Time-depth Conversion Research Articles

Accounting for the Different Propagation Velocities for the Focusing and Time–Depth Conversion in a Layered Medium

This contribution deals with the focusing and time–depth conversion of GPR data in a two-layer scenario. In a layered scenario, the different propagation velocities of the electromagnetic waves in different layers can induce distortion in the reconstruction. In particular, it is not possible, with common data processing, to achieve the focusing of both the targets embedded in the upper and deeper layers at the same time. We will present a strategy for mitigating this problem. The strength points of this strategy are its simplicity and the fact that it only requires a commercial code for the GPR data processing plus some steps easily implementable through a homemade code. This work extends a contribution of the same authors at the 20th International GPR conference held in Chanchung, China, in June 2024, and the conclusions are updated with three-dimensional numerical results, including depth slices determined directly in the spatial domain. Both numerical and experimental data will be shown.

Implications of depth conversion on fault geometries and fault-risk assessment in the Smeaheia CO 2 storage site, northern North Sea

Assessments of fault geometries and fault-risk parameters are crucial when evaluating the integrity of a structurally controlled CO 2 storage site. To perform these assessments, seismic data, recorded in time, must be converted to depth. The velocity models used for this time-depth conversion influence the final depth image and, consequently, the geometry of the interpreted faults. Against this background, we create four velocity models for depth conversion, assess the impact on fault throw, dip, and thickness of the primary seal, and, subsequently, fault-risk assessment of the Vette Fault Zone in the Smeaheia CO 2 storage site. We find that fault throw and thickness of the primary seal are more influenced by depth conversion compared to fault dip. In contrast, the overall assessment of membrane seal presence and geomechanical integrity shows less sensitivity to the depth conversion process. Consequently, we suggest that a relatively robust fault-risk assessment can be made with a variety of velocity model designs and data input. Nevertheless, we find a mean difference of 2% shale gouge ratio, 4% slip tendency, and 9% dilation tendency for the Vette Fault Zone emphasizing the importance of accounting for the influence of depth conversion to optimize structural assessments in potential CO 2 storage sites.

An innovative time-depth conversion for the management of buried scenarios with strong discontinuities

This paper proposes a time-depth conversion accounting for discontinuities of the investigated buried scenario, which reverse in discontinuities of the propagation velocity of the waves. The focus is on the imaging of cavities in the framework of GPR prospecting. We show that a time-depth conversion accounting for the different propagation velocity of the waves in the cavity and in the surrounding soil can significantly improve the imaging in an easy way. Both simulated and experimental results will be proposed.

GPR Mapping of Cavities in Complex Scenarios with a Combined Time-Depth Conversion.

The paper deals with a combined time-depth conversion strategy able to improve the reconstruction of voids embedded in an opaque medium, such as cavities, caves, empty hypogeal rooms, and similar targets. The combined time-depth conversion accounts for the propagation velocity of the electromagnetic waves both in free space and in the embedding medium, and it allows better imaging and interpretation of the underground scenario. To assess the strategy's effectiveness, ground penetrating radar (GPR) data referred to as an experimental test in controlled conditions are accounted for and processed by two different approaches to achieve focused images of the scenario under test. The first approach is based on a classical migration algorithm, while the second one faces the imaging as a linear inverse scattering approach. The results corroborate that the combined time-depth conversion improves the imaging in both cases.

Fine characterization of interbedding sand-mudstone based on waveform indication inversion

Abstract Owing to the increasing challenges associated with coal mine exploration and development, extremely precise surveys with high-resolution images are required to support production. Conventional inversion methods cannot provide sufficiently precise images of the complex lithologies in coal measure strata. Accordingly, this study performed research in Qiyuan mining area, Shanxi Province, China, and predicted the complex lithology on the basis of facies control using waveform indication inversion and waveform indication simulation. Horizontal changes in seismic waveforms were used to reflect lithologic assemblage characteristics for facies-controlled constraints, and the vertical mapping the connection between seismic waveform and logging curves was shown. Moreover, high-resolution inversions of wave impedance and natural gamma parameters were conducted. Combined with lithologic shielding and accurate time–depth conversion, the inversions enabled the precise characterization of the lithological assemblage distribution in the study area. Our results showed that waveform indication inversion could distinguish between coal seams and limestone, whereas waveform indication simulation based on natural gamma could effectively distinguish between sandstone and mudstone. Furthermore, the horizontal resolution was improved and the vertical resolution extended to a thickness of 2–3 m. In addition, the inversion results were highly consistent with drilling results, with an error <0.1 m. Therefore, waveform indication inversion and simulation could be applied to coal mines for safe and efficient production.

