The tensor radio-magnetotelluric (RMT) method has extensively been used in near-surface investigations to obtain resistivity models of the subsurface. The main objective of this thesis is to further develop the RMT survey technique for a less paid attention and challenging environment namely on shallow water bodies and in the urban environment. The other objective is to develop a new processing technique to enhance the resolution and sensitivity of the tensor RMT method. For the first time a data acquisition system called ‘boat-towed RMT’ is introduced that has the capability to measure tensor RMT data on water bodies like lakes and rivers. A RMT survey carried out on Lake Mälaren near the city of Stockholm shows the capability and efficiency of the boat-towed RMT system. The resistivity models obtained from the RMT data are consistent from one line to another and show good correlation with the existing geological and drill core data. In general, a three-layer resistivity model was obtained that has a conductive layer interpreted as lake sediments, which is sandwiched between high resistive bedrock and resistive water column. A coherent discontinuity of low resistivity zone was observed in bedrock across all the lines. It was interpreted to originate from a major fracture zone striking in the direction of water bodies. However, due to the lack of penetration, RMT method alone was insufficient to provide a conclusive interpretation of this. Synthetic analysis was performed and showed that lower frequencies using controlled-sources are required to obtain the desired penetration depth. We took the advantage of the Swedish winters and carried out controlled-source RMT measurements on frozen lake at the same location. The new controlled-source models have enough depth penetration to delineate fractured bedrock. Furthermore, in order to improve the resolution and sensitivity of tensor RMT data, a new processing technique was developed that preserves the identity of each transmitter and allows improved resistivity model of the subsurface. These new acquisition and processing techniques should be useful in many different applications for urban infrastructure planning projects especially in Scandinavia where 7% of the land is covered by fresh water bodies and is poorly explored for these purposes.

digitalisering@umu.se

<p> The Taranaki Basin is the only known producing basin within New Zealand. Since the drilling of the first well in 1865, the Taranaki basin has remained relatively underexplored. The Arawa-1 well was drilled in 1992 using 2D seismic lines as a control. New Zealand has started an exploration initiative by publicly releasing all geological and geophysical information gathered on and offshore New Zealand. The gathered information includes the Parihaka 3D survey, which directly overlaps with the Arawa-1 well and original 2D lines. This study focused on the Miocene-Pliocene Mangaa Formation, which exhibited reservoir quality within the Arawa-1 well. Seismic attributes have been used to locate an area of interest within the Mangaa Formation. A Coherence attribute was useful for identifying geomorphological features as well as faults. An average energy volume was used to emphasize brighter amplitudes from background signatures and to define lateral boundaries of the reservoir. Upon mapping an area of interest within the Mangaa Formation, the amplitude anomalies were conformable to structural highs. Results were compared to an analog well, Karewa-1, where amplitude anomalies were relatively identical. Amplitude versus offset analysis was conducted for the amplitude anomaly within the Mangaa Formation and found a class 4 anomaly. The interpreter performed fluid replacement modeling with the assumption of 100% gas, derived from the analog, Karewa-1. The interpreter compared the resulting model to the observed trends inside and outside of the amplitude anomaly. The gas model signature resembled that of the amplitudes inside of the amplitude anomaly, and the amplitude signature of the original water saturation resembled that of the amplitudes outside of the anomaly. The results allow the interpreter to use the correlation of amplitude signatures and fluids in place to assist in de-risking prospect potential. </p><p>

We combine the principles of sequence stratigraphy and syntectonic (growth) stratigraphy to assess deformation of the central Los Angeles (LA) basin from the late Pleistocene to the present. Sequence stratigraphy provides temporally correlative horizons that define the activity and structural kinematics of folds and underlying blind-thrust faults. Using these insights, we demonstrate that the slip rate on the western segment of the Puente Hills blind-thrust fault system (PHT), which lies directly beneath downtown Los Angeles, has accelerated from the late Pleistocene through the Holocene. This increase in slip rate implies that the magnitudes and/or the frequency of earthquakes on this fault segment have increased over time, challenging the characteristic earthquake model and presenting an evolving and potentially increasing seismic hazard to metropolitan Los Angeles. To assess the slip rate on the LA segment probabilistically, we developed a new method for estimating uncertainty in the true depths of interpreted geologic features from seismic reflection data. We achieved this by simulating the effects of varying the order and proportionality of interval velocities using an autoregression model based on nearby wellbore velocities. We further assessed the impact of resolution uncertainty on the true depth of interpreted geological features. Combined with age uncertainties for geologic horizons, this approach yields robust assessments of the slip rates on blind-thrust faults. Finally, we map temporally correlative sequence boundaries across the Los Angeles basin from the late Pleistocene to present. This enables us to determine changes in accommodation space in time and from them infer changes in deformation. Our results show persistent deepening in the central trough of the Los Angeles basin, activity of the Compton and PHT faults, and lateral growth of the LA segment of the PHT. / Earth and Planetary Sciences

