Journal of the Earth and Space Physics
Year:2002 | Volume:28 | Issue:2|Papers Count:13

A detailed mapping and short-term micro earthquake survey in a small region covering the Sahneh fault segment of the Main Recent Fault of the Zagros Mountains, in western Iran, is conducted to study the seismotectonics of the Sahneh fault. It is found that the Sahneh fault is an active second order structure of the transpression (positive flower structure) type with temporarily low level of seismic activity. Meizoseismal regions of historical earthquakes, macroseismic epicenters of pre-instrumental (1900-1963) earthquakes, as well as the location of microearthquakes together with the structural geology of the region, strongly suggest that the study region should be considered as a localizing structure, so that the seismic activity of the region cannot reliably be correlated to any known active faults. Present microseismic activity is concentrated in the area bounded by the southern part of the Sahneh fault and the northern elongation of the Nahavand fault, that covers the meizoseismal regions of the Farsinaj earthquake of 13 December 1957 (Ms=6.7). Focal mechanism of earthquakes and structural analysis confirms the prominent right-lateral strike-slip deformation along the Sahneh fault. This is consistent with relative motion between the Arabian Plate and the Central Iran Microcontinent. Hypocenters of earthquakes recorded from August to September 1998, by the local seismic network, show that earthquakes in this region originate mainly in depths of about 10 km.

In this research, gravity surveying is used for exploration of petroleum reservoirs in the Chenaran region, northeast of Iran. For this purpose a gravity survey is conducted by making gravimeter readings at 816 sites, located along 26 parallel lines of profiles. The recording sites are at equal distances of I km and the lines of profiles are spaced at equal intervals of 2 km. After estimating the surface density value, two dimensional and three dimensional Bouguer anomaly maps are made in which all the relevant corrections, including the free-air correction, the Bouguer correction, the latitude correction, the terrain correction, the drift correction, and the tidal correction, have been applied. For interpretation of these maps, local and regional anomalies have been separated by using two of the most widely used schemes that involve computation of weighted averages, namely upward continuation method and the second vertical derivative respectively. The local patterns of Bouguer gravity variation reveal the presence of buried anticlinal structures and fault zones in the region. Due to the favourable geological conditions for petroleum accumulation in the region, it is very likely that petroleum reservoirs have been trapped by these buried structures. Finally the results indicate that gravity survey, particularly, Bouguer anomaly separation, plays ae important role in petroleum exploration.

One of the exclusive beneficial characteristics of VSP data is the possibility of identifying multiple reflections, which exist in up coming waves. As shown in this paper, powerful predictive deconvolution operators can be designed from down going waves in order to suppress the above-mentioned multiples. In this study VSP data from the south of Iran was used to suppress multiples from nearby surface 2D seismic data. The designed operators suppressed multiples from surface seismic data properly without creating any destructive side effects on the rest of the record. The results indicate that the application of deconvolution operators not only suppressed the multiples from 2D seismic data but also produced a better correlation between VSP data and the final 2D seismic section. Seismic data and required software packages for this study were provided by the National Iranian Oil Company.

Application of seismic attributes is one of the most common way of studying subsurface variations. Dip and true thickness as seismic attributes play an important role. For instance, in detection of structural events like faults and imaging edges of strati graphic changes. In this study, Isopach and dip maps of the reservoir layer in a 3D seismic project in northeast Iran were calculated. Then the true thickness map of the layer was computed. The true thickness map shows subtle features such as minor faults and alluvial fan in the reservoir layer. Also displacement of some faults is deduced from the true thickness map.

During last few decades, the acquisition of seismic reflection data was performed using the common mid point method. In order to remove the effect of offset between sources and receivers from reflection times, Normal Moveout Correction is applied. This operator does not perform well on non-horizontal reflectors. Also, the CMP stacking process acts as a dip filter, tending to enhance reflections having a particular slope in the stack, while more or less attenuating reflections having different slopes. Therefore, additional processing steps are required in the usual processing algorithms. For instance, Dip Moveout operator will be able to remove the effect of dip from stacking velocity. In this paper, the effectiveness of DMO operator in removing the effect of dips from stacking velocity was examined over two synthetic models when interval velocities are constant or increase gradually with depth. The models consist of first set of selected horizontal layers conflicted with dipping layers. The velocity of the models is assumed to be constant. The second selected model consists of dipping layers with different velocities. DMO operator was also examined over several 2-D seismic lines from central Iran. According to the results of this study, DMO by Fourier transform is a powerful tool for improving processing of seismic data. The results are as follows: (1) Stacking velocities become independent of dip, so that correct stacking of simultaneous events with conflicting dips is made possible. (2) Velocity analysis is improved, and provides velocities that are more appropriate for migration as well as stacking. (3) Continuity of reflectors will be improved. (4) Ties are improved because a zero offset trace is the same regardless of the direction of the offsets from which it is derived. (5) Coherent noise with impossibly steep dip is removed, without the artificial alignments often associated with dip filters. (6) The signal-to-noise ratio is improved.