3D Check shot Modeling for Time-Depth Conversion

Time-depth conversion is one of the keys to successful drilling with a specific target. However, if the drilling target is incorrectly defined, it can cause significant material losses. For example, several cases found that the hydrocarbon depth was deeper than the expected target. As a result, the well will be considered dry even though it still has hydrocarbon reserves. In this study, we intend to develop 3D check shot modeling in which check shot and formation marker data are referred to and taken at the well to obtain a precise time-depth relationship to overcome this problem. The well data with check shot available that can be used for velocity modeling, for the time-depth conversion process is very limited. However, in development fields with dozens or even hundreds of wells, the time-to-depth conversion process becomes a challenge where it is not enough when using one single velocity. In addition, the velocity model must accommodate lateral heterogeneity so the depth structure map can match the well formation markers. On the other hand, seismic data can indirectly measure subsurface velocities vertically and horizontally. This research combines well-check shot data with seismic semblance velocity data to build a velocity model calibrated by well data. The time-to-depth conversion process with the calibrated velocity model has an average residual of less than 5 meters compared to only using check shot data, with an average residual is 25-meters.

A posteriori insertion of information for focusing and time–depth conversion of ground‐penetrating radar data

AbstractThis paper deals with ground‐penetrating radar prospecting and exploits a semi‐heuristic strategy to account for inhomogeneous background media, empty cavities or topography of the surface. We assume here that no more than a commercial processing software is available. Customarily, commercial codes assume a homogeneous soil and a flat interface in order to achieve the focusing of the data. Therefore, this is also the model exploited here, whereas the data are referred to an inhomogeneous soil or to a non‐flat interface. The proposed strategy exploits the principle that ‘the data speak’, even if with some ambiguity and some reticence, and they can reveal or at least suggest important features of the underground scenario. On this basis, heuristically, we exploit features derived from the data themselves as a posteriori information, improving the available focusing and time–depth conversion even having at disposal a basic model of the background scenario.

Precise time-depth conversion of coal measure strata based on velocity splicing: a case application in Qinshui basin

Precise time-depth conversion of coal measure strata based on velocity splicing: a case application in Qinshui basin

Application of Time-Frequency Decomposition and Seismic Attributes for Stratigraphic Interpretation of Thin Reservoirs in Onshore Fuba Field Niger-Delta, Nigeria

Application of time-frequency decomposition and seismic attributes for stratigraphic interpretation of thin reservoirs in Onshore Fuba Field Niger Delta, Nigeria, are here presented, using Well-log and 3D Seismic data. Well-to-seismic tie, fault mapping, horizon mapping, time surface generation, depth conversion and seismic attributes generation were carried out using Petrel software. Structural interpretation of seismic data reveals a highly faulted field. Two distinct horizons were mapped. Fault and horizon interpretation shows closures that are collapsed crestal structures bounded by two major faults. All the interpreted faults are normal synthetic and antithetic faults which are common in the Niger Delta basin. The depth structure maps reveal anticlinal faults. Reservoirs are found at a shallower depth from 6500 to 7500 ft and at a deeper depth ranging from 11500 to 13000 ft. The variance and chaos values range from 0.0 to 1.0. The Variance edge and chaos analysis were used to delineate the prominent and subtle faults in the area while the produced spectra were used to delineate the stratigraphy and thickness of the thin-bedded reservoirs. The results of spectral decomposition at frequencies between 12Hz and 35Hz indicate some thin pay sand zones reservoirs which were characterized by of low frequency and high amplitude associated with known hydrocarbon zones, meandering channels, lobes and the presence of small scale faults in the field. Six new probable zones (Prospects A, B, C, D, E and F) of hydrocarbon accumulation were identified. The results of the study will help in the recovery of more hydrocarbon as by-passed zones and subtle structures are revealed in the area of study.

