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The development of a tensor-amt system
These measurements were characterized by unusual features of the source field in terms of extremely high degrees of polarization and low values of the e~pticity. A comparison with the results from audio-rn~e~~ll~i~ measurements at other locations shows remarkably large values of the power spectra in the measurements from the Tostrup area. The analysis of the field characteristics highly indicates that contributions from artificial sources are the dom~at~g part of the fields. Some diffic~ties and limitations in the geophysical interpretation of these measurements will be discussed.
MAGNETOTELLURIC
MEASUREMENTS IN JUTLAND, DENMARK
JOHANNISSEN Dept. of Applied Geophysics,
University of Lule&.S-951
87 Luiea (Sweden)
In the summer 1981 magnetotelluric measurements along a north--south striking profile in Jutland were performed. In the southern part, furthermore, some geoebctrical depth soundings (Schiumberger array, spacing up to L/2 = 5 km) were carried out. A onedimensional interpretation of the magnetotelluric as well as the geoelectrical data was made. By comparing with some of the deep tests, two main results were obtained. The “magnetotelluric basement”, defined as the substratum where the resistivity increases by at least a decade, is formed by the Zechstein Formation or, in the absence of that formation, by the Precambrian basement. The “geoelectrical basement”, on the other hand, is constituted by an intermediary high resistive layer. The resistivity cross-section, derived from the interpretation of the magnetotelluric data, will be presented. THE DEVELOPMENT OF A TENSOR-AMT SYSTEM JENS BACH=SEN Lab. of Geophysics,
University of Arhus, Finlandsgade
6-8, 8200 Aarhus N (Denmark)
Sveriges Geologiska AB, Lulell, and the laboratory of Geophysics, IIrhus have co-operated in the development of a Gchannel, micro-processor-based AMT-equipment. The instrument is broadband operated, and the overall signal processing is performed in the field. The digital processing unit is expected to be finished in the spring 1983. In order to test the magnetic and electrical sensors analogue recordings were carried out in Vriisterbotten, Northern Sweden, in October 1981. System noise calculations together with some results, i.e. power spectra and apparent resistivities, will be presented. Furthermore, results from an ordinary
292
field set up are compared with results where a pseudo-reference station technique has been used. Based upon these measurements, recommendations on the final equipment are given.
TWO CASE STORIES: LONG PERIOD MT NEAR AARHUS AND SHORT TO MEDIUM PERIOD MT ON THE FAEROE ISLANDS BO HOLM JACOBSEN Laboratoriet (Denmark)
for Geofysik,
Aarhus
Universitet,
Finlandsgade
6-8,
DK-8200
Aarhus N
Recording of MT-signals with periods longer than 1 hour was attempted north of Aarhus using induction coils and PbCl-electrodes. Electric reference fields were recorded simultaneously 30 km south of the main station using CuSO, -electrodes. Signal quality is evaluated. MT-signals with periods 5OOs-2s were recorded on the Faeroe Islands (October 1982 in cooperation with P.H. Nielsen DGU, Thorshavn). Special attention is paid to the ocean effect and telluric distortions.
PRINCIPAL RESPONSES AS THE FOUNDATION INTUITION
OF GEOPHYSICAL
BO HOLM JACOBSEN Laboratoriet (Denmark)
for Geofysik, Aarhus Universitet, Finlandsgade
6-8,
DK-8200
Aarhus N
The concept of principal responses (P.R.) is defined both through its relation to the Popperian philosophy of science and through its more humble presence in the everyday geophysical practice. It is stated that: (1) P.R. is a characterizing element in the geosciences; (2) conscious application of P.R. does make experimental design and interpretation faster and safer; (3) P.R. may be a didactic tool in the teaching and training of geophysicists.
