Anolog Sığ Deniz Sismiği Verilerinin Sayısallaştırılması Ve Su Tabanı Tekrarlı Yansımalarının Bastırılması

Abstract

Kağıt üzerine kaydedilen veri, veri işlem teknikleri için uygun olmamasından dolayı kullanışlı değildir. Bilindiği gibi, kayıtlarda yorumcunun doğru ve güvenilir bir nitelikte yorum yapmasını olumsuz yönde etkileyecek bazı faktörler bulunur. Su tabanı tekrarlı yansımaları, tabaka arası tekrarlı yansımaları, hayalet tekrarlı yansımaları, profil dışı üçüncü boyuttan yansıma ve saçılmalar bunlar arasındadır. Şimdiye kadar tekrarlı yansımaları yok etmek üzere, ters evrişim yönteminden yararlanılmıştır. Fakat bu işlemin uygulanması için de verinin sayısal hale dönüştürülmesi gereklidir. Bu çalışmada sayısallaştırmak suretiyle, kayıttan elde edilen zaman serisi harmoniklerle ifade edilerek veri işlem tekniklerinin uygulanmasına elverişli hale getirilmiştir.

Analog recorders were the basic storage and display media of the early seismic data. Advances in the electronics during the last two decades introduced the digital recording of the seismic data into the magnetic tapes. This new recording technology is widely accepted by the petroleum companies in the acquisition of multi-channel seismic reflection data for deep explorations. The single channel shallow marine seismic data, however, have been recorded in analog form on paper. The main reason for this is the lack of interest of the petroleum industry to the shallow prospects. Meanwhile, the field of engineering geophysics has paid considerable attention to the shallow seismic prospection techniques. Once the seismic data is recorded solely on paper, there is not much to do to improve the quality of the data. The operator has to do all the adjustments (e.g. filters, contrasts, scaling) simultaneously with the data flow. Analog recording gives no chance to playback the data for any modification. Therefore, when digital recording is not available, digitization of analog record is required. From the view of interpretation, existance of multiples in analog records affects interpreters in a negative way. Determining how the actual event proceeds underneath the multiple is not always clear. Eliminating multiples by using deconvolution methods are well known. However, in this work, a simple summation operation was applied to the primary and the multiple, so, the effect of multiple was striven to eliminate or diminish. Whatever the method is, if the data are not digital, processing is often difficult or even impossible. Therefore, the analog record is digitized to be defined in terms of sinusoidal curves so that data-processing techniques can be used on the record. It is better to define how an analog recorder operates before introducing the work done under the scope of this thesis. In marine seismic measurements, the operation of the recorder is as follows: A small device called stylus leaves a trace onto a thermal paper. Also, there is a rotating band that belongs to this recorder system. The thermal paper is wrapped around two cylinders. Each time the stylus is completed its one cycle, the stylus leaves a trace. This provides us roughly with the information about the period of the wiggle. If the recorder is set to + (positive) side, the traces that the stylus draws are the periods of wiggles with positive polarity. There is a threshold level that the recorder can record the data. If the wiggle energy is greater than the threshold level, the recorder starts to record. By deciding whether the traces belong to the same event or represent different events, some parts that the recorder does not record between the sequential traces are thought as wiggles with negative polarity. With this idea, these two traces and the gap are combined with cosine curves. However, the combination operation depends on the range between two traces. If the range is large, the combination operation is not done because traces are thought as parts of different events. Defining the analog record by cosine curves, some processing techniques can be applied to the data. In order to do this, first some triangles are formed from the traces. Then, the top points of these triangles are combined with half periodical cosine curves However, some assumptions were made for conversion from digital state to wiggle state. These are as follows: 1. The traces in analog records are considered as the periods of the positive peaks of sinusoidal signal. 2. The maximum points of the wiggles with positive polarity is in the middle of each trace. 3. There is a reverse proportionality between the periods and amplitudes. 4. If the length between two sequential traces (in pixels) is greater than the sum of the lengths of two traces, these two traces will represent two independent events. 5. The combination point of the continuing borders of triangles formed on the traces will give the maximum point of the wiggle with negative polarity. 