Cross-validation of time-depth conversion and evaluation of different approaches in the Mesopotamian Basin, Iraq

The time-depth conversion process is a significant task in seismic interpretation to establish the link between geophysical information in the time domain and geological information in the depth domain at/away from well locations. Selecting the suitable velocity model for time-depth conversion to generate an accurate depth map is difficult if the accuracy of these models is unknown. In the current study, the cross-validation technique is used as a tool to diagnose and evaluate the performance of time-depth conversion at/away from well controls to predict the depth of Top Hartha and Zubair reservoirs using the dataset of East Baghdad Oil Field. To test this technique, four common velocity model approaches used for time-depth conversion with different scenarios of velocity parameters (initial velocity V 0 and depth gradient (K)) were applied to produce ten velocity models (1–10). According to the gradient variation of velocity with depth, check shot analysis, the velocity models (1–10) include three key velocities layer-cakes: Layer 1 (Middle Miocene-Upper Cretaceous), Layer 2 (Upper Cretaceous), and Layer 3 (Lower Cretaceous) with 18 horizons from Middle Miocene down to Lower Cretaceous. The cross-validation analysis reveals that the velocity model with a variable surface initial velocity and constant depth gradient (Model 9) was the most accurate with fewer mistie between actual and predicted depth. Consequently, this model is used to construct the depth map of the Hartha and Zubair reservoirs. Finally, this study progresses a workflow that can be applied to the region with any geological setting to investigate time-depth conversion uncertainty.

3D seismic interpretation and prospect identification of the dembe field offshore, Niger delta, Nigeria

3D seismic interpretation and prospect evaluation has been carried out in a hydrocarbon bearing field, standard method of seismic interpretation which includes fault interpretation, well-seismic tie, and horizon mapping were carried out with the aid of seismic attributes such as variance edge and root mean square attributes. Well-seismic-tie established a reasonable tie and indicated that the identified reservoirs were of low impedance as the top of this reservoirs coincided with the troughs. TWT vs Depth plot showed a linear trend which aided in creating a velocity model for the time-depth conversion. Fault mapping revealed that the area was divided into 8 blocks by regionally extensive faults associated with several minor faults which are normal and listric in geometry. Petrophysical analysis showed that the reservoirs has an average porosity of 0.24, water saturation of 0.35, and net-to-gross of 0.78 which are favourable reservoir conditions. A lead Dembe-3, and 2 prospects Dembe-1 and Dembe-2 were identified. Volumetric estimation shows that the prospects boasts of a combined petroleum reserve of about 134 MMSTB of recoverable oil and about 167 BCF of recoverable gas. The results from this study show that, away from currently producing zones of the Dembe field, additional lead and prospects exist which could be further explored to optimize hydrocarbon production in the field. Â

Some results of seismic travel-time reflection tomography study

Velocity model is essential for seismic data processing as it plays an important role in migration processes as well as time depth conversion. There are several techniques to reach that goal, among which tomographic inversion is an efficient one. As an upgrade version of handpicked velocity analysis, the tomography technique is based on the reflection ray tracing and conjugate gradient method to estimate an optimum velocity model and can create an initial high quality model for other intensive imaging and modelling module such as reverse-time migration (RTM) and full-waveform inversion (FWI). For the mentioned benefit, we develop a seismic travel-time reflection tomography (SeisT) module to study the accuracy of the approach along with building the technical capability in seismic processing. The accuracy of the module has been tested by both synthetic and real seismic field data; the efficiency and the accuracy of the model have been proven in terms of development method as well as field data application.

Imaging and depth conversion of low relief fields

Abstract A huge proportion of the world’s known hydrocarbon accumulations reside in low-relief structures (see, for example, (Macgregor, 1996)). Capturing top-reservoir undulation for low-relief fields is crucial, impacting reserve calculations and development decisions. We present a workflow and case study of a greenfield development from onshore the Arabian Plate (Sultanate of Oman) where reprocessing and depth conversion velocity model building led to better characterisation of the reservoir and depth uncertainties. With the subtle structural variation captured in two-way-time, time-depth conversions based on direct velocity data allowed the team to make explicit examination of the consequences of the uncertainty to these low-relief reservoirs. In both the reprocessing and depth conversion, horizontal wells played a novel role in the quality control.

