Journal of the Earth and Space Physics
Year:2003 | Volume:29 | Issue:1|Papers Count:18

One of the parameters derived from field measurements of Sub-Audio Magnetics (SAM) technique is called Total Field Magnetometric Resistivity (TFMMR). In general, SAM is a high resolution technique that obtains subsurface electrical and magnetic properties through measuring total magnetic field including the synthetic geomagnetic field associated with the low frequency (less than 200 Hz) galvanic current flowing in the earth. In a previous paper by one of the authors of this paper, the anomalous TFMMR responses due to simple geometrical targets have been presented in details. However, as was pointed out in that paper, the simple models are ideal models that are rarely exists in nature and hence the need to find the solution for complex structures using numerical methods is inevitable. In this paper, the theoretical basis of the TFMMR parameter and the behavior of the governing electric and magnetic fields over 2-D structures in both spatial and wave number domains are derived first. Next the selection of the proper numerical technique for TFMMR forward modeling over 2-D structures via employing 3-D sources (point source electrodes) is followed by addressing the numerical difficulties encountered in the course of obtaining valid responses including 1) the singularity associated with the source term (current electrode) in the governing partial differential equations, 2) the singularity associated with the inverse Fourier transform and 3) singularities associated with computing different magnetic field components. Finally, some methods for resolving the numerical singularities are presented and the validity and accuracy of the numerical results against the available simple analytical solutions are evaluated.

Series of experiments is carried out to investigate the influence of a mountain on air flows around and over it. Experiments were concentrated on the role of orography in the adjustment process. Based on the results of experiments, we find that: (1) the final equilibrium state of the atmosphere is not geostropically balanced. The height of mountain influences the adjustment period; higher mountains make the adjustment period longer. (II) Fronts are not observed during geostrophic adjustment, unlike what was suggested before. Although frontal features appear on the maps, their length scales are shorter than those of the atmospheric fronts. (III) The ratios of kinetic energy to the potential energy released during adjustment are smaller and larger than those predicted by the theory when, kinetic energy is computed from respectively geostrophic and actual winds. (IV) For small Froude numbers, the relation between mountain drag and Froude number is more complicated than what has already been suggested. (V) In the nonlinear regime studied here, splitting of flow and vertically propagating internal gravity waves are found as other major mechanisms for drag development in addition to the three possibilities suggested by Smith for linear regimes.

The Fork earthquake occurred in Zagros Mountains at a distance about 120 km from Darab city in Fars province, producing extensive destruction but relatively low rate of human loss. Field investigation and the distribution of aftershocks suggest an east-west trend faulting with a reverse mechanism having a small strike-slip component. The locally recorded aftershock activity was extended to a length of about 40 km and a depth of about 30 km. The majority of aftershocks took place at a depth range 10-20 km and was scattered indicating a complex mode of faulting. The result of waveform inversion indicated that the mainshock had mainly reverse mechanism and the source process included two main fault slip. The total seismic moment was calculated to be Mo=3.1 x 1025 dyne cm. The calculated maximum dislocation was about 50 cm and the obtained moment magnitude was Mw=6.2. The average stress drop was estimated to be 25 bars and the average dislocation was 25 cm. The Fork earthquake is one of the rare events that have occurred in Zagros suture zone with magnitude greater than 6. Therefore, the ground-motion characteristics during the mainshock should be considered for the high safety design of structures in the damaged area.

Well logs and seismic sections provide main data for oil reservoir modeling. Generating synthetic seismogram is a method to calibrate these data. Characterization of Bangestan reservoir by well log analysis and seismic modeling was the goal of the present study. Bangestan is one of the Ahwaz Oil Field reservoirs. Ahwaz Field is located at the north side of Ahwaz city in the south of Iran. The major reservoirs in this field are Asmari, Bangestan and Khami Formations. In the first step of this study, several well logs (Neutron, Density, Sonic and Gamma ray) were digitized for more than fifty wells. These logs provided petrophysical information. Seismograms were generated by Geosyn software using sonic and density logs along the field on a seismic line. Petrophysical parameters calculated for this study using Excel software. Finally a synthetic seismic section was created and compared with petrophysical data such as porosity, volume of shale, lithology, etc. Interpretation of reflector's behavior, study of the level of seismic parameters to capture geologic heterogeneities in synthetic seismograms, recognizing problems and prevents, analysis of the petrophysical parameters along the section and giving guidance for next studies on this reservoir constitute main results of this study. Porosity variation was recognized the most important factor in appearance of reflectors on the section and reflectors were shown that present zonation of Bangestan reservoir might be changed.