A major section of the present research is concerned with the identification of the rainfall temporal patterns of five selected meteorological stations in Khorasan province in the north east of Iran using the Pilgrim and Huff methods. The first section provides a brief review and discussion about the identification methods of rainfall patterns. A comparative analysis is presented whereby the Pilgrim method results are assessed by direct comparison with results from the Huff method. It was found that the Huff method is an appropriate choice. The main result is that the rainfall temporal pattern of Khorasan is more correctly modeled by the Huff method. Although the region is semi-arid, the patterns are specific and different from other regions like Illinois, although in the same latitude but with different total rainfall patterns.

In recent decades, the problem of air pollution, especially the particulate type has created health problems for people in large cities such as Tehran. Here, from a different perspective, the relationship between the precititation processes and the air pollution in Tehran has been investigated. The climatic changes in Tehran from 1950 to 2000 are considered to be mainly due to urbanization, as well as the topographic setting of this city. This study shows that the annual mean maximum and minimum temperatures in Tehran are greater than those of Karaj, a smaller city to the west of Tehran. The same is true for the mean annual minimum temperature for Varamin a city just east of T ehran. Although the annual maximum temperature in Varamin is higher than that of T ehran which is mainly due to the topographic effect. The records indicate that the visibility in Tehran gradually decreased around the 1975s, which is considered as the start of an accelerating city expansion, hence an increase in air pollution. The frequency of smaze events has increased substantially since the 1975s. The percipitation records also indicate that the mean annual precipitation has increased relatively over Tehran more than over Karaj and Varamin, although local precipitation (based on the sudden increase in local relative humidity) has been reduced which could be due to overseeding of the clouds by pollutant aerosols.

Our eyes can recognize the arrival of the signal by comparing the recorded amplitude in various seismic traces. In this article we tried to imitate this natural ability to find a mathematical tool instead of the eyes. Fractals are mathematical models, which are able to feel these changes. An algorithm is introduced for the automatic picking of seismic first arrivals. It detects the presence of a signal by analyzing the variation in fractal dimension along the trace. Among different methods, the "divider method" was found to be the most suitable method for calculating the fractal dimension of a seismic trace. A change in dimension was found to occur close to the transition from the noise part of a trace to the signal plus noise part of it, that is the first arrival. The nature of this change varies from trace to trace, but a detectable change is always found to occur. The algorithm has been written in MATLAB environment and is slower than the other non-fractal methods.

In this paper the distribution of P-wave velocity in regionally metamorphosed rocks of the Zayandeh-rud dam has been investigated. The data collection has been carried out by a 24 channel digital device. Data analysis shows that P-wave velocity in metamorphic rocks increases with depth continuously following the linear equation v(z)=3200+6.5.z. The P-wave velocity in accordance with metamorphic grades from slate to schist changes from 3200 m/s at surface up to 5475 m/s at 350 m depth. High velocity gradient in metamorphic rocks of the study area is a consequence of changes in metamorphic grades from surface to the investigated depth and the effects of surface weathering. The calculation of porosity based on Wyllie (1965) equation indicated that the porosity has changed from 5.5% at the surface to less than 1% at the 350 m depth continuously, which is in agreement with lithological changes from slate to schist.

The effect of topographical masses on geoid height determination is described by three terms: the direct topographical effect on gravity, the primary indirect topographical effect on potential and the secondary indirect topographical effect on gravity. Martinec (1998), Sjoeberg and Nahavandchi (1999) and Nahavandchi (2000a) derived different equations to compute these effects. These different approaches for computing the effects over a rugged terrain area (Rocky Mountains) are compared.