6. As a result of combining the top points of the triangles by cosine curves, linear (digital) events are converted into wiggles. VI In order to obtain the wiggles derived by using triangular elements, the necessary formulas were derived. Combination point of the continuing borders of triangles, drawn by making use of the linear wiggle traces with positive polarity, gives the top point of the wiggle with negative polarity among two positive events. By using the top points of the triangles, as a result of drawing half periodical cosine curves between two top points of sequential traces recorded on the thermal paper, conversion process from linearity to wiggle state can be done. This conversion process was done by a Fortran program written for this work. Also, a config program was prepared that executes depending on a main Fortran program. Previously, Nicholas Blake and Charlie Hewlett (1993) worked together to recover digital information from old, lost or corrupted records in order to load new information to a work station. They dealed with already digitized data by using their vectorizing techniques. However in this work, non-digital data are used. Therefore, first, the data are digitized. The purpose is to help interpreters deal with shallow marine seismic analog records that can not be easily interpreted. The work by N. Blake and C. Hewlett (1993) can be used not only in geophysics but also in visual platforms such as photographs, films. In this work, we devised a technique: First to digitize the paper shallow marine seismic sections; second to remove or diminish the influence of severe sea bottom multiples. To accomplish these tasks, two Fortran programs were developed and presented. To digitize the paper shallow marine seismic sections, the original analog data are scanned with a flat-bed optical scanner and saved into a pcx extended file. Then, this file is converted into a binary file formed by the numbers 0 and 1. Scanned data have two colors, black and white. In the binary file, 0 means the dark pixels and 1 means the light ones. Subsequently, the digitized data were processed by using Fortran program that creates the sinusoidal events. As a result, the parameters created were determinedin the main Fortran program and a file called "Config" that interpreters can change the parameters to improve the visibility. In the config program, there are three parameters that users can change to improve the quality of the record image. The first parameter is the scroll amount that the multiple will be given against the primary signal. If the parameter is chosen as zero, operator using the program should mark the horizons on the seismic sections so carefully that the result is optimized. The second parameter determines the frequencies of the wiggles to be formed. If the parameter is chosen to be a large number, there exists less number of wiggles, otherwise, so many wiggles are formed. The third parameter is a kind of coefficient used in the summation of primary and multiple. In other words, it exaggerates the effect of the primary. The numbers 1, 2, and 3 were tried for this parameter in this work. vu As a result of all trials, the values that give the optimized result are the values chosen as zero for the first parameter, one for the second parameter, and two for the last one. Actually, the prgram that operators will use is as follows: The main computer program presented under the scope of this thesis includes several parts such as convertion from analog state to wiggle state, processing step and re-conversion to analog state after some required processes are done. After the required settings or changes are done and exitting from the config file with saving, at DOS prompt, type in "d" to run the executable file of the main program. Then, click.pcx file you want on the first coming screen. After that, notice the messages that will appear on the top right side of the openning.pcx file. When you open a graphic file (.pcx extended file) a "PRIMARY" statement comes appear on the top right side and then disappears. At this stage, user should left-click with mouse on the point of the primary signal that he wants to draw and specify the location of the primary signal to the main program by left-clicking on the different points of the same horizon. When the marking of the primary is completed, user can pass to another step by right-clicking any point with mouse. After that, to mark the horizon where the tail that follows the primary ends, a "TAIL" statement appears as process message. This process can be completed as done in the first process. If user wants to pass to another process, he should just right-click with the mouse. Later, the turn is to mark the horizon where the multiple starts. In order to do this, "MULTIPLE" statement comes and then, disappears. There is an opportunity to re-edit the horizons for mistakes done during the marking process. By pressing "m" key on keyboard, the location of a point where user will left-click with mouse can be changed. With "i" key, you can insert a point on the location you click on. If you want to delete a point from a horizon, just click on the point you want to delete and then,.pcx file is presented, you should only press the "q" key. By pressing the key "p", the process step can be past to message "PROCESS" comes appear. The state of the image after process can be saved by pressing "s" key. After or before saving, you can refresh the screen image by pressing the "r" key. In the config program, there are three parameters that users can change to improve the record image. The first parameter is the scroll amount that the multiple will be given against the primary signal. If the parameter is chosen as zero, operator using the program should mark the horizons so carefully mat the V1U result is optimized. The second parameter determines the frequencies of the wiggles to be formed. If the parameter chosen to be a large number, there exists less number of wiggles, otherwise, too many wiggles are formed. The third parameter is a kind of coefficient used in the summation of primary and multiple. In other words, it exaggerates the effect of primary. The mentioned technique requires a personal computer (4 MB RAM, SVGA color screen), a mouse, a flat-bed optical scanner, and the software package developed under the scope of this thesis.