JOURNAL OF THE EARTH
Year:2009 | Volume:4 | Issue:1|Papers Count:10

Although strong ground motion networks are expanding, near-source strong motion recordings are still sparse .In this article it is planned to characterize the level and variability of strong ground motion in near field of large earthquakes due to source effects. We have developed a stochastic rupture model that characterizes the variability and spatial complexity of slip as observed in past earthquakes. We model slip heterogeneity on the fault plane as a spatial random field for 21 near source earthquakes. The data follows a von Karman autocorrelation function (ACF), for which the correlation lengths (a) increase with the source dimensions .These stochastic slip distributions are used to develop the temporal behavior of slip using physically consistent with stochastic-dynamic earthquake source models .It means that we can use this model to simulate realistic strong ground motion in order to characterize the variability of source effects in the near-field of large earthquakes. For earthquakes with large fault aspect ratios, we observe substantial differences of the correlation length in the along-strike (ax) and downdip (az) directions. Increasing correlation length with increasing magnitude can be understood using concepts of dynamic rupture propagation. The power spectrum of the slip distribution can also be well described with a  fractal distribution in which the fractal dimension D remains scale invariant, accounting for larger ‘‘asperities ’’ for large-magnitude events.Our stochastic slip model can be used to generate scenario earthquakes for near-source ground motion simulations.

Construction in near fault regions is of specific importance for which safety should be considered against seismic faults. Fault rupture zone is a region that civil construction receives highest damage from near fault zone effects such as surface fractures, strong ground motion, displacement and landslide in the regions of great slope and rough topography. In this study, first, the three dimensional topography and slope map of the studied area are prepared. The fault rupture zone of North Tehran Fault is calculated after developing the faults density and magnitudes gained from three probable mobile scenarios. Present study suggests a fault rupture zone of 2.2 to 1 km for North Tehran Fault.

Classification of data sets helps to understand their hidden nature. Among different existing classifiers, neural network is more frequently used, and despite its complexities, it has gained widespread applications. Due to significant capabilities of neural network, a good number of softwares have been developed to facilitate its usage. Unfortunately, unawareness of the logic of this technique, hidden in these softwares, leads to partially wrong interpretation of data sets. This paper, first present a new logic for joint set classification by neural network, and then attempt to discuss the uncertainties in performances of the MLP neural network classifier. In order to study the applicability and advantage of the new method in joint set classification, 8 sets of synthetic joints were developed, each with 4 defined characteristics (dip, dip direction, infilling type and infilling rate). The spatial distribution of joints were selected in a way that with two parameters (dip and dip direction), and traditional tools (rose diagram and stereonet), only a limited number of sets could be differentiated. A computer program, using neural network, was developed and the synthetic data was classified in four dimensional spaces. Present study showed that this new technique can easily differentiate and classify all the 8 synthetic sets.

It is a common experience in processing land seismic reflection data to be confronted with data containing weak reflection signals submerged by coherent, strong linear noise such as ground roll or air waves. Ground roll is a type of Rayleigh wave that has high amplitude, low frequency and low velocity. Ground roll is also dispersive and overwhelm the desired reflection signals unless special steps are taken in data acquisition and processing to get rid of it. Various methods to filter ground roll have been proposed and applied in recent years. Coherent linear noise is recognized by lateral continuity and its predictability from nearby traces. Therefore, to eliminate them, it is requested a kind of prediction filter which can predict coherent energy from trace to trace. Since linear events are coherent in space, f-x prediction filter is used to eliminate these events from seismic data in frequency-space domain. In the frequency-space domain, for a given frequency, coherent noise is predictable. In this paper, complex Wiener filter theory has been used to predict coherent noise. In the frequency-wave number domain signal and ground roll can be separated. But when the difference between the apparent velocities of signals and noise is small, part of signals exist in the ground roll rejoin. To retain this part of signals we can use f-x prediction filter. This method is named f-k and f-x filtering. This method was tested on synthetic and real shot records contaminated with ground rolls. Results in reducing ground rolls are acceptable.

Seismic interferometry is the process of cross-correlating seismic traces recorded at different locations at the Earth's surface with the aim of retrieving information about the subsurface.The field of interferometry changes our opinion about parts of seismic data that is usually discarded due to being considered as noise. Some examples of such noises are seismic codas (the multiply scattered parts of seismic waveforms) and background noise (whatever is recorded when no identifiable active source is emitting, and which is superimposed on all recorded data). In this study the efficiency of interferometry is shown trough some examples.

