Dissertations / Theses on the topic 'Synthethic seismograms'

The aim of this thesis is to develop procedures for the modelling and inversion of teleseismic P and S waveforms which are as flexible as possible. This flexibility is necessary in order to obtain accurate source depth and mechanism estimates for small to moderate size events, such as those that are relevant in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). ¶ The main challenge for extending source depth and mechanism inversion methods to smaller events is to ensure that sufficiently accurate synthetic seismograms are available for comparison with observed records. An accurate phase-adaptive reflectivity method has therefore been developed, against which the performance of less computationally intensive approximations can be judged. The standard reflectivity method has been modified to allow for different crustal and upper mantle structures at the source and receiver, and the full effects of reverberations and conversions in these structures can be allowed for. Core reflections and refractions can also be included; these phases can become important at certain distance ranges. A slowness bundle approach has been developed, where a restricted slowness integration about the geometric slowness for the direct wave is undertaken at each frequency, allowing accurate results to be obtained whilst avoiding the expense of a full reflectivity technique. ¶ Inversion using the neighbourhood algorithm (NA) is performed for source depth, mechanism and time function, by modelling direct P and S and their surface reflections (pP, sP and pS, sS) at teleseismic distances. Both SV and SH data are exploited in the inversion, in addition to P data, in order to obtain improved constraints on the source mechanism, including any isotropic component. Good results are obtained using a simple generalised ray scheme, however, the use of a flexible derivative-free inversion method means that more accurate synthetics are able to be used in the inversion where appropriate. The NA makes use of only the rank of the data misfits, so that it is possible to employ any suitable misfit criterion. In the few cases where control on the source mechanism is limited, good depth resolution is still usually obtained. ¶The structures near the source and receiver play an important role in shaping the detail of the teleseismic waveforms. Although reasonable results can be achieved with simple synthetics and a standard velocity model, significant improvement can be made by modifying the representation of structure near the source and receiver. In the case of sub-oceanic events it is important to allow for the effects of water reverberations. The crustal structure near the receiver can also have quite a large influence on the waveforms through reverberations and conversions. This is exploited in receiver function inversion, which is again accomplished using the NA approach.

On construit un modèle de vitesse qui correspond a la structure du manteau supérieur de l'Europe en utilisant des stations européennes les ondes de volume p et pp et des séismes qui ont lieu dans le domaine égéen. Pour cela on utilise la méthode des faisceaux gaussiens. Il est alors possible pour des séismes dont D<30**(O) de déterminer leur mécanisme au foyer et leur intensité

The study area, Dhurnal oil field, is located 74 km southwest of Islamabad in the Potwar basin of Pakistan. Discovered in March 1984, the field was developed with four producing wells and three water injection wells. Three main limestone reservoirs of Eocene and Paleocene ages are present in this field. These limestone reservoirs are tectonically fractured and all the production is derived from these fractures. The overlying claystone formation of Miocene age provides vertical and lateral seal to the Paleocene and Permian carbonates. The field started production in May 1984, reaching a maximum rate of 19370 BOPD in November 1989. Currently Dhurnal‐1 (D-1) and Dhurnal‐6 (D-6) wells are producing 135 BOPD and 0.65 MMCF/D gas. The field has depleted after producing over 50 million Bbls of oil and 130 BCF of gas from naturally fractured low energy shelf carbonates of the Eocene, Paleocene and Permian reservoirs. Preliminary geological and geophysical data evaluation of Dhurnal field revealed the presence of an up-dip anticlinal structure between D-1 and D-6 wells, seen on new 2003 reprocessed data. However, this structural impression is not observed on old 1987 processed data. The aim of this research is to compare and evaluate old and new reprocessed data in order to identify possible factors affecting the structural configuration. For this purpose, a detailed interpretation of old and new reprocessed data is carried out and results clearly demonstrate that structural compartmentalization exists in Dhurnal field (based on 2003 data). Therefore, to further analyse the available data sets, processing sequences pertaining to both vintages have been examined. After great effort and detailed investigation, it is concluded that the major parameter giving rise to this data discrepancy is the velocity analysis done with different gridding intervals. The detailed and dense velocity analysis carried out on the data in 2003 was able to image the subtle anticlinal feature, which was missed on the 1987 processed seismic data due to sparse gridding. In addition to this, about 105 sq.km 3D seismic data recently (2009) acquired by Ocean Pakistan Limited (OPL) is also interpreted in this project to gain greater confidence on the results. The 3D geophysical interpretation confirmed the findings and aided in accurately mapping the remaining hydrocarbon potential of Dhurnal field.

