| dc.description.abstract | The survey area covers a portion of the Urema Graben in the northern part of the Inhaminga license, which is located north of Beira province and south of the Zambezi River. The purpose of this thesis was to do the field processing 2D seismic signal using Processing software and make a basic structural interpretation of the processed data set.
The data sets for this project were obtained from DNO International ASA, and for this research were only used some parts of the seismic data in GEG-Y format of the Lines DNO-07-01 and DNO-07-01 with row shots with geometry in headers as well as acquisition report for both lines, a processed line DNO-MZ-07-01 with a tentative interpretation done by DNO International ASA based on correlating the well information via other existing lines, partly low quality, and also the available information about Urema Graben area.
The focus on the processing was given to the Corrections of time, amplitude, frequency-phase content, data compressing or Stacking, and data positioning or migration. The sequences of the processing flows used on this work can vary for others processing depending on the actual data set and seismic data processor. As an example deconvolution can be applied to the data both before and after stack, for this work was done before. And also seismic migration, the imaging step, can be applied both before (pre-stack migration) and after (post-stack migration) stack. In every so often two or more steps were done in the same time, or in a way not discrete.
A datum elevation of 250m above the mean sea level and velocity VR of 2.500 m/s were used, and the datum elevation was determined by the taking an average of elevations of all stations. Deconvolution was done in order to remove the effect of the wavelet from the seismogram and produce a wavelet with a simple phase character. Band-pass filter was applied to the edited data for removing some low frequency ground roll and cultural noise, and high frequency ambient noise, and the four values of frequencies of Ormsby filter was (4-8-90-100Hz). NMO correction was applied to get the zero-offset travel time of each reflector, when applied this correction to a CMP gather the hyperbolic reflection events become flat event. To set stacked section we first apply NMO correction to a CMP gathers, then stack or sum all the traces of the CMP gather and position the subsequent trace at its particular CMP location. In order to increase the amplitude of weak signals several times during the processing were applied Automatic Gain Control (AGC).
On the interpretation section was focused on the interpretation of 2 horizons and also recognition of the major occurrences of faults. To interpret selected horizons (Lower Grudja and Maputo formation) of the seismic Line DNO 07-01 was based on a comparison of the seismic reflection character and the thicknesses (in TWT - two way travel time) of the formations of this line with a seismic section processed and interpreted by DNO International ASA, based on correlating the well information via other existing lines (partly low quality) of the Urema Graben area, combining with the existing interpretation and explanation of the Lower Grudja and Maputo formation. Since seismic signature of faulting is variable, to define faults we were based on general ideas such as parallel reflections terminate against a planar or curvy-planar trend (fault planes rarely produce reflections) and are uniformly offset in the adjacent fault block. In unmigrated data, diffractions that line up can be indicators of faulting. And also subtle dips in seismic reflections that line up from profile to profile could be indicators of small offset faulting.
Two preliminary stacks of the Line 07-01 and Line 07-05 showing evident reflectors that can be boundary layer transition or even secondary events are results of the processing, and two horizons top Lower Grudja formation and top Maputo formation marked on the seismic sections, and a group of normal and tilted fault with no significant displacement on the seismic section of the both lines, are the results of the tentative of structural interpretation. | nb_NO |
