Directional EM-measurements in boreholes

149

HIGH LEVEL GAMMA APPLICATIONS

SPECTROMETRY

-

CONSEQUENCES

AND

A.H. LINDlkN and H.O. MELLANDER

Geological

Survey of Sweden,

Box 670, 751 28 Uppsala (Sweden)

Airborne gamma spectrometers are traditionally calibrated over artificial pads with known contents of potassium, uranium and thorium. Usually the calibration procedure also includes multi-level test flights over a single strip with laboriously estimated ground values of the three radio-elements. Although sufficient in the normal case (i.e., window-technique), the method fails to give full knowledge of the shape of the entire gamma spectrum as a function of flight altitude and varying ground-contents. This paper presents one possible method to solve the problem. Three separate test-strips were chosen, each one dominated by one of the three elements. Multi-level test flights were carried out during October 1980. A special TV-computer technique separated spectra from potassium, uranium and thorium. The method provides the shape of the gamma spectra for different altitudes up to 240 m. It facilitates the use of a much larger part of the spectrum, thereby increasing the precision, and opens new applications, among them snow-water equivalent measurements and detection of artificial gamma-sources.

UPPERMOST CRUSTAL PROFILE, NORTHERN

STRUCTURE ALONG THE BLUE ROAD SWEDEN, FROM SURFACE WAVE ANALYSIS

C.-E. LUND and K. _&STRoM

University (Sweden)

of Uppsala, Department

of Solid Earth Physics, Box 556, S-751 22 Uppsala

Short period surface waves recorded along the Blue Road Profile have been used for the determination of velocity--depth models along the profile, for the uppermost 2-3 km of the crust. Calculated dispersion curves have been brought into agreement with measured dispersion data by changing the model parameters. The obtained velocity--depth models agree with results obtained by refraction data from the same profile.

DIRECTIONAL LENNART

Boliden

EM-MEASUREMENTS

MALMQVIST

and ROBERT

Mineral AB, S-936 00 Boliden

IN BOREHOLES

PANTZE

(Sweden)

The objective of geophysical exploration direction and distance to promising targets.

from boreholes is to indicate This three-dimensional informa-

150

tion about the surrounding of the borehole must be collected in the onedimensional hole. Generally borehole EM equipments only have the capabilities to measure the EM field parallel to the hole. To improve the possibilities for 3 D interpretation of the surrounding of an exploration hole a 3-component EM-probe has been developed and tested by Boliden. The probe measures the EM-field parallel to the hole and the horizontal field perpendicular to the hole. The third component is perpendicular to the two other components. The three components create a right-hand system. The amplitude and phase are recorded for each one of the components. Phase reference is a separate loop which can be arbitrarily placed on the ground. The advantage of a continuous wave system is the fact that an efficient noise reduction can be made. This makes measurements in mines possible also when mining operations take place. A test program has indicated a considerable potential for this system when combined with a large loop transmitter at the ground. The potential range for the detection of a large conducting target is at least 75 m. Geophysical borehole measurements thereby have become an important part of any exploration program for massive sulphides.

FOURIER AMPLITUDE SPECTRA OF AEROMAGNETIC FIELDS: A STRUCTURE ANALYSIS OF THE MID AND NORTH PART OF NORWAY AND THE CONTINENTAL SHELF HANS

P. MOXNES

Geological

Survey of Norway,

P.O. Box 3006, N-7001

Trondheim

(Norway)

Depth to the basement and the Curie point isotherm is estimated by white depth. There is a positive correlation between this depth and the results from different geophysical surveys in the said areas.

THE NEW AEROGEOPHYSICAL SURVEY OF FINLAND JUKKA

MULTALA

and JOUKO

EQUIPMENT

OF THE GEOLOGICAL

VIRONMAKI

Geological Survey of Finland, Department Espoo 15 (Finland)

of Geophysics,

Kivimiehentie

1, SF-02150

In 1979, a renewal became necessary in the aerogeophysical surveying equipment of the Geological Survey of Finland. The main reasons for the renewal were the increasing difficulties in obtaining spare parts and fuel for the old DC-3 aircraft, and the rapid development in performance, weight and reliability of electronic instruments during the seventies. The new installation in a DHC-6 Twin Otter, STOL aircraft was designed and built in 1979-60,