y s a n d t h e m e a n s of i n t e r preting the results. W i t h i n t h e f r a m e w o r k of i n t e r n a t i o n a l c o o p e r a t i o n , u n i f i e d m a g n e t i c m a p s of c e n t r a l a n d southeastern Europe are being assembled, common palaeomagnetic and archaeomagnetic inv e s t i g a t i o n s of s a m p l e s f r o m i d e n t i c a l g e o l o g i c a l p e r i o d s a r e b e i n g m a d e , a n d c o n c l u s i o n s on the t r a j e c t o r i e s of the p a l a e o m a g n e t i c p o l e s a n d on c o n t i n e n t a l d r i f t h a v e a l s o b e e n m a d e . A 56
V. BUCHA
GRAVIMETRY IN CZECHOSLOVAKIA G r a v i m e t r i c r e s e a r c h in Czechoslovakia is being followed at institutes of the Academy of S c i e n c e s (Geophysical Institute of the ~.S.A.V. in Prague and the Geophysical Institute of the S.A.S. in Bratislava), at u n i v e r s i t i e s (Technical University in Prague, C h a r l e s University in Prague, Technical College in Ko~ice), in institutes of applied geophysics (Institute of Applied Geophysics in Bruo and other e s t a b l i s h m e n t s of l e s s e r significance), and in the geodetic s e r v ice ( R e s e a r c h Institute of Geodesy and Cartography and the Geodetic and Cartographic Institute in Prague). The Institutes of the Academy of S c i e n c e s and the institutes of the u n i v e r s i t i e s , as well as institutes of the geodetic s e r v i c e , undertake the b a s i c r e s e a r c h in g r a v i m e t r y , w h e r e a s the o t h e r e s t a b l i s h m e n t s deal in applied g r a v i m e t r y and in the consistent mapping of Czechoslovakia. Basic r e s e a r c h in g r a v i m e t r y may be divided as follows: (1) D e t e r m i n a t i o n of the shape of the e a r t h which involved, f i r s t of all the problem of s u b s t i tuting a m a t h e m a t i c a l model for the actual earth. The question of the m o s t suitable shape and size of this model was studied as well as its position with r e s p e c t to the earth. Mathematical f o r m u l a e w e r e d e r i v e d for t r a n s i t i o n f r o m one plane of r e f e r e n c e to another u n d e r absolutely g e n e r a l conditions of t r a n s f o r m a t i o n . The effects of various quantities onthe magnitude of s o called absolute deviations of the v e r t i c a l w e r e studied. The shape of the geoid for the t e r r i t o r y of Czechoslovakia was d e t e r m i n e d in g r e a t detail for the t e s t i n g a r e a s in Czechoslovakia and in the W e s t e r n Alps. (2) F o r investigating t h e o r e t i c a l c o n s i d e r a t i o n s , two g r a v i m e t r i c testing a r e a s w e r e surveyed in high mountain regions. The f i r s t is designed for testing the methods of d e t e r m i n i n g the shape of the e a r t h by g r a v i m e t r i c methods, the second for verifying the methods for d e t e r m i n i n g the shape of the e a r t h by a combination of gravitational m e a s u r e m e n t s and t r i g o n o m e t r i c levelling. (3) The e a r t h tides a r e r e c o r d e d by a s y s t e m of horizontal pendulums. The r e c o r d s are conc u r r e n t l y subjected to h a r m o n i c analysis and a r e regularly published. M e a s u r e m e n t s of the v e r t i c a l cornponentusing the Gs. 11 g r a v i m e t e r a r e in an e x p e r i m e n t a l stage. The i n s t r u m e n t s have been placed in the d e e p e s t tidal station of the world in B@ezov4 Hory n e a r P g i b r a m , at a depth of 1,300 m. (4) Methods of interpretation. F o r p u r p o s e s of applied g r a v i m e t r y , solution of the d i r e c t and i n v e r s e p r o b l e m by using models, substituting the r e a l e a r t h gravitational f i e l d b y a s y s t e m of m a t e r i a l points, etc., a r e being investigated. Methods of an analytical extension of the function, the use of higher d e r i v a t i v e s of gravity potential, especially with a view to d i r e c t application in oil r e s e a r c h , have also