3D seismic attribute analysis of the Jurassic Entrada/Curtis interval within the North Hill Creek (NHC) survey has been useful in delineating reservoir quality eolian-influenced dune complexes. Amplitude, average reflection strength and spectral decomposition appear to be most useful in locating reservoir quality dune complexes, outlining their geometry and possibly displaying lateral changes in thickness. Cross sectional views displaying toplap features likely indicate an unconformity between Entrada clinoforms below and Curtis planar beds above. This relationship may aid the explorationist in discovering this important seismic interval. Seismic and well log attribute values were cross plotted and have revealed associations between these data. Cross plots are accompanied by regression lines and R2 values which support our interpretations. Although reservoir quality dune complexes may be delineated, the Entrada/Curtis play appears to be mainly structural. The best producing wells in the survey are associated with structural or stratigraphic relief and the thickest Entrada/Curtis intervals. Structural and stratigraphic traps are not always associated with laterally extensive dune complexes. Time structure maps as well as isochron maps have proven useful in delineating the thickest and/or gas prone portions of the Entrada/Curtis interval as well as areas with structural and stratigraphic relief. We have observed that the zones of best production are associated with low gamma ray (40-60 API) values. These low values are associated with zones of high amplitude. Thus, max peak amplitude as a seismic attribute may delineate areas of higher sand content (i.e. dune complexes) whereas zones of low amplitude may represent areas of lower sand content (i.e. muddier interdune or tidal flat facies). Lack of significant average porosity does not seem to be related to a lack of production. In fact, the best producing wells have been drilled in Entrada/Curtis intervals where average porosity is near 4 %. There are however zones within the upper portion of the Entrada/Curtis that are 40 ft. (12.2 m) thick and have porosities between 14% and 20%. By combining derived attribute maps with observed cross plot relationships, it appears that the best producing intervals within the Entrada/Curtis are those associated with high amplitudes, API values from 40-60 and structural relief.

The aim of this thesis is to develop procedures for the modelling and inversion of teleseismic P and S waveforms which are as flexible as possible. This flexibility is necessary in order to obtain accurate source depth and mechanism estimates for small to moderate size events, such as those that are relevant in the context of monitoring the Comprehensive Nuclear-Test-Ban Treaty (CTBT). ¶ The main challenge for extending source depth and mechanism inversion methods to smaller events is to ensure that sufficiently accurate synthetic seismograms are available for comparison with observed records. An accurate phase-adaptive reflectivity method has therefore been developed, against which the performance of less computationally intensive approximations can be judged. The standard reflectivity method has been modified to allow for different crustal and upper mantle structures at the source and receiver, and the full effects of reverberations and conversions in these structures can be allowed for. Core reflections and refractions can also be included; these phases can become important at certain distance ranges. A slowness bundle approach has been developed, where a restricted slowness integration about the geometric slowness for the direct wave is undertaken at each frequency, allowing accurate results to be obtained whilst avoiding the expense of a full reflectivity technique. ¶ ...

