MODELLFORSØK

Vestlig litteratur om emnet er gjennomgått. Et modellsystem for IP er utviklet, tildels basert på eksisterende komponenter. Målinger med pulstransientmetoden foretas i en stor elektrolytt-tank, hvor malmlegemer simuleres ved ledende materiale påsprøytet et spesielt grafittbelegg. Målingene kan utføres med akseptabel nøyaktighet og reproduserbarhet, og for enkelte geometrier får man god overenstemmelse med teoretiske modeller. Systemet kan brukes for vilkårlige geometrier til å simulere strukturer i jordskorpen.

Det ble gjort gravimetri for å kunne beregne størrelsen på en pegmatittkropp som var omgitt av amfibolitt og glimmerskifer. Det ble målt tre profiler over kroppen. Ved hjelp av de målte anomalikurvene og modellberegninger har en funnet at pegmatittkroppen har et dyp (tykkelse) på ca. 300 m.

Denne undersøkelsen ble gjort for å se om gravimetri kunne bidra til å løse problemene med å kartlegge de store strukturene i området. Det ble målt langs alle veiene mellom Narvik og Æfjorden. Det viste seg at Rånamassivet gir en kraftig tyngdeanomali. Egenvekta for alle bergartstypene ble målt og geologiske modeller ble prøvd. En modell som gir samme anomali som den målte blir presentert.

User's manual for the finite element program TURAM is described. With the program we can calculate theoretical anomaly curves for real geological two- dimensional structures in the field of a line source (Turam cable). The program is user oriented and it generates the finite element grid auto- matically. It is based on a program for two-dimensional calculation of elec- tromagnetic fields in power engineering applications, and is written in Fortran IV for the computer Univac 1108 of the Norwegian Institute of Technology.

User's manual for the finite element program PLANE WAVE is described. With the program we can calculate theoretical anomaly curves for real geological two- dimensional structures in the field of a plane wave. The program contains three plane wave cases: TE and TM mode magnetotellurics and VLF. The program is user oriented and it generates the finite element grid auto- matically. It is based on a program for two-dimensional calculation of elec- tromagnetic fields in power engineering applications, and is written in Fortran IV for the computer Univac 1100/62 of the Norwegian Institute of Technology.

A program for layered model(one-dinmensional) interpretation of audiomagneto- telluric (AMT) measuremants is described. The program is based on least - squares fit and on hyperparabolic optimization. The essential feature of the optimization method is that up to four parameters can be optimized simul- taneously. The program uses nine fixed frequencies from 3700 to 8 Hz, but it can easily be changed for any other frequencies. The program is written in Fortran IV for the computer Hewlett-Packard 3000 of the Geological Survey of Norway.

A finite element program for two-dimensional modeling of Turam is described. The program is user oriented and has automatic grid generation. The output includes printout and plots of normalized flux dendities along a surface profile or in a bore hole. It is possible to choose between a plot of the total magnetic field or of the normalized magnetic field, i.e. of the ano- malous field. The program has been used to simulate three-dimensional models. This has been achieved by specification of the total current through the conductor in relation to the total current through the half space. By reducing the amount of current in the conductor, a reduction of the length of the conductor is simulated. Examples which demonstrate the effect on the anomaly curves are given. By reducing the gathered current (also called conductive current), the electromagnetically induced edge current will dominate the anomaly curves. A case history is also given.

A program for the Hewlett Packard-41 CV pocket calculator for calculating audiomagnetotelluric (AMT) apparent resictitivity curves for layered model is described. The program calculates the apparent resistivity at 10 predesti- nated frequencies ranging from 10 000 to 8 Hz.

User's manual for the finite element program TURAM is described. With the program we can calculate theoretical anomaly curves over real geological two- dimensional structures in the field of a line source of current for either the conventional Turam or the so-called Elfast version. The program is user oriented an uses automatic grid generation. It is based on a program for two dimensional calculation of electromagnetic fields in power engineering applications, and is written in Fortran IV for the computer Univac 1100/62 of the Norwegian Institute of Technology. This report replaces the earlier user's manual for the TURAM program (Report No. 1765/1).

A regional gravimetric survey has revealed a strong anomaly south of Kauto- keino. Later detailed measurements shows that the anomaly consists of N-S and E-W trending limbs. The anomaly coincides with a vulcano-sedimentary sequence, largely greenstone. The residual gravimetric anomaly has a maximum of 25 mgal. The greenstone consist of metadiabase, metabasalt, tuffite and highly conductive graphite schists. A quartz monzonite instrusion is situated on the border between the greenstones and the surrounding quartz-feldspathic gneisses. The gneisses are assumed to be the basement of the greenstones. The density contrast between the greenstones and the gneiss is 0,31 1o3 kg/m3. A interpretation of the gravity anomaly shows that the greenstone belt may have a maximum depth of 5,8 km. This thickening of the greenstones is thought to coincide in time with doming of the basement. The investigation was carried out by a team consisting of geophysicists, bedrock- and quaternary geologists in an area of exceedingly poor exposure.