This thesis presents applications of geophysical methods in the fields of environment and hydrogeology. In relation to environmental problems, two different geophysical surveys have been carried out; one to study an arsenic contaminated area in the southern Thailand (paper I) and a second to locate shallow faults in Quaternary sediments in an area around the Ongkharak Nuclear Research Centre, central Thailand (paper II). For hydro-geological problems, surveys were conducted in southern Sweden (paper III) and in the Vientiane basin, Laos (paper IV). In the arsenic contaminated area, tin and associated minerals, i.e. arsenopyrite and pyrite, have been extracted from granites and natural processes and the mining activities led to arsenic contamination in the environment. Electrical resistivity and self potential (SP) have been used to define the distribution of arsenic contamination in the groundwater. Resistivities of 25 - 100 ohm-m and a positive SP anomaly of 66.0 mV were observed in an area where the arsenic content in auger water at 3.5 to 5.0 m depths was high, 0.5 - 5.0 mg/l. Integrated interpretation of resistivity, seismic refraction, GPR, and gravity data gave a clear image of subsurface structures at a depth to 30 m. There was a good correlation between the resistivity and the gravity data. A subsurface rise was found, which possibly acts as a naturally buried dam, separating a high contaminated area from a low contaminated area. This study has demonstrated that the combination of geophysical methods is successful in delineating contaminated areas and contributes to the understanding of a possible mechanism for the distribution of arsenic. In the Ongkharak Nuclear Research Centre area combined GPR and resistivity pseudosections (dipole-dipole and pole-pole arrays) have given a good image of shallow faults in Quaternary sediments, faults that were originally indicated from regional remote sensing interpretations. Horizontal discontinuities of reflected signals obtained by GPR and images of lateral resistivity variation have been correlated to faults or subsurface movement identified by geological mapping in trenches. This signature of the faults is caused by contrasts in dielectric permittivity and/or in resistivity, which originates from vertical displacement at sedimentary layers and from sediments filling the faults. The positions of sub-faults as identified by GPR and their strike directions obtained from the trenching data (N60°W - N70°W, N65°W - N70°W, and N30°W) agree with the general NW-SE trend of the major faults, the Mae Ping Fault Zone, the Nakhon Nayok-Prachinburi and the Ongkharak faults in central Thailand. Thermoluminescence (TL) dating showed that sub-faults in the area have been active at about 7,500-2,400, 4,800- 1,750, and 9,700-2,300 years ago. Thus, these sub-faults have been classified to be of the same generation and they are defined as "capable faults" with reference to the criteria of U.S. Nuclear Regulatory Commission. Magnetic Resonance Sounding (MRS) has been successfully tested for detecting groundwater and in combination with Vertical Electrical Sounding to characterizing aquifer in southern Sweden and in the Vientiane basin, Laos. The combination of MRS and VES in the southern areas of Sweden shows that low resistivity layers interpreted as clay are sometimes identified close to the surface. The results here have shown that the MRS signals penetrate through the clay and that deeper aquifer can still be detected. The MRS data suggest aquifers that are not only hosted in soft sediment materials (moraine, sand, and mixed materials), but also hosted in basement rocks. Based on the MRS and borehole pumping test data, the hydraulic conductivity of aquifers has been estimated and the results agree with yield, average water content and subsurface geological data. The results from the measurement in the Vientiane basin have shown that there is usually two - three water bearing layers, and the "best aquifer" is found at depths between 15 and 25 m, with regard to high water content, permeability and resistivities indicating fresh water. MRS has also shown to be an important tool in constraining layer thickness and distinguishing low resistive layers of impermeable rock from what could have been interpreted as water in the VES interpretation. MRS data also suggest a clay layer at depths between 30 and 50 m, which is overlying halite deposits. This clay layer may act as a naturally sealing layer to protect the fresh water in above aquifer from salt contamination. On the basis of this drilling for fresh water is recommended not to penetrate deeper than 20 to 40 m, depending of the local depth to the clay layer. / Godkänd; 2007; 20071126 (ysko)

Gas bubbles in ice hold relevance to the origin and process of formation, distinguishing between an atmospheric or dissolved origin. An extraction line was built to isolate gases from ice, and tested using glacial, aufeis and ground ice from the Canadian Arctic. A newly developed mass spectrometry technique was used to analyze the gases for gas ratios of three principal atmospheric gases (N2, O2, and Ar) as well as 18O and 15N of these gases. O2/Ar and N2/Ar ratios demonstrate differences between atmospheric gas in glacial ice and gases exsolved from freezing water, due to the difference in their relative solubilites. Glacial and ground ice contained higher oxygen-18 values than the atmosphere and aufeis ice. Results for glacier ice show that isotopic and gas ratios of atmospheric gas entrapped during glacial ice formation change through gravitational settling. Aufeis ice bubbles originate as dissolved gas with alteration by excess air addition and/or mixing with snow pack. In the ground ice sampled, low oxygen concentrations and high delta 18O values, indicate respiration prior to formation.