博士
國立成功大學
資源工程學系
102
The direct cross-plotting technique that involves a graph of resistivity or neutron log data versus sonic log data is often used to create a pseudo-sonic log. With the resulting transform, a pseudo-synthetic seismogram can then be generated to aid seismic interpreters. However, pseudo-synthetic seismograms generated in this way often do not match the standard synthetic seismograms (made from the sonic and density logs) if either the “gas effect” for the neutron log versus the sonic log cross-plot or the “hydrocarbon effect” for the resistivity log versus the sonic log cross-plot is not corrected. The compensated neutron log (CNL) and the short normal resistivity log (SN) were carefully compared to a borehole-compensated sonic log (BHC) from the same well in Taiwan. Semi-empirical relationships from CNL versus BHC and SN versus BHC plots were reviewed from the literature. Then, we developed a method to transform a neutron log into a pseudo-sonic log and a resistivity log into a pseudo-sonic log by splicing together several continuous sections of the best-fit line functions according to formation tops. Additionally, sandstones in which the gas effect or hydrocarbon effect caused the data to be scattered were separately plotted. The presented examples show that the new method produces a pseudo-sonic curve that would generate the pseudo-synthetic seismogram most similar to the standard synthetic seismogram according to frequency, amplitude and polarity. Blind tests in northwestern Taiwan were performed in two wells to test the effectiveness of the new techniques, and the results were good. The method has also been proven to be applicable in carbonate-dominated log sections, as evidenced by application of the techniques to a well in Kansas, USA. The techniques presented in this dissertation can be used to improve the conventional transform of a neutron log into a pseudo-sonic log or a resistivity log into a pseudo-sonic log by employing the direct cross-plotting technique. This, in turn, may be particularly useful for oil and gas exploratory and development areas where neutron logs or resistivity logs are more prevalent than sonic logs.

碩士
國立成功大學
資源工程學系
104
The seismic profile is useful to identify different subsurface formations in the geophysical exploration for hydrocarbons. The surface conditions may not be appropriate for making a seismic reflection survey in some situations, and the results may be affected by several geological factors. In this study, various logs such as gamma ray, spontaneous potential, and resistivity from three wells in southwestern Taiwan consisting primarily of clastic rocks, were used to calculate pseudo-synthetic seismograms for portions of wells where sonic log quality was poor or missing and for entire wells where the sonic and/or density logs were never run. In this thesis, the gamma ray log is modified with the trend of a pseudo P-wave velocity log that is converted from the resistivity log by the Faust equation to make an alternative pseudo-sonic equation. The thesis includes several methods for generating pseudo-synthetic seismograms: well logs comparison, adjusted Faust equation for pseudo P-wave velocity, zones classification, logs trend modification, regular and alternative cross-plot techniques, and pseudo gamma ray from spontaneous potential log by shale volume conversion. According to the construction results for three wells, the pseudo-synthetic seismograms correlate well with seismic reflection profiles in two wells; the other well’s correlation is less certain due to the poor quality of the seismic reflection profile, despite the good match between the original P-wave velocity log and the pseudo P-wave velocity log. Hopefully these valuable techniques can be applied to aid the seismic interpreter, resulting in enhanced efforts for seismic interpretation.

