In 1992, a comprehensive three-component ocean bottom seismic surveywas performed in the central and northern area of the Voring Basin, offshore mid-Norway, NE Atlantic. An important part of the data acquisition program consisted of a local survey with 20 Ocean Bottom Seismographs (OBS) dropped at approximately 200 m interval in 1300 m water depth. The main purpose of the local survey was to acquire densely sampled P- and S-wave reflection data above a seismic flatspot anomaly observed earlier, in order to more accurately predict if hydrocarbons could be related to it. The conventional reflection data processing methods applied to the vertical components included predictive deconvolution in order to attenuate low frequency ringing, near offset mute and a series of constant velocity stacks in order to obtain the optimal velocity function. The final result is a “trouser” shaped, high resolution V_z stacked section with minor influence of water multiples. The inline (V_x ) component contains no strong multiples, and extensive near trace muting was hence not necessary to apply for this component. Velocity analysis together with ray-tracing modelling indicate that P-S-converted shear waves (reflections) represent the dominant mode. The results of the interpretation and modelling indicated a V_p/V_s -ratio of approximately 2.6 in the overburden, which suggests domination of partly unconsolidated shale, while the V_p/V_s -ratio in the assumed reservoir was approximately 1.8, which indicates a more sand dominated facies. Outside the flatspot area a higher V_p/V_s -ratio ratio (approximately 2.0) was estimated, indicating that hydrocarbons could be present in the assumed reservoir.

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Slip complexity of earthquake faults is studied based on an N-degree-of-freedom dynamical spring-slider system in the presence of slip-law-type, velocity- and state-dependent friction. Simulation results based on such a friction law show that slip complexity depends on the inhomogeneous distribution of the breaking strengths (including its pattern and degree) along the fault and nonlinear velocity- and state-dependent friction. However, for the given model parameters the former is more important than the latter in controlling slip complexity. Frictional effects obviously appear only when the distribution of the breaking strengths is inhomogeneous. In addition, the stiffness ratio, defined as the ratio of the coil spring strength, K_c, to the leaf spring strength, K_l, is also a factor in controlling slip complexity.

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Seismic velocity structure of volcanic region is highly heterogeneous so that seismic ray tracer used in this field should be robust for the velocity heterogeneity. From this view point a three-dimensional robust seismic ray tracer, effective in any complicated velocity structure, is developed by using a hybrid scheme of the shortest path calculation and the downhill simplex optimization method. The node configuration necessary for the three-dimensional shortest path calculation is newly presented. Validity and efficiency of the calculation are examined by synthetic tests. A travel time accuracy of less than 0.1%, and a rms ray path error of 0.05 km are achieved. Calculations for the synthetic velocity models and checkerboard testing show the effectiveness of this ray tracer in practical situations. The present ray tracer is recommended to be used in travel time tomography in highly heterogeneous velocity structure such as volcanic regions.

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In order to reconsider the mechanism of the self-reversal of TRM (SRTRM) of hemoilmenite in dacite pumice, magnetic properties of newly collected Haruna dacite pumice near the historical sampling site (more than 50 years ago) were investigated again. All Haruna dacite pumice samples collected showed SRTRM or at least partial SRTRM. This time, the methods of microprobe analysis and the Bitter technique which were not available 50 years ago, were added to examine hemoilmenite crystals. Contrary to the result which was first reported by Hoffmann and Fehr on the basis of the micromagnetic observation of hemoilmenite crystals in Pinatubo dacite pumice, our hemoilmenite crystals are considerably more homogeneous in TiO₂-content in a single crystal than those of Hoffmann and Fehr. Also, we could not find any “shell structure” with two phases having different magnetism and compositions in a single crystal of hemoilmenite in Haruna dacite pumice. We came to the conclusion that the intergrown shell-structure itself in Hoffmann and Fehr's model may not be essential but only the coexistence of the intergrown two phases in a single crystal of hemoilmenite must be essential for the acquisition of SRTRM of hemoilmenite. Both models, one by Ishikawa and Syono and the other by Hoffmann and Fehr are basically similar to each other, regardless of the structure involved.

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The unique network of geomagnetic observatories along 145°E geomagnetic longitude extending from the magnetic equator to the north pole has enabled to study the latitudinal profiles of Storm Sudden Commencement (SSC) amplitudes in the three components H, Y and Z of the geomagnetic field separately for the daytime and nighttime events. An abnormally large positive impulse of Z is observed at the equatorial stations with maximum at Trivandrum during the daytime as well as the nighttime hours suggesting large induced current within the earth's crust south of Indian continent. The daytime enhancement of SSC (H) at the extended equatorial latitudes is undoubtedly due to the disturbed electric field generated by the magnetopause current communicated to the equator through polar latitudes. A prominent decrease of SSC (H) during night hours and the ‘induction vector’ at SSC frequencies at equatorial latitudes are indicative of the concentration of induced current from source fields extended in altitudes.

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In recent years, quantitative analysis of the magnetosphere-ionosphere coupling and electrodynamics of the polar ionosphere received much attention. Though remarkable progress has been made in this field by using a variety of magnetogram inversion techniques in order to infer the global ionospheric current distribution, there is still a need for modeling ionospheric currents locally, over a certain region, for comparison with other geophysical ground-based and satellite observations. This paper presents a simple method for estimating equivalent ionospheric currents using magnetic field observations along a meridian chain of ground-based vector magnetometers. The method can be applied in an automatic fashion to any available magnetometer chain data, for example, from the DMI Greenland west coast chain. We first describe how we separate contributions to the observed geomagnetic variations from external (ionospheric) and internal (induced) sources. We then model the ionospheric electrojet by a sequence of narrow current strips and apply the Biot-Savart law to formulate an inversion problem. Using a regularization technique, we find a stable distribution of the equivalent ionospheric currents crossing the magnetometer chain in eastward and westward direction. Simulation tests and a case study (20 March 1999) are discussed in order to illustrate properties of the solution to the inverse problem and to present a practical tool, which is accessible through the DMI World Wide Web site.

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An upward beam of hot ions which originates in the ionosphere is commonly observed in and above the region of auroral particle acceleration. Past simulations of the weak double layer (WDL) have only included magnetospheric hot electrons and ionospheric cold ions. We simulated the formation of the WDL numerically and more realistically using a one dimensional electrostatic code. We modeled magnetospheric hot electrons, ionospheric cold ions and hot ionospheric beam ions in the system. The existence of the hot ion beam modifies the characteristics of the WDL, its magnitude of the potential drop and lifetime. We investigated the variation in the character of the WDL for different fractions of the hot ion beam component. The magnitude of the potential jump of the WDL was found to decrease but the lifetime of the WDL increase as this proportion increased. The existence of a critical value of the ratio of the hot beam ions to the cold ions, above which electrons cannot be accelerated effectively and which places an upper limit on the altitude of auroral particle acceleration region has also been shown.

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