In this paper the effects of permeable bed on wave run up, bed shear stress and internal currents inside the bed, the shape of streamlines and trajectories of fluid elements inside the bed are studied. The experiments have been done in Hydraulic Laboratories of Sea and Coast in "Soil Conservation and Watershed Management Research Center". General properties of flow in rivers, estuaries, and coastal seas are highly dependent on the bed morphology. These include mainly bed shear stress, velocity profile, and turbulence. We consider the effects of suction (W>0) and injection (W<0) on the properties of flow, particularly the bottom shear stress. Four types of bottom permeabilities with different size of sand have been tested. The results indicate a substantial reduction and enhancement of the bed stress under injection and suction respectively as has been observed by others on wave motion in shallow seas. For this study we considered 5 groups of waves oncoming to shore in the steepness ranges between 0.015 and 0.05. Observations of streamlines inside the bed were taken using a video camera embedded inside the bed, which recorded the pattern of injected dye. Speed of current is computed in two locations: 3 and 13 cm depth by introducing Ṽ=W/U (relative dimensionless suctions/injection velocity) we found that for the suctions case, when Ṽs increases, the stress are enhanced. While for injection case (Ṽi), the stress is reduced. Typically, TI TO varies between 2.5 for Ṽs≈-0.4. Hence, relative bed stress can vary by a factor of 5 which can have a substantial impact on the bed morphology.

Double diffusive convection (DOC) is a common phenomenon which is often associated with temperature inversion (increase of T with depth) (TI) for the greater part of the world's oceans. The fact that TI with widely varying properties (thickness, intensity, and stability) occurs quite often in the ocean and that they are, as a rule, hydrostatically stable, has attracted special attention. Here we investigate the structure of temperature inversions in the Persian Gulf and Oman Sea. TI regions in the Persian Gulf is" observed in the winter time at depth of about 40 m, but are observed in the Persian Gulf outflow in Oman sea at a depth of about 250 m in summer and winter. In the Persian Gulf, the diffusive regime of DOC is often observed and the density ratio is in the range of 0.2-0.7. In Oman sea the finger as well as the diffusive regimes are observed with density ratio of about 0.4 TI in the Persian Gulf is also found to be more intensive than that of the Oman's. TI often appear in the boundary of outgoing water of the Gulf and incoming water of open sea, a region prone to instability (large scale) and eddy for motion. Using a relation of ΔS= aΔT + b for TI regions, we found that a and b are respectively 0.92, 0.34 and 0.37, 0.04 for the Persian Gulf and Oman sea, indicating that the flow in these regions is isopycnal in Oman sea but highly non isopycnal in the Persian Gulf. Therefore, mixing mechanisms are expected to be different in the two situations. Here the DOC fluxes have been estimated to be about 22 W/m2 for 0.5<Rp <1 and 2 W/m2 for 0.1<Rp <0.5. These fluxes can have substantial effects on thermohaline circulation in this region and should be considered in any modeling problems.

Low-frequency, high-amplitude surface wave (ground roll) is an old problem in land-based seismic field records. Conventional processing modules are to suppress ground- rolls, such as frequency and velocity filters, which use the Fourier transform. All of these techniques assume that the frequency contents of seismic trace are stationary, but in reality seismic traces have time varying frequency contents. In this study an alternative method to the Fourier transform known as the wavelet transform was used to suppress ground rolls. The key advantage of the wavelet transform over the Fourier transform is that it does not assume that the trace is stationary, and it can also localize the target information in the time-scale domain. The wavelet transform decomposes the seismic trace using basis functions that have finite extent in both frequency and time. In this research, an algorithm was prepared to take the wavelet transform of seismic traces of a shot record. Then a filter for ground roll suppression was designed based on the results of wavelet transform. Finally, the efficiency of this filter was compared to that of the band-pass and F-K filters.

In a multilayer earth model, if the thickness of a layer is smaller than a theoretical limit, its refracted signals would not be recorded as first arrivals and the layer will be hidden. This event is known as blind zone. Also in a multilayer earth model if velocity of a layer is smaller than the surrounding layers (below and above layers), no refracted signal of their interfaces would appear as first arrivals. This event is called velocity inversion and the layer that has this property is said hidden layer. Due to these conditions, some errors would occur in calculation of depth and velocity of underlaying layer in seismic surveys. In this study it was shown that lateral arrivals such as postcritical reflections are useful for identifying blind zone and its parameters. Results of using this method on synthetic data are presented. It was also shown that if there is any undulation on the surface of refractor, it could be used for identifying hidden layer and calculating its parameter by determination of the horizontal spacing between effects of these undulations on travel-time curve.

Normal mode initialization algorithms of first and second order accuracy are formulated for a regional barotropic model. The initialization algorithms are implemented on the actual data of 500hPa level in different spatial resolutions using a potential enstrophy conserving Eulerian scheme due to Sadourney for time integration. If the linearized Potential vorticity qℓ which is left unchanged during the normal mode initializations, is constructed based on the initial geopotential height Z only, and if the geostrophic wind associated with Z is used to define the boundary conditions needed to solve the modified Helmholtz equations involved, then the initialization algorithm will exhibit an undesirable sensitivity to spatial resolution. This sensitivity reflects different behavior of small and large scales as regards balance. But if qℓ is constructed based on the distribution of both Z and relative vorticity ζ of the initial wind field, and if the geostrophic wind associated with the qℓ thus constructed is used to define the boundary conditions, then consistent results at different spatial resolutions will be obtained, Qualitatively, the results of the first and second order 1fIgorithms are indistinguishable. By measuring the time variation of an Euclidean norm for the departure of actual states from their approximately balanced-state counterparts determined by the two initialization algorithms during 48 hours numerical integrations, the superiority of the second-order algorithm is demonstrated, particularly for the first 24 hours of integration.