博士
國立成功大學
地球科學系
107
The “conventional” synthetic seismogram (CSS) that is used in oil and gas interpretation is generated from sonic and density logs. Alternatively, gamma-ray and spontaneous potential logs can be combined with a density log to create a pseudo-synthetic seismogram (PSS) by using different methods as demonstrated from the logs of wells in Kansas, USA. Herein we introduce several new methods for constructing pseudo-synthetic seismograms by editing (Method-I), by considering major shifts in the velocity log curve (Method-II), by deleting the outliers both from gamma-ray and sonic (ITT) logs by statistical formulas (Method-III), the borehole corrections of gamma-ray logs (Method-IV) (due to extremely high gamma-ray readings), by combining the gamma-ray (GR) and self-potential (SP) logs to produce the spontaneous ray (SR) log (Method-V), and by deleting the outliers by using the statistical formulas resembling Method-III but applied only to the outliers of the gamma-ray log (Method-VI). The values were cross-plotted against the sonic ITT values to determine a linear transform for producing a pseudo-sonic (PS) log and, ultimately, a pseudo-synthetic seismogram. After generating the pseudo-sonic logs, there were several ways to compare the pseudo-sonic with the sonic ITT. The first four methods (Method-I to Method-IV) were compared visually, and the second two were compared statistically between the conventional and pseudo-sonic logs. The Nash-Sutcliffe efficiency (NSE) formula was used in Method-V and the Pearson product-moment correlation coefficient (PCC) was used in Method-V and Method-VI to compare between the pseudo-sonic and the sonic (ITT) (Quadir et al. 2018a; Quadir et al. 2018b). The range for the Nash-Sutcliffe efficiency (NSE) acceptable value for the pseudo-sonic logs of three wells was from 78% to 83% (Method-V). This technique (Method-V) was tested on three wells, one of which was used as a blind test well, with satisfactory results. The Pearson product-moment correlation coefficient (PCC) value between the composite PS (SR) log with low density correction and the conventional sonic (CS) log was 86% (Method-V). To demonstrate a wider-range application of our method (Method-VI), the procedure was applied to wells from the Hugoton Embayment (HE), Central Kansas Uplift (CKU), Sedgwick Basin (SGB), Salina Basin (SB), Forest City Basin (FCB) and Nemaha Uplift (NU). The correlation coefficient between the PS and the conventional sonic (ITT) was 0.75, 0.92, 0.86, 0.91, 0.77 and 0.70, respectively. Also, the match between the resulting conventional synthetic seismogram (CSS) and the pseudo-synthetic seismogram (PSS) from a blind test well for each area was quite good. Because of the common occurrence of spontaneous potential and gamma-ray logs in many of the hydrocarbon basins of the world, this inexpensive and straightforward technique (Method-V) could hold significant promise in areas that are in need of alternate ways to create pseudo-synthetic seismograms for seismic reflection interpretation. Provided the outliers have been properly treated (Method-VI), the GR log is a viable tool for creating pseudo-sonic logs and pseudo-synthetic seismograms for exploration in oil and gas basins where there are few wells with sonic logs or where sonic log quality is poor. The economical and straightforward technique presented here for creating the pseudo-synthetic seismograms from the gamma-ray and SP logs can be useful for exploration in oil and gas basins where there are few wells with sonic logs and where high gamma-ray log may cause anomalous pseudo-velocities.

This thesis integrated geology, geophysics, and petroleum engineering data to build a detailed reservoir characterization models for three gas pay sands in the Grand Isle 33 & 43 fields, offshore Louisiana. The reservoirs are Late Miocene in age and include the upper (PM), middle (QH), and lower (RD) sands. The reservoir models address the stratigraphy of the upper (PM) sand and help delineate the lower (RD) reservoir. In addition, this research addresses the partially depleted QH-2 reservoir compartment. The detailed models were constructed by integrating seismic, well log, and production data. These detailed models can help locate recoverable oil and gas that has been left behind. The upper PM model further delineated that the PM sand has several areas that are shaled-out effectively creating a flow barrier within reservoir compartments. Due to the barrier in the PM-1 reservoir compartment, an area of potentially recoverable hydrocarbons remains. In Grand Isle 33, the middle QH sand was partially depleted in the QH-2 reservoir compartment by a series of development wells. Bottom hole pressure data from wells in Grand Isle 32 & 33 reveal that the two QH fault compartments are in communication across a leaking fault. Production wells in the QH-1 compartment produced reserves from the QH-2 compartment. The lower RD sand model helped further delineate the reservoir in the RD-2 compartment and show that this compartment has been depleted. The RD model also shows the possible presence of remaining recoverable hydrocarbons in the RD-1 compartment. It is estimated that about 6.7 billion cubic feet of gas might remain within this reservoir waiting to be recovered. A seismic amplitude anomaly response from the QH and RD sands is interpreted to be a lithologic indicator rather than the presence of hydrocarbons. Amplitude response from the PM level appears to be below the resolution of the seismic data. A synthetic seismogram model was generated to represent the PM and surrounding sands. This model shows that by increasing the frequency of the seismic data from 20 Hz to a dominant frequency of 30 Hz that the PM and surrounding sands could be seismically resolvable. Also the PM-1 compartment has possible recoverable hydrocarbons of 1.5 billion cubic feet of gas remaining.