THE THERMAL CONDUCTIVITY OF SOME SEDIMENTARY ROCKS FROM SCANDINAVIA
CRYSTALLINE
AND
JAN I. KRISTIANSEN Laboratoriet (Denmark)
for Geofysik, Aarhus
Universitet,
Finlandsgade
6-8,
DK-8200
Aarhus N
Some available information on thermal conductivity of earth materials from the Scandinavian area is collected. The mean conductivities as reported
MAGNETOTELLURIC
MEASUREMENTS IN JUTLAND, DENMARK
JOHANNISSEN Dept. of Applied Geophysics,
University of Lule&.S-951
87 Luiea (Sweden)
In the summer 1981 magnetotelluric measurements along a north--south striking profile in Jutland were performed. In the southern part, furthermore, some geoebctrical depth soundings (Schiumberger array, spacing up to L/2 = 5 km) were carried out. A onedimensional interpretation of the magnetotelluric as well as the geoelectrical data was made. By comparing with some of the deep tests, two main results were obtained. The “magnetotelluric basement”, defined as the substratum where the resistivity increases by at least a decade, is formed by the Zechstein Formation or, in the absence of that formation, by the Precambrian basement. The “geoelectrical basement”, on the other hand, is constituted by an intermediary high resistive layer. The resistivity cross-section, derived from the interpretation of the magnetotelluric data, will be presented. THE DEVELOPMENT OF A TENSOR-AMT SYSTEM JENS BACH=SEN Lab. of Geophysics,
University of Arhus, Finlandsgade
6-8, 8200 Aarhus N (Denmark)
Sveriges Geologiska AB, Lulell, and the laboratory of Geophysics, IIrhus have co-operated in the development of a Gchannel, micro-processor-based AMT-equipment. The instrument is broadband operated, and the overall signal processing is performed in the field. The digital processing unit is expected to be finished in the spring 1983. In order to test the magnetic and electrical sensors analogue recordings were carried out in Vriisterbotten, Northern Sweden, in October 1981. System noise calculations together with some results, i.e. power spectra and apparent resistivities, will be presented. Furthermore, results from an ordinary
292
field set up are compared with results where a pseudo-reference station technique has been used. Based upon these measurements, recommendations on the final equipment are given.
TWO CASE STORIES: LONG PERIOD MT NEAR AARHUS AND SHORT TO MEDIUM PERIOD MT ON THE FAEROE ISLANDS BO HOLM JACOBSEN Laboratoriet (Denmark)
for Geofysik,
Aarhus
Universitet,
Finlandsgade
6-8,
DK-8200
Aarhus N
Recording of MT-signals with periods longer than 1 hour was attempted north of Aarhus using induction coils and PbCl-electrodes. Electric reference fields were recorded simultaneously 30 km south of the main station using CuSO, -electrodes. Signal quality is evaluated. MT-signals with periods 5OOs-2s were recorded on the Faeroe Islands (October 1982 in cooperation with P.H. Nielsen DGU, Thorshavn). Special attention is paid to the ocean effect and telluric distortions.
PRINCIPAL RESPONSES AS THE FOUNDATION INTUITION
OF GEOPHYSICAL
BO HOLM JACOBSEN Laboratoriet (Denmark)
for Geofysik, Aarhus Universitet, Finlandsgade
6-8,
DK-8200
Aarhus N
The concept of principal responses (P.R.) is defined both through its relation to the Popperian philosophy of science and through its more humble presence in the everyday geophysical practice. It is stated that: (1) P.R. is a characterizing element in the geosciences; (2) conscious application of P.R. does make experimental design and interpretation faster and safer; (3) P.R. may be a didactic tool in the teaching and training of geophysicists.
THE THERMAL CONDUCTIVITY OF SOME SEDIMENTARY ROCKS FROM SCANDINAVIA
CRYSTALLINE
AND
JAN I. KRISTIANSEN Laboratoriet (Denmark)
for Geofysik, Aarhus
Universitet,
Finlandsgade
6-8,
DK-8200
Aarhus N
Some available information on thermal conductivity of earth materials from the Scandinavian area is collected. The mean conductivities as reported