been elaborated. (5) Concerning the determination of gravity topographic c o r r e c t i o n s , special c a r e was given to the calculation of gravity topographic c o r r e c t i o n s with a v i e w to the c o n s i d e r a b l e height det a i l s in Czechoslovakia. Maps of gravity c o r r e c t i o n s w e r e constructed for the entire state t e r r i t o r y , including the influence of r e g i o n s f r o m 5 to 166.7 km with an accuracy of m o r e than 0.5 regal. F c r calculating the influence of the i n t e r n a l region, s e v e r a l methods w e r e suggested using models, and the a p p r o p r i a t e aids w e r e derived. (6) For the g r a v i m e t r i c survey of Czechoslovakia, a basic net of the f i r s t and second o r d e r was s u r v e y e d using N o e r g a r d and Gs. 11 and Gs. 12 Askania g r a v i m e t e r s . Connection to the international s y s t e m of P o t s d a m was established. The whole of Czechoslovakia was g r a v i m e t r i c a l l y s u r v e y e d with a gravity point density of at l e a s t one point p e r 5 km 2, and maps of Bourguer anomalies with and without topographic c o r r e c t i o n s to ~ s c a l e of 1:200,000 w e r e constructed. A unified p u n c h - c a r d pool of all gravity m e a s u r e m e n t s in Czechoslovakia was established. (7) Concerning i s o s t a s y r e s e a r c h , m a p s of i s o s t a t i c anomalies for the t e r r i t o r y of Czechoslovakia have been derived. M. PICK SEISMOLOGY IN CZECHOSLOVAKIA In Czecho~,~lovakia t h e r e a r e six main institutions working in the field of seismology: the Geophysical Institute of the Czechc31ovak Academy of Sciences in P r a g u e , t h e Geophysical Institute of the Slovak Academy of Sciences in B r a t i s l a v a , the Institute of Geophysics of the C h a r l e s University in Prague, the Institute of Applied Geophysics in Brno, the Mining R e s e a r c h Institute in Prague and the I.G.H.P. ( D e p a r t m e n t of Engineering Geology) in Prague. The f i r s t t h r e e institutions deal with b a s i c r e s e a r c h and o p e r a t e s e i s m o l o g i c a l stations, the last t h r e e institutes a r e active in geophysical p r o s p e c t i n g and in the application of s e i s m o l o g i c a l methods in mining and engineering. Seismological stations in Prague, Prfihonice ( c e n t r a l station), Ka~persk~ Hory, Bratislava, ~rob~irov~i HL,rbanov5 and Skalnat~ P l e s o f o r m the Czechoslovak s e i s m o l o g i c a l network and a r e
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run by the f i r s t t h r e e i n s t i t u t e s . T e n - y e a r , annual and monthly station r e p o r t s containing s e i s m o g r a m a n a l y s e s and m a c r o s e i s m i c o b s e r v a t i o n s a r e d i s t r i b u t e d r e g u l a r l y to s t a t i o n s and c e n t r e s all o v e r the world. B a s i c r e s e a r c h in s e i s m o l o g y is c o n c e n t r a t e d on the following p r o b l e m s : (1) The t h e o r y of s e i s m i c w a v e s in l a y e r e d media, with s p e c i a l e m p h a s i s on d y n a m i c p r o p e r t i e s of r e f l e c t e d , r e f r a c t e d head w a v e s and s u r f a c e w a v e s of the SH type. (2) S e i s m i c m o d e l s of the c r u s t and u p p e r m a n t l e with p o s i t i v e and n e g a t i v e velocity g r a dients; a s p e c i a l m o d e l l i n g technique ( m u l t i c o m p o n e n t g e l s , u l t r a s o n i c a p p a r a t u s , t r a n s d u c e r s , m o d e l l i n g bench) h a s been developed. (3) S t r u c t u r e of the c r u s t and u p p e r m a n t l e u s i n g k i n e m a t i c a s well a s d y n a m i c p r o p e r t i e s of s e i s m i c w a v e s r e c o r d e d up to 3,000 k m f r o m the focus. The e a r t h ' s c r u s t is studied u s i n g explosion r e c o r d s along the m a i n profile c r o s s i n g C z e c h o s l o v a k i a f r o m the Ore Mountains (Kru~n~ Hory) to South Slovakia. P r e l i m i n a r y r e s u l t s indicate