MAGNETISME

NGU conducted an airborne geophysical survey in Selbusjøen area, in Klæbu, Trondheim, Malvik and Selbu municipalities in Trøndelag County, as part of NGU’s general airborne mapping program. The data acquisition was carried out from August 10th to September 9th, 2023.
This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical survey consists of 2841 line-km data, covering an area of 591 km2, flown from the base at Vassfjellet ski centre. The last flight consisting of re-flown lines to improve the magnetic data was flown from the base in Okkenhaug near Levanger.
The NGU modified Geotech Ltd. Hummingbird frequency domain electromagnetic system supplemented by an optically pumped caesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter were used for data acquisition. The data collected were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and levelled using Geosoft micro-levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA). Electromagnetic data were filtered and levelled using both automated and manual levelling procedures. Apparent resistivities were calculated from in-phase and quadrature data for three coplanar frequencies (880 Hz, 6.6 kHz and 34 kHz), and for two coaxial frequencies (980 Hz and 7 kHz) separately using a homogeneous half-space model. All data were gridded using cell size of 50x50 meters and presented as 40% transparent grids on top of topographic maps.

This report describes the work done for the 1-year collaboration project between NGU and Equinor:
Seafloor massive sulfide (SMS) geophysical model. The project aims to establish a synthetic model for
the purpose of optimizing subsea acquisition and detection limits for SMS bodies at the seafloor via
AUV (autonomous underwater vehicle) geophysical surveys.
At the first stage of the project, a 3D layered geo-model is built for the study purpose. We call this model
the SYM model. An ultra-slow spreading mid-ocean ridge from the North Atlantic has been referenced
to establish the geological setting for this synthetic model. The SYM model size is ~100 km x 150 km
laterally with 100 m resolution, and 10 km vertically with 500 m resolution. Real bathymetry is used from
a region between Mohns Ridge and Knipovich Ridge as the starting point of the model design, based on
which a sediment layer, the pillow basalt and gabbro layer, the Moho, together with an intra mantle layer
to simulate mantle variations and/or Curie depth, have been built into the model with varying thickness –
thinner at the ridge, thicker further away from the ridge. Last but not least, 17 basalt-hosted SMS bodies
and 1 ultramafic-hosted SMS body have been imbedded into the model at or near the seafloor.
For the second part of the project, the model was transferred into an indexed voxel model to facilitate an
easy change between different petrophysical model parameters. Subsequently, forward modelling of
gravity and magnetic potential fields (and their gradients) has been conducted schematically on the
synthetic model to determine the optimal survey design for AUV acquisition of geophysical data.
Conclusions drawn from our synthetic tests describe what should be expected in an ideal environment
using AUV surveys for SMS detection. In this project, we also discuss challenges in practice regarding
instrumentation limitation, positioning precision, survey efficiency considerations, processing errors, etc.
Possible extensions of the project are discussed. For example, including more types of geophysical
data – namely, SP (self-potential) and EM (electromagnetic) – in addition to the potential fields. The 3D
synthetic voxel model could be used for benchmarking a variety of theoretical algorithms for deep sea
geological understanding, such as Curie-depth determination, hydrothermal studies, seismic imaging,
multi-geophysical data modelling and integration

AS a part of the Base 2 project NGU has performed geophysical borehole logging in four boreholes at Smøla, Møre and Romsdal county and three at Bømlo, Vestland county. The purpose of the drilling and logging was to study onshore deep weathered basement rocks. Such rocks can be a potential oil reservoir offshore, on the continental shelf.
The logged parameters were resistivity, porosity, seismic velocity, water conductivity, total gamma radiation, spectral gamma, magnetic susceptibility, Induced Polarisation, and Self Potential. All boreholes were logged using acoustic and optical televiewer. Flow measurements and pumping were performed. Lugeon test were done by the drilling company.
The results show heavily fractured rock in all boreholes. Especially in Bh4 at Smøla the resistivity was very low. Combined with low p-wave velocity and fracturing it can be interpreted as weathered and altered rock. IP anomalies may indicate clay.
The Lugeon tests showed quite low values and correlates well with the fracture frequency. From the Lugeon numbers the hydraulic conductivity and permeability are calculated. Calculated apparent porosity from the resistivity log, using Archi's law, is in the same range as laboratory porosity measurements on cores performed by NTNU. Most of the porosity values are in the range of 1 – 4 %.
Flow measurements showed no vertical flow in the boreholes and no in/out flow from fractures. Pumping indicated very low water capacity in all wells except Bh3, Bømlo. Near surface water just below the casing flowed into the borehole.
Bh2 at Bømlo was stuck at 27 m depth. Reopening of the borehole failed, and masses (sand, clay) flowed into the borehole and blocked it.