The surface section of Kuh-e-Bandobast has been investigated for the Paleocene deposits and evaluation changes throughout the Cretaceous - Tertiary boundary. This studied area is located at 50 km southeastern of Jahrum city. This surface section has 36m. thickness and lithologically consists mainly of grey marls with cream-coloured to brown argillaceous limestone which belong to the lower part of the Pabdeh Formation. The Paleocene strata conformably rest on the Cretaceous deposits, whereas they gradually change to the Eocene deposits with no sharp lithological changes. The biostratigraphical investigation has been carried out on 15 thin-sections. These samples contain some index planktonic foraminifera such as: Globoconusa daubjergensis (Early Paleocene) and Morozovella velascoensis (Late Paleocene) which reveal that the boundary of the Cretaceous / Tertiary is quite continues in this part of Fars area.

The South Pars gas field is one of the Southwestern fields of Iran in Zagros sedimentary basin and consists of two Kangan and Dalan reservoirs. The Kangan and Dalan Formations belong to Dehram Group and are the most important gas reservoirs in Persian Gulf area. Petrophysical characteristics in the studied well were evaluated by the data of the well logs and cores. For an accurate evaluation, the necessary corrections were applied on logs by considering the places of wash out, environment effects, gas and shale effects. After that, the lithology of the Kangan and Dalan Formations were determined by using of cross plots of neutron-density, neutron-sonic, M-N plot, MID plot. On the based of Lithology, shale volume, presence of evaporite sediments and porosity, the Kangan Formation has been zoned to K1 and K2, and also the Dalan Formation has been zoned K3 and K4. The results show that the zone K2 of the Kangan Formation and the zone K4 of the Dalan Formation have an apporiate reservoir in terms of gas saturation level, porosity and Lithology. Kangan Formation, Dalan Formation, South Pars field, Petrophysical evaluation, Water saturation and gas

The studied area is located at 18 kilometers north-east of Bostan Abad township (east Azarbaijan province). According to structural geology, volconic rocks are situated in Oromieh-Dokhtar belt, and faults observe in same direction to this system with NW-SE trend that these are cut off with new faults with NE-SW trend. The results show that kaolinite alteration trend with silica and propylitic veins are same direction with NE-SW faults in this area. Therefore, these faults with these trends can be considered as the mineralization control for determination of the alterations. In this project aster data (B level) are provided and process and spectral analysis for recognizing intensity and kind of argillic, propylitic, iron oxide and silica alteration relation to metal mineralization of the area. For recognizing different alterations of the study area, some primary spectral analysis have been done for recognizing minerals are present all over the area. Finally the result of aster data spectral analysis (VNIR, SWIR, TIR) compose together and introduce a final map that shows 4 promising areas containing 10 target for the continuation exploration programs.

The studied area lies in south of Mashhad and has several thrust faults. The most important of them is the Ultra-Basic Mashhad unit thrust fault which is an imbricate type of thrust fault and consists of turbiditic rocks. Another important fault contact is the Sangbast-Shandiz thrust fault which causes the thrusting of Paleozoic turbidits into Lower-Middle Jurassic conglomerates. This shows that this thrust fault is a leading imbricate type. The age of this fault is post-Jurassic. This fault approximately has a 140 to 145 degree dip in a NE direction and also consists of fault-propagation and folding. The axis of these folds is parallel to front normal thrust and its’ surface axis is in a NE direction. The movement of this fault has been determined to be dextral. By calculating the parameters and special vectors and graphing them on related diagrams, it was specified that the joints in the area are front normal thrust and front parallel thrust kind.The studied area is also affected by thrusting and a dextral oblique-slip displacement from a NE-SW direction.

The Asmari Formation of Oligomiocene age is the main reservoir in Dezful Embayment. The Asmari Formation was studied around the northern flank of Bangestan Anticline and at the north west of Bonjileh village. The thickness of  the Asmari Formation in  the studied region is 198.5 meters. The formation is overlain and underlain with disconformable contacts, by the Pabdeh and Gachsaran formations respectively. Field studies led to recognition of three lithofacies: limestone, shale and marly limestone. Derived results from microscopic studies of thin sections led to recognition of 14 microfacies that in comparison with standard microfacies were precipitated in 4 depositional environments, including supratidal, lagoon, barrier and open marine. Based on these results, the Asmari Formation deposited in different parts of a homoclinal carbonate ramp. Several parameters such as: environmental condition, surface water penetration in to the reservoir, diagenetic processes, and the rate of fracture development are responsible for the quality of this reservoir.