GEOFYSIKK

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.