Geomagnetic micropulsations are described and possible origins discussed. Previous work in Canada is reviewed with particular attention to the normal daytime Pc oscillations with periods from 10 - 30 seconds. A description is given of the work done correlating the amplitude and direction of the exciting vector at two stations. Field work at Ralston, Alberta during July -August 1959 is described in detail. A digital computer is used to obtain the auto-correlation coefficient and the covariance of the horizontal components of the vector. From these two paramenters the dominant frequency, amplitude, and polorization of the vector are obtained. The conclusion is reached that normal daytime Pc's seem to occur in a small band centred around a dominant frequency. Two such dominant frequencies may occur simultaneously and the vectors can have different polarization. The amplitude and direction of a given Pc seems to follow a closed diurnal pattern reaching a maximum slightly before local noon. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

Two facets of paleomagnetism are discussed as part of this study; the use of paleomagnetic records to improve our understanding of magnetic field reversals, and the use of paleosecular variation in volcanology studies. A detailed volcanic record of the Matuyama-Brunhes (MBR) reversal (780 ka) suggests that the reversing magnetic field remains dipolar throughout the transition. This record, taken from the Tatara-San Pedro Volcanic Complex located in the Andean region (36$\sp\circ$S, 71$\sp\circ$W) of central Chile, is recorded in 29 andesite flows. The record is exceptionally well constrained temporally and geochemically as well as paleomagnetically. The lavas record a quick change from reversed polarity to a period of transitional stability, with virtual geomagnetic poles (VGP) forming a cluster centered in Australia, followed by a quick swing to normal polarity. The Chilean data demonstrate that the dipole assumes an intermediate subequatorial position before completing the transition. VGP data from the four other available MBR lava records are largely coincident with the Chilean grouping and further strengthen the suggestion of a dipolar Matuyama-Brunhes reversal field geometry. The integration of paleomagnetism with geochemistry offers volcanologists an efficient and independent correlation tool for studying geochemical trends and growth morphology in volcanic centers. The Tatara-San Pedro Volcanic Complex, a Quaternary arc volcanic center in the Chilean Andes (Singer et al., in press), serves as an excellent model of complex stratocone evolution and lends itself well to a demonstration of this technique. Three fundamental relationships between paleomagnetic and geochemical data are demonstrated within the TSPVC: an absolute correlation, a paleomagnetic dependent correlation, and a geochemical dependent correlation. These correlations provide a degree of temporal insight into the rates of magma productivity that would be difficult to gain through conventional radiometric dating techniques.

No description available.

Strain distribution across broad plate boundaries is inadequately understood. California's Mojave Desert has been adjacent to Tertiary convergent and transform plate margins and its general geology is fairly well known, making it an excellent location for crustal motion studies. Models for the Cenozoic history of the Mojave Desert predict crustal motions, but paleomagnetism is the primary method for quantifying such motions. The Barstow, California area has good exposure of appropriate rocks ranging from Oligocene-earliest Miocene to Pliocene in age. Standard paleomagnetic sampling and analysis of these volcanic units yielded primary paleomagnetic directions, which spatially and temporally constrain crustal motions in the Barstow area. Observed magnetic directions were compared with expected directions for cratonic North America allowing determination of crustal rotations and translations. Paleomagnetic directions from the Lane Mountain Quartz Latites and Jackhammer Formation suggest 55$\sp\circ$ to 75$\sp\circ$ of clockwise rotation of the Barstow area in earliest Miocene time which may be related to oroclinal bending of the southern Sierra Nevada batholith. These units may also record a geomagnetic reversal. Twenty-three degrees of counterclockwise rotation of the Pickhandle Formation coincides with early Miocene northeast-southwest extension and detachment faulting in the Mojave Desert. Other parts of the desert experienced variable senses and amounts of rotation concurrently. Drag along transfer zones or detachment surface geometry appear to have caused upper plate rotations in the extended terranes, while the lower plates remained unrotated. These results suggest that extension in the Mojave Desert is related to similarly oriented Miocene extension in adjacent areas. This study, like most paleomagnetic work in the western and central Mojave Desert, shows no evidence for post-18 Ma rotation, suggesting that post-10 Ma right-lateral faulting there has produced relatively little crustal deformation. The exception to this generalization is the northeast corner of the Mojave Desert which underwent about 28$\sp\circ$ of post-18 Ma clockwise rotation. This enigmatic and structurally different northeastern area has apparently experienced atypical post-middle Miocene activity. Paleomagnetic flattening data and structural constraints indicate that post-Oligocene north-south translation of crustal blocks in the Mojave Desert has been insignificant.