GEOFYSIKK

Rapporten er ment som et kompendium for undervisning ved NTNU, men også ved andre universiteter i
Norge. Andre interesserte inviteres også til å studere muligheter og begrensninger ved elektriske
målinger. Det gis en detaljert oversikt over:
• Ord og uttrykk
• Grunnleggende teori inklusive materialegenskaper
• Faktorer som påvirker elektriske målinger
• Vertikale elektriske sonderinger (VES), datainnsamling, inversjon og tolking
• 2D resistivitet (ERT), datainnsamling, inversjon og tolking
• Muligheter og begrensninger ved 2D resistivitet (Modellering)
• Selv Potensial (SP), datainnsamling, inversjon og tolking
• Indusert Polarisasjon (IP), datainnsamling, inversjon og tolking
• Oppladet potensial (CP), datainnsamling, inversjon og tolking
• Elektriske borehullsmålinger, ERT, SP, IP og CP
Eksempler på gode resultater innenfor:
• Forundersøkelser for tunneler
• Kartlegging og karakterisering av leire
• Kartlegging av grunnvann og grunnvannsforurensing
• Kartlegging av sand- og grusforekomster
• Kartlegging av ustabile fjellparti
• Kartlegging av permafrost
• Resistivitet i kombinasjon med andre metode

The Stuoragurra Fault Complex (SFC) constitutes the Norwegian part of the larger Lapland Province of postglacial faults in northern Fennoscandia. The 90 km long SFC consists of two separate fault systems: the Máze Fault (revised) System in the southwestern area and the Iešjávri Fault System to the northeast. The distance between the fault systems is 12 km. The faults dip at an angle of 30–75° to the SE. Here we present data from trenching of different fault sections. The trenching reveals deformed overburden in all 11 sites and inclusions of peat and organic bearing soil in the deformed and partly overrun loose deposits on the footwall in most of the sites. Radiocarbon dating of macro remains of plants located in buried and severely deformed sediment horizons indicates late Holocene ages for the (final) formation of the different fault segments, more specifically that the faulting at Fitnajohka and Máze in the Máze Fault System formed during earthquakes younger than c. 470-500 years ago.

NGU har utført undersøkelser med bakkegeofysikk i Kristiansand kommune for å følge opp enkelte områder som kan inneholde jarositt. Valgt metodikk er ERT, en metode som NGU har god erfaring med for å påvise forvitret fjell. Basert på områder hvor det påvist jarositt ble det valgt ut fire områder for videre oppfølging. Disse ble valgt basert på resultatet fra prøvetaking gjort i tidligere undersøkelser. Områdene ble i tillegg sammenliknet med NGU sitt AMAGER-kart for området. Dette er et produkt av topografi og magnetiske data hvor resultatet er et aktsomhetskart som viser potensielle områder med oppsprukket fjell eller forvitring.

NGU conducted an airborne geophysical survey in Forollhogna area, in Rennebu, Midtre
Gauldal and Holtålen municipalities in Trøndelag County, and Tynset, Tolga and Os
municipalities in Innlandet County, as part of NGU’s general airborne mapping program. The
data acquisition in Forollhogna area was carried out from June 23rd to July 20th, 2021.
This report describes and documents the acquisition, processing and visualization of the
acquired datasets and presents them in maps. The geophysical survey consists of 8214 linekm data, covering an area of 1609 km2, flown from the base near Orvos (Flight 1-38) and from
the base near Frigården (Flight 39).
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 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 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 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.

Near-surface geophysical applications employing multiple 2D methods that are mapping multiple properties along co-located profiles, enable new possibilities in exploring subsurface structures. Diverse data sets can now be cooperatively interpreted with the use of cluster analysis which is an unsupervised machine learning method that helps to explore patterns in data and separate them into groups based on similarity. In this report, we present our first attempt in testing this novel 2D method by applying cluster analysis on our previously published results from parts of the Knappe-tunnel.

The now completed 3.8 km long Knappe tunnel west of the city of Bergen is a site where multimethod-based exploration is available but also a test area where resulting clustering interpretations can be directly compared to bedrock quality estimations carried out after its completion. In this framework, resistivity and P-wave velocity values for co-located profiles were automatically classified with the use of Fuzzy C-means clustering method. Two independent implementations of this method were utilized: one in-house ran by NGU-researcher Ying Wang, and another provided and ran by Dr Beatriz Benjumea of the Geological Survey of Spain. Hence, a large variety of 2D results were produced, enabling inherently joint interpretation of possible weak zones, in addition to those already extracted directly from the ERT and Refraction Seismic inversion results. Through this procedure we were able to test the performance of the Fuzzy C-means method independently and evaluate how systematic clustering results are, coming from two different algorithms.

Our results indicate that we were not able to utilize cluster analysis in full in this first attempt due to limited knowledge concerning applicability of the method on vertical 2D geophysical profiling. However, new ideas and approaches were inspired that will be pursued in the future.