In many cases, inversion in 2D gives a better description of the subsurface compared with 1D inversion, but, computationally, 2D inversion is expensive, and it can be hard to use for large-scale surveys. We have developed an efficient hybrid 2D airborne frequency-domain electromagnetic inversion algorithm. Our hybrid scheme combines 1D and 2D inversions in a three-stage process, in which each step is progressively more accurate and computationally more expensive than the previous one. This results in an approximately [Formula: see text] speedup compared with full 2D inversions, and with only minor changes to the inversion results. Our inversion structure is based on a regular grid, in which each sounding is discretized individually. The 1D modeling code uses layered models with derivatives derived through the finite-difference method, whereas our 2D modeling code uses an adaptive finite-element mesh, and it uses the adjoint-state method to calculate the derivatives. By incorporating the inversion grid structure into the 2D finite-element mesh, interpolation between the different meshes becomes trivial. Large surveys are handled by using local meshing to split large surveys into small sections, which retains the 2D information. The algorithm is heavily optimized and parallelized over the frequencies and sections, with good scalability even on nonuniform memory architecture systems, on which it is generally hard to achieve a satisfactory scaling. The algorithm has been tested successfully with various synthetic studies as well as field examples, of which results from two synthetic studies and a field example are shown.
In a majority of geotechnical projects and in geohazard studies, knowledge of sediment thickness is crucial, along with information about sediment types such as possible occurrence of sensitive clay. We test the recently improved system response (SR) method on data from an airborne electromagnetic (AEM) survey carried out in Norway to support ground investigations for linear infrastructure projects. In geotechnical projects, especially the upper metres are important to resolve. The acquisition systems, calibration and data processing and inversion are continuously improved to increase the sensitivity of the AEM systems. SR is applied in the inversion of time-domain AEM data making it possible to utilise the very earliest time gates, which provide information about the shallower layers, aiming to increase the near-surface resolution of the models. We test this new method on data from a site where comparably small resistivity contrasts (5–10 Ωm embedded in 10–50 Ωm) are crucial to resolve, to successfully identify hazardous quick clay. The AEM field data inverted with the full SR provide models with more pronounced structures in the near surface, better reflecting true structures observed in resistivity borehole measurements. We see the same outcome when conducting synthetic modelling. In the synthetic models where these early gates were included, thinner layers with smaller resistivity contrasts are resolved. This is a promising result considering to further use AEM in projects where high resolution in the near surface is essential.
The inner Aurland fjord and the adjacent Flåm valley (Western Norway) are subject to potential rock slides comprised of creeping rock- and debris masses. Based on indications that precipitation drives the sliding movements, the local municipality and regional hydroelectricity company are evaluating the option to drain the unstable area with a more than 10 km long drainage tunnel to a nearby hydropower reservoir. Both rockslides and tunnel corridor encounter phyllite, a low grade metamorphic rock type that is potentially reworked to clay in disturbed zones. Water saturated clay is a strong conductor and thus an ideal target for an electromagnetic (EM) survey. We conducted an airborne EM mapping survey to find indications for the sliding planes and to assess the tunnel corridor for potential tunnelling hazard areas. Spatially constrained inversion of the 250 line km data set reveals extended conductive zones that we interpret as sliding planes and/or gneiss / phyllite interface. Detailed follow up of initial results is planed with limited percussion drilling and ground resistivity surveys.
Summary The aim of our study is to assess whether resistivity may be transformable into an engineering parameter such as the Q-value and can substitute seismic investigation when not practicable. ERT has the advantage of being efficient, reliable, silent and non-destructive, therefore suitable in urban areas. We have integrated Q-values derived from core drillings and geological logging during tunnelling and resistivity derived from surface ERT on two tunnel projects in Norway. In both cases, it is qualitatively evident that lower resistivity points towards weaker rock. Indeed, preliminary results can partially show the expected correlation where resistivity data co-align with boreholes or the tunnel.
Oslo municipality is presently planning bus and car tunnels to facilitate its accessibility and increase traffic efficiency. Urban environment is usually a challenge for geophysical pre-investigations because of the various sources of noise, vibrations and restriction both in time and space. These technical challenges were overcome with the use of a newly developed seismic streamer specifically designed for noisy urban areas, from an industry-academia partnership. A total of 3.5 km long seismic data along 14 profiles were acquired for the tunnels pre-investigation with the main goals of (1) obtaining information about depth to bedrock, (2) detecting potential weakness zones, and (3) optimizing the number of drillings and their locations for a follow-up study. In addition, six electrical resistivity tomography profiles were acquired near the planned tunnel alignments. Inversion of first breaks and electrical resistivity data provides a seamless depth to bedrock interface that is in most places in good agreement with the nearby geotechnical soundings. In addition, the geophysical sections reveal the bedrock undulation character and provide some indication of weakness zones. This case study also illustrates that if the pre-investigation had been based only on boreholes, it would have overseen a potential difficulty during excavation.
Quick clay may be described as highly sensitive marine clay that changes from a relatively stiff condition to a liquid mass when disturbed. Extended quick clay layers account for a lot of geo-hazards in Scandinavia and North-America and hence their occurrence and extent need to be mapped. Geophysical methods have been tested for small scale quick-clay mapping at a research site (Vålen) close to Oslo, Norway. By scrutinizing results from Electric Resistivity Tomography (ERT) and Multi-channel Analysis of Surface Waves (MASW) and integrating them with geotechnical borehole data with the help of a resistivity logging tool (RCPTu), we confirm the value for such integrated studies in for quick-clay hazard zonation. Geophysical investigations allow indeed interpolation in between limited borehole results and thus provide a more cost-efficient and extended result than with boreholes alone.
The inner Aurland fjord and the adjacent Flam valley (Western Norway) are subject to a potential rock slide comprised of creeping rockand debris masses. From repeated GPS measurements we understand that rock and debris movements are constrained by precipitation and snow melt. Based on this assumption the local municipality and regional hydroelectricity company are evaluating the option to drain the unstable area with a more than 10 km long drainage tunnel to a nearby hydropower reservoir. We conducted an airborne electromagnetic (AEM) mapping survey to find indications for the sliding planes and to assess the tunnel corridor for potential tunneling hazard areas.
The Inhaminga hydrocarbon exploration licence in central Mozambique sets the location for a multi-method airborne geophysical survey. The size of the Inhaminga block, spanning some 16 500 km2 from Beira to the Zambezi, limited available data and a tight exploration schedule made an airborne survey attractive for the exploration portfolio. The aim of the survey was to map hydrocarbon seepage zones based on the evidence that seepage may create resistivity, radiometric and sometimes magnetic anomalies. The survey involved a helicopter-borne time domain electromagnetic induction system (AEM) and a fixed wing magnetic gradiometer and radiometer.Our data analysis highlights an anomaly extending some tens of kilometres through the survey area along the eastern margin of the Urema Graben. The area is imaged by AEM as a shallow resistive unit below a strong surface conductor and shows high Uranium and low Potassium concentrations (normalised to mean Thorium ratios). A seismic dimming zone on a 2D seismic line crossing the area coincides with the resistivity and radiometric anomaly. The geological exploration model expects seepage to be linked to the graben fault systems and an active seep has been sampled close to the anomaly. We thus interpret this anomaly to be associated with a gas seepage zone. Further geological ground work and seismic investigations are planned to assess this lead.Airborne data has further improved the general understanding of the regional geology allowing spatial mapping of faults and other features from 2D seismic lines crossing the survey area.