the depth of Moho v a r i e s between 28-36 km. A s c h e m e of a p o s s i b l e u p p e r m a n t l e s t r u c t u r e in s o u t h e a s t e r n E u r o p e h a s been d e r i v e d u s i n g an a n a l y s i s of the o s c i l l a t o r y c h a r a c t e r of amplitude c u r v e s of body waves. The m a i n c h a r a c t e r i s t i c s of the s t r u c t u r e a r e the low-velocity channel at a depth of about 100 k m and the d i s continuity at a depth of about 400 km. Between the bottom of the c r u s t and the low-velocity channel t h e r e a r e s e v e r a l d i s c o n t i n u i t i e s of the f i r s t and s e c o n d o r d e r s . Below the low-velocity channel, two d i s c o n t i n u i t i e s m a y e x i s t at a depth of about 150-200 km. (4) E a r t h q u a k e activity and t e c t o n o p h y s i c s . The s e i s m o c i t y of Europe and the M e d i t e r r a n e a n a r e a i s studied u s i n g e p i c e n t r e s and i n t e n s i t y m a p s , s t r a i n r e l e a s e c u r v e s , m a g n i t u d e - f r e quency r e l a t i o n s and total e n e r g y r e l e a s e . A m i g r a t i o n of e p i c e n t r e s of s t r o n g e s t s h o c k s w a s o b s e r v e d along the North Anatolian fault s y s t e m (1930-1953) and in s o u t h e r n Italy (1783-1785, 1908-1920). A detailed s e i s m o t e c t o n i c i n v e s t i g a t i o n of s o m e r e c e n t e a r t h q u a k e s w a s p e r f o r m e d , e.g., Skopje (26 J u l y 1963), Varto (19 A u g u s t 1966), e a s t e r n Alps (1963-1964), etc. (5) M i c r o s e i s m s in the p e r i o d r a n g e T= 0.1-9 s e c a r e investigated. Short p e r i o d noise h a s a p r o n o u n c e d m a x i m u m n e a r T = 0.5 sec. The s t o r m m i c r o s e i s m s m o s t l y display f r e q u e n c i e s of T = 5 sec. The origin and d e v e l o p m e n t of s t o r m m i c r o s e i s m s w a s found to be c l o s e l y connected with a rapidly moving p r e s s u r e s y s t e m in the North Atlantic. (6) T h e o r y and c o n s t r u c t i o n of i n s t r u m e n t s . The t h e o r y of e l e c t r o m a g n e t i c s e i s m o g r a p h s w a s developed in the field of calculation of the coupling coefficient and the p a r t i a l c o n s t a n t s . F o r r e j e c t i o n of d i s t u r b i n g s e i s m i c n o i s e , a g a l v a n o m e t r i c a m p l i f i e r with a r b i t r a r y t r a n s m i s s i o n c h a r a c t e r i s t i c s and a s y s t e m of r e j e c t i o n f i l t e r s w a s designed. Using the p r i n c i p l e s of the p i e z o p h o t o g r a p h i c method, a s i m p l e cell f o r r e c o r d i n g e x p l o s i o n - g e n e r a t e d p r e s s u r e s n e a r the s o u r c e (7.5-22,000 k g / c m 2 ) w a s c o n s t r u c t e d . A p r e c i s e u l t r a s o n i c a p p a r a t u s f o r s e i s m i c m o d elling and l a b o r a t o r y m e a s u r e m e n t s on rock s a m p l e s w a s developed ( r e s o l u t i o n power 0.01 s e c , two t i m e c h a n n e l s and t h r e e s i g n a l channels). (7) E l a s t i c p r o p e r t i e s of r o c k s . Velocity and attenuation of u l t r a s o n i c w a v e s in the Mc r a n g e u n d e r high p r e s s u r e s w e r e i n v e s t i g a t e d . Recently, the e l a s t i c a n i s o t r o p y of i g n e o u s and m e t a m o r p h i c r o c k s h a s been studied. S e i s m o l o g i c a l m e t h o d s play an i m p o r t a n t role in the r e s e a r c h of r o c k p r e s s u r e and rock b u r s t s in deep m i n e s . T h e r e f o r e , s e i s m i c p i c k - u p s have been i n s t a l l e d in the coal m i n e s n e a r Kladno and in the o r e m i n e s n e a r Pf~fbram. Some r e s e a r c h w a s done in the g e n e r a t i o n of s e i s m i c waves. Simple t y p e s of s o u r c e s w e r e s i m u l a t e d in the l a b o r a t o r y and the p r o p a g a t i o n of p r e s s u r e w a v e s in s o i l s w a s studied in situ. V.
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