NGU conducted an airborne geophysical survey in Røros, Trøndelag county as part of
NGU’s general airborne mapping program in 2020.
This report describes and documents the acquisition, processing and visualization of the
acquired datasets and presents them in maps. The geophysical surveys consist of 2584 linekm data, covering an area of 523 km2 flown between June 15th to June 22nd, 2020.
The NGU modified Geotech Ltd. Hummingbird frequency domain EM system supplemented
by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels
RSX-5 spectrometer mounted on a AS350-B3 helicopter was used for data acquisition.
The survey was flown with 200 meters line spacing, azimuth 90o
. Average speed was
95 km/h, and average height clearance of the EM bird was 47 m and 77 m for the
spectrometer.
Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total
magnetic field data were corrected for diurnal variation and levelled using standard levelling
algorithm. Radiometric data were processed using standard procedures recommended by the
International Atomic Energy Association (IAEA).
EM data were filtered and levelled using both automated and manual levelling procedures.
Apparent resistivity was calculated from in-phase and quadrature data for three coplanar
frequencies (880 Hz, 6.6 kHz and 34 kHz), and for two coaxial frequencies (980 Hz and 7 kHz)
separately using a homogeneous half space starting model of 1000 ohm-m.
All data were gridded using cell size of 50x50 meters. Magnetic and radiometric data is
presented as 40% transparent grids with shaded relief on top of topographic maps. EM data is
presented as 30% transparent grids on top of topographic maps.

In the frame of the GREENPEG project NGU and terratec Geophysical Services have compiled a petrophysical database
of European pegmatite ores and wall rocks from so far four different sites with different types of pegmatites in different
settings.
With the aim to develop toolsets for hidden pegmatite exploration, the GREENPEG project wants to overcome the lack
of exploration technologies for pegmatite ore deposits. Here, a comprehensive petrophysical database contributes with
knowledge on petrophysical contrast between pegmatites and wall rocks facilitating a right choice of geophysical
methodologies and a more efficient exploration.
Petrophysical measurements were therefore performed on hand specimens, drill cores and in selected boreholes at two
of the three project demonstration sites Tysfjord (Norway), Wolfsberg (Austria) and Leinster (Ireland). In addition, hand
specimens from a pegmatite test site in Portugal/Spain were included as well.
The measurements on specimens were carried out at the NGU petrophysical laboratory, acquiring density, susceptibility,
remanence, thermal conductivity and radiometric isotopes content for U, Th and K.
Specimens represent samples from both pegmatites and wall rock to map potential contrasts in the petrophysical
parameters between these two lithologies as a basis for geophysical exploration.
Within the GREENPEG project two wireline downhole geophysical logging campaigns were performed and
measurements done in several boreholes at the demonstration sites Leinster and Wolfsberg. The borehole probes
measured with a sample interval of 1 to 10 cm over the entire depth of the borehole and therefore provide a highresolution
in-situ data set of the rock formation.
The database is uploaded and available on the open data repository Zenodo (www.zenodo.com) under a Creative
Commons Attribution 4.0 International (CC BY 4.0) license. This repository is used by the GREENPEG project to give
access to data collected and acquired by the project.

This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical survey consists of 8500 line-km data, covering an area of 1700 km2, with 4600 km (920 km2) flown in June from Stiklestad, Verdal, and 3900 km (780 km2) flown in August from the base in Gaulstad in Ogndalen.

NGU’s modified Geotech Ltd. Hummingbird frequency domain EM system supplemented by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter was used for data acquisition.

The survey was flown with 200 meters line spacing, azimuth 100o, average speed was 116 km/h, average height clearance of the bird was 55m and 85m for the spectrometer.

Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and leveled using Geosoft micro-levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA).

EM data were filtered and leveled using both automated and manual levelling procedures. Apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880Hz, 6.6kHz and 34kHz), and for two coaxial frequencies (980Hz and 7kHz) separately using a homogeneous half space model.

All data were gridded using cell size of 50x50 meters and presented as 40% transparent grids with shaded relief on top of topographic maps.

NGU conducted an airborne geophysical survey in Øksfjord area, in Kvænangen, Loppa og Alta municipalities in Troms and Finnmark, as part of NGU’s general airborne mapping program. The data acquisition in the Øksfjord area was carried out from 20th to 31st of August 2019. Two flights from the same area, flown September 1st, 2014, were also included in the data set for processing and presentation.

This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2155 line-km data, covering an area of 813 km2, with 245 km (49km2) flown in 2014 from Ystneset, near Øksfjord community center, and 1910 km (764 km2) flown in 2019 from Alteidet.

NGU’s modified Geotech Ltd. Hummingbird frequency domain EM system supplemented by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter were used for data acquisition.

Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and leveled using a micro-levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA).

EM data were filtered and leveled using both automated and manual levelling procedures. Apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880Hz, 6.6kHz and 34kHz), and for two coaxial frequencies (980Hz and 7kHz) separately using a homogeneous half space model.

All data were gridded using cell size of 100x100 meters and presented as 40% transparent grids with shaded relief on top of topographic maps.

NGU conducted an airborne geophysical survey in Stjørdal, Selbu, Malvik and Meråker municipalities, as part of NGU’s general airborne mapping program.
This report describes and documents the acquisition, processing and visualization of the acquired datasets and presents them in maps. The geophysical surveys consist of 2655 line-km data, covering an area of 535 km2 flown on July 21st to 29th 2021 and August 30th to September 6th 2021.
The NGU modified Geotech Ltd. Hummingbird frequency domain electromagnetic system supplemented by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels RSX-5 spectrometer mounted on a AS350-B3 helicopter was used for data acquisition.
The survey was flown with 200 meters line spacing, azimuth 100o. The average speed was 106 km/h, and the average height clearance of the bird was 45.7 m for the magnetometer and electromagnetic probe and 75.7 m for the spectrometer.
Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total magnetic field data were corrected for diurnal variation and levelled using Geosoft micro-levelling algorithm. Radiometric data were processed using standard procedures as recommended by International Atomic Energy Association (IAEA).
The electromagnetic data were filtered and levelled using both automated and manual levelling procedures. The apparent resistivity was calculated from in-phase and quadrature data for three coplanar frequencies (880 Hz, 6600 Hz and 34133 Hz), and for two coaxial frequencies (980 Hz and 7000 Hz).
All data were gridded using cell size of 50 m x 50 m and presented as 40% transparent grids with shaded relief on top of topographic maps. The electromagnetic data are presented as 30% transparent grids on top of topographic maps.

NGU conducted an airborne geophysical survey in Hemnes and Hattfjelldal municipalities,
as part of NGU’s general airborne mapping program. The data acquisition in the Okstindan
area was started and completed in August 2021.
This report describes and documents the acquisition, processing and visualization of the
acquired datasets and presents them in maps. The geophysical surveys consist of 2039 linekm data, covering an area of 394.5 km2
flown from the base at Vesterli Skistadion in Korgen,
Hemnes
The NGU modified Geotech Ltd. Hummingbird frequency domain EM system supplemented
by an optically pumped Cesium magnetometer and the Radiation Solutions 1024 channels
RSX-5 spectrometer mounted on a AS350-B3 helicopter was used for data acquisition.
The survey was flown with 200 meters line spacing, azimuth 90o
, average speed was 90
km/h, average height clearance of the bird was 65 m and 93 m for the spectrometer.
Collected data were processed at NGU using Geosoft Oasis Montaj software. Raw total
magnetic field data were corrected for diurnal variation and leveled using Geosoft microlevelling algorithm. Radiometric data were processed using standard procedures as
recommended by International Atomic Energy Association (IAEA).
EM data were filtered and leveled using both automated and manual levelling procedures.
Apparent resistivity was calculated from in-phase and quadrature data for three coplanar
frequencies (880Hz, 6.6kHz and 34kHz), and for two coaxial frequencies (980Hz and 7kHz)
separately using a homogeneous half space model.
All data were gridded using cell size of 50x50 meters and presented as 40% transparent
grids with shaded relief on top of topographic maps

This report describes data processing and interpretation of magnetic and gravity data acquired in Brattbakken area, Verdal, Trøndelag county. The survey was carried out in September 2020. This work is part of the project named NIKKEL I TRØNDELAG, a project aiming at understanding the lateral and vertical extents of the nickel deposits in Trøndelag region.
The interpretation was carried out by incorporating 3D VOXI inversion of magnetic data and 2D forward modelling of the magnetic and gravity data. The areal geology indicates the presence of Gula group dominating the survey area. The group is comprised of Gula schist intercalated with Gula amphibolite and banded iron formation (BIF). The Gula group, namely the amphibolite is intruded by gabbro and pyroxenite. The south-eastern part of the survey area is covered by Funnsjø greenstone.
The results from 3D inversion and 2D modelling suggest the intrusions to be about 215 – 260 m wide and 90 -270 m thick.
The ore zone within the pyroxenite intrusion was further examined. An inversion with denser grid cell size was carried out. The interpreted result suggests the identified ore prospect to be between 20 - 30 m thick with susceptibility of 0.12 SI. However, there is a potential for significant larger thickness and up to 145 m with varying susceptibility from 0.06 SI to 0.12 SI across the prospect area. The variation of susceptibility values could indicate varying ore grades or that some anomalies are caused by other high magnetic sources.