TOLKNING

In this account we describe GMAP, a combined palaeomagnetic database, geographic mapping and palaeoreconstruction package. GMAP is written for IBM AT compatible computers and the package contains approximately 1 Mbyte of geographic data, i.e. continental grid data, political boundaries and magnetic anomalies from the Central and Northern Atlantic. Large palaeomagnetic data- bases from Scandinavia, the British Isles and th USSR are also included. In addition selected palaeomagnetic data from Spitsbergen, Central Europe and North America/Greenland are included. The palaeomagnetic data and files containing information on finite rotation poles are used for palaeorecon- structions, but GMAP also allows interactive translation of continents.

Result of a helicopter geophysical survey in the Meråker area have been inter- preted with the goal of locating mineralization. A comparison of geophysical data over known mines and showings has determined that mineralization is correlated with 70 ppm or greater EM anomalies in the high frequency channel. Associated magnetic anomalies are either weak (under 25 nT) or non-exists. Results indicate that two zones are favourable for mineralization. One is along the contact between Sulåmogruppen greenstones and meta-sandstones and the second is along an axis connecting Dudu gruve with Langsund gruve and extending north to Hammerskallen skjerp.

En foreløpig tolkning av aeromagnetiske og gravimetriske data på sokkelen utenfor Troms mellom 69o30,N og 71oN presenteres. Det er fokusert på å finne et estimat av dyp til magnetisk basement og forkastninger i basement. I til- legg er det utført aeromagnetisk og gravimetrisk modellering langs profiler. Harstadbassenget kan være adskilt i en sørlig og en nordlig del. Det er mulig at den proterozoiske Bottenvika-Senjaforkastningssonen går i NV-SØ retning under bassenget. Dyp til magnetisk basement sør og nord for denne mulige for- kastningen er henholdsvis ca. 3 km og ned mot 10 km. Mot øst avgrenses Harstad bassenget av Troms-Finnmarkforkastningskomplekset i form av forkastnings- blokker i basement på 5-9 km dyp. Disse blokkene kan være roterte. Det finnes visse likhetstrekk mellom dette forkastninskomplekset og Vestfjorden-Vanna- forkastningskomplekset. Fra gravimetriske data kan opptil 6 km mektige sedi- menter tolkes på Finnmarkplattformen. En mulig gravimetrisk kanteeffekt er observert sørover fra 17oØ, 71oN i overgangen mellom kontinentalskråningen og kontinentalsokkelen. Denne anomalien kan være overlagret en anomali som representerer en sørlig fortsettelse av Senjaryggen. Ved videre undersøkel- ser anbefales det å utføre regionalfeltseparasjon for både Bouguer- og fri- luftsdata, samt å intensivere prøvetaking av bergartsprøver på land slik at modelleringen på kontinentalsokkelen kan forbedres.

IMP is an interactive, user-friendly, and pseudo 3D modelling program for magnetic and gravity data, and it forms a key-element in the IMP system developed over a total of 4 years. IMP is a forward modelling program which handles all the basic routines normally associated with such systems. In addition, the program provide statistical routines and on-line information on the characteristics of the local Earth's magnetic field (Norway). IMP can further display geographic maps(vector data) and/or magnetic/gravity grid images together with profile-lines and geological bodies in plane view (XY plane). Pdepth, an inversion program to calculate depth to magnetic basement can also run inside IMP, hence withnessing the integrated nature of the IMP system which brings it ahead of other systems. A gravity inversion program is currently also under development to run inside IMP.

PDEPTH is a program for automatic calculation of depth to magnetic basement based on a total magnetic field or anomaly profile, and PDEPTH forms an inte- grated part of the IMP system developed by T.H.Torsvik. PDEPTH utilizes Phillips method (1975,1978,1979) for calculating magnetic basement depths and is adapted from an original mainframe Fortran program written by Phillips (ADEPTH). Conversely, PDEPTH is written for IBM compatible computers. A program named IMDEPTH is also available; this program however handles grid-data for automatic scanning and calculation of magnetic basement depths. This report describes how to use PDEPTH and outlines the basic software theory.

IMPPROF is an interactive database system for storage and manipulation of magnetic and gravity profiles. Profiles can be displayed together with gridded image files and/or geographic vector data. The system principally aims to extract and manipulate profiles interactively on the screen. A selected profile can subsequently be exported for pseudo-3D forward modelling and/or interactively subjected to magnetic or gravity inversion.