Purely numerical methods based on the Finite Element Method (FEM) are becoming increasingly popular in seismic modeling for the propagation of acoustic and elastic waves in geophysical models. These methods o er a better control on the accuracy and more geometrical exibility than the Finite Di erence methods that have been traditionally used for the generation of synthetic seismograms. However, the success of these methods has outpaced their analytic validation. The accuracy of the FEMs used for seismic wave propagation is unknown in most cases and therefore the simulation parameters in numerical experiments are determined by empirical rules. I focus on two methods that are particularly suited for seismic modeling: the Spectral Element Method (SEM) and the Interior-Penalty Discontinuous Galerkin Method (IP-DGM). The goals of this research are to investigate the grid dispersion and stability of SEM and IP-DGM, to implement these methods and to apply them to subsurface models to obtain synthetic seismograms. In order to analyze the grid dispersion and stability, I use the von Neumann method (plane wave analysis) to obtain a generalized eigenvalue problem. I show that the eigenvalues are related to the grid dispersion and that, with certain assumptions, the size of the eigenvalue problem can be reduced from the total number of degrees of freedom to one proportional to the number of degrees of freedom inside one element. The grid dispersion results indicate that SEM of degree greater than 4 is isotropic and has a very low dispersion. Similar dispersion properties are observed for the symmetric formulation of IP-DGM of degree greater than 4 using nodal basis functions. The low dispersion of these methods allows for a sampling ratio of 4 nodes per wavelength to be used. On the other hand, the stability analysis shows that, in the elastic case, the size of the time step required in IP-DGM is approximately 6 times smaller than that of SEM. The results from the analysis are con rmed by numerical experiments performed using an implementation of these methods. The methods are tested using two benchmarks: Lamb's problems and the SEG/EAGE salt dome model.
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The Gulf of Mexico Basin Opening project (GUMBO) is a study of the lithological composition and structural evolution of the Gulf of Mexico (GoM) that uses Ocean Bottom Seismometer (OBS) data from four transects in the Northern GoM. I examine 39 OBS shot records in the easternmost transect for shear wave arrivals and pick shear wave travel times from the 11 usable records. I then carry out a tomographic inversion of seismic refraction travel times. I use the resulting shear-wave velocity model in conjunction with a previously constructed P-wave model to examine the relationship between Vp and Vs. I compare velocities in the sediment and basement with empirical velocities from previous studies for the purpose of constraining lithological composition below the transect and make an interpretation of the structural evolution of the eastern GoM. The seismic velocities for crust landward of the Florida Escarpment are consistent with normal continental crust. Seaward of the Escarpment, velocities in the upper oceanic crust are anomalously high (Vp = 6.5 – 7 km/sec; Vs = 4.0 – 4.6 km/sec). A possible explanation for this observation is that GoM basalt formation consisted of basaltic sheet flows, forming oceanic crust that does not contain the vesicularity and lower seismic velocities found in typical pillow basalts. Increased magnesium and iron content could also account for these high velocities. Seismic refraction and reflection data provide a means of investigating the nature of the Moho in the northeastern GoM. I use a finite difference method to generate synthetic record sections for data from eight instruments that are part of the two easternmost GUMBO seismic lines (lines 3 & 4). I then vary the thickness of the Moho in these synthetic models and compare the results with the original receiver gather to examine the effects this variability has on amplitudes. The data from the instruments chosen for these two lines are representative of continental and transitional crust. The finite difference models indicate that the Moho beneath GUMBO 3 is ~1500 m thick based on the onset and amplitudes of PmP arrivals. All five instruments display consistent results. The instruments along GUMBO 4 suggest a Moho almost twice as thick as GUMBO 3 on the landward end of the transect that grades into a Moho of similar thickness (1750 m) in the deep water GoM. The three instruments used to model the Moho in this area show that the Moho ranges from ~1750 to 3500 m in thickness. The sharper boundary beneath continental crust in GUMBO Line 3 supports other evidence that suggests magmatic underplating and volcanism in the northern GoM during the mid-Jurassic. The thicker Moho seen on the landward end of GUMBO Line 4 that is overlain by continental crust was likely unaffected by GoM rifting. Therefore, the Moho beneath the Florida Platform might be as old as the Suwannee Terrane, and complex Moho structure is not uncommon for ancient continental crust.
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