An interpretation of aeromagnetic, gravimetric and petrophysical data from the Norwegian Sea between 67°N and 71°N is reported. Most of the magnetic and gravity anomalies within the survey area may be interpreted in terms of a fault-related basement relief. Depth to basement estimates based on gravity and aeromagnetic data are 5 km and 10 km, respectively, in the Ribban Basin. The discrepancy between the two estimates is most likely caused by downfaulted Caledonian Nappes, Devonian sediments or amphibolite-facies gneisses. The gravity modelling indicates that the Lofoten - Vesterålen area constitutes a basement high within the rifted and thinned crust of the continental shelf, similar to the basement highs within the Vøring Basin to the South. The depth to the basement is calculated to c. 10 km in northern part of the Træna Basin and c. 9 km in the Røst Basin. A correlation between Jurassic-Cretaceous faulting and underlying basement topography indicates that the deep structural setting may have guided the location of the later faulting. Within the Utrøst Ridge - Ribban Basin the basement map shows a complex pattern with several up- lifted and rotated basement blocks. The Vestfjorden Basin seems to consist of two sub-basins or half grabens; one to the north with a boundary fault to the east and one to the south with a boundary fault to the west. To the west of the Bodø High, and northwards along the shelf edge, there is a continuous zone of high frequency anomalies interpreted as the eastward termination of intrasedi- mentary volcanic rocks. The aeromagnetic and gravity data indicate that the Precambrian rocks in the Lofoten-Vesterålen area, despite Mesozoic reactivation along possible Caledonian zones of weakness, are situated within the margin of the Baltic Shield. They probably constitute a deep section of the Transscandi- navian Granite-Porphyry Belt.

An integrated study of geophysical data from the offshore and onshore areas of northern Norway between 67° and 71°N was carried out as a collaboration project between Elf Petroleum, Norsk Hydro, The Norwegian Petroleum Direc- torate, Statoil and the Geological survey of Norway. The interpretation of aeromagnetic, gravimetric and petrophysical data was integrated with mapping and palaeomagnetic dating of post-Caledonian faults on land.

Ved hjelp av gravimetriske data er det gjort en beregning av tykkelsen på dekkebergartene i området Tysfjord - Ofoten - Harstad. Den gravimetriske modelleringen er gjort langs 11 øst-vest gående profiler over Narvikdekke- komplekset og de utenforliggende grunnfjellsområdene. Beregningen viser at dekkebergartene danner et trau som er opptil 7 km dypt. Kanten faller brat- test på vestsiden, mer slakt i øst. De største dyp er i Ballangenområdet. Resultatene av undersøkelsen er vist som profilsnitt og som dybdekotekart sammenstilt med geologisk kart. Datagrunnlaget fra undersøkelsen er et gravimetrisk residualanomalikart som er basert på tyngdemålinger utført av NGU og andre i det aktuelle området over mange år, samlet i NGU's database for gravimetri, og petrofysiske data (bergartenes densitet) fra NGU's database for petrofysikk. Aeromagnetiske anomalikart er også framstilt over området til denne undersøkelsen. Resultatet av en gravimetrisk undersøkelse som ble gjort i 1970-årene i Ballangen i forbindelse med malmundersøkelser i Råna og som ikke tidligere er gjort offentlig tilgjengelig i rapportform, er her tatt med som bilag.

An interprtation of the Nordland Aeromagntic Survey (NAS-94) in the Norwegian Sea between from 64° 30' and 68°N is reported. The NAS-94 survey has revealed several sets of high frequency anomalies not discernible in the old NGU data sets. This is due to the use of more sensitive magnetometers, closer line spacing, more accurate navigation and improved processing. When compared with petrophysical measurements on cores from IKU's shallow drilling programme, the anomaly pattern is interpreted to represent three types of magnetic sour- ces; 1) Quaternary overburden of varying thickness and bathymetric features, 2) buried magnetic sedimentary rocks, subcropping sedimentary units and pos- sible magnetic fault zones, 3) magnetic basement in the Lofoten Horst, Nord- land Ridge, southern part of the Trøndelag Platform and mainland Norway.