Calibration Method of Petroleum Underground Horizon Based on High Precision Gravity and Magnetic Exploration

Because the high-precision calibration results of the petroleum underground layer are of great significance for oil production efficiency, research on the calibration method of the petroleum underground layer based on high precision gravity and magnetic exploration is researched. The gravity magnetic model is used to retrieve the bedrock depth, and the results of the basement structure and sedimentary rock distribution of the gravity and magnetic geology in the petroleum underground horizon of the Tongbai basin are obtained. On this basis, the geological data, logging data, seismic data, and VSP data are comprehensively used, and the layered calibration method is used to calibrate the petroleum underground layer of the Tongbai basin. Considering the seismic datum and the core elevation in the area, the rock formation is divided by various logging curves. The average time difference and density of the divided rock layers are interpolated at equal depth intervals to obtain velocity sequences and density sequences at equal time intervals and finally realize time-depth conversion. When the drilling geological horizon is unified, the synthetic record of the seismic reflection layer is compared with the geological horizon to realize the horizon calibration of the seismic reflection layer. When the local stratification is not uniform, the seismic reflection layer is calibrated by tracking the seismic reflection layer, high-precision velocity analysis, and various synthetic records to verify the reliability of the geological horizon. The results show that the proposed method can accurately survey the geological conditions of the Tongbai basin. It detected 14 basement faults, and the NW-trending and NE-trending faults controlled the basin, while the north-south faults controlled the later evolution of the basin. The method can be used for the horizon calibration of inclined wells, which is suitable not only for anisotropic media but also for formations with a less lateral variation of local formation lithology. Moreover, its usage is flexible, and it can be corrected by multiple speed data.

Combined Migrations and Time-Depth Conversions in GPR Prospecting: Application to Reinforced Concrete

In this paper, we propose the combination of different migration results achieved on the same data in order to account for different values of the propagation velocities of the electromagnetic waves within the considered Ground Penetrating Radar (GPR) profile. These different values can be the result of a variable lithological composition or (more probably for short Bscans) the results of different moisture levels, or both. Here, we separately consider the two cases of horizontal or vertical variability of the propagation velocity with a transition zone between two zones with constant propagation velocity. Moreover, we also propose a time-depth conversion accounting for these different values of the propagation velocity along the considered GPR Bscan. The method is applied to real data gathered in the field with regard to a concrete coverage containing liner layers.

Hydrothermal Dolomite Paleokarst Reservoir Development in Wolonghe Gasfield, Sichuan Basin, Revealed by Seismic Characterization

Hydrothermal dolomite paleokarst reservoir is a type of porous carbonate reservoir, which has a secondary porosity and can store a large amount of oil and gas underground. The reservoir is formed by magnesium-rich hydrothermal fluids during the karstification and later stages of the transformation. Due to the strong heterogeneity and thin thickness of hydrothermal dolomite paleokarst reservoirs, it is a real challenge to characterize the spatial distribution of the reservoirs. In this paper, we studied the hydrothermal dolomite paleokarst reservoir in the Wolonghe gasfield of the eastern Sichuan Basin. First, based on detailed observations of core samples, the characteristics and storage space types of the dolomite reservoir were described. Secondly, the petrophysical parameters of the paleokarst reservoirs were analyzed, and then the indicator factor for the dolomite reservoirs was established. Thirdly, using the time–depth conversion method, the geological characteristics near boreholes were connected with a three-dimensional (3D) seismic dataset. Several petrophysical parameters were predicted by prestack synchronous inversion technology, including the P-wave velocity, S-wave velocity, P-wave impedance, and the hydrothermal dolomite paleokarst reservoir indicator factor. Finally, the hydrothermal dolomite paleokarst reservoirs were quantitatively predicted, and their distribution model was built. The 3D geophysical characterization approach improves our understanding of hydrothermal dolomite paleokarst reservoirs, and can also be applied to other similar heterogeneous reservoirs.

Physical properties and core-log seismic integration from drilling at the Danube deep-sea fan, Black Sea

Abstract Drilling, coring, and geophysical logging were performed with the MARUM-MeBo200 seafloor drilling rig to investigate gas hydrate occurrences of the Danube deep sea fan, off Romania, Black Sea. Three sites within a channel-levee complex were investigated. Geophysical log data of P-wave velocity, electrical resistivity, and spectral gamma ray are combined with core-derived physical properties of porosity, magnetic susceptibility, and bulk density. Core- and log physical property data are used to define a time-depth conversion by synthetic seismogram modeling, which is then used to interpret the seismic data. Individual polarity reversed reflectors within the stratigraphic column drilled are linked to reduction in P-wave velocity and bulk density. Those reflectors (and associated reflection packages) are accompanied by distinct and systematic changes in sediment porosity, magnetic susceptibility, and electrical resistivity. Overall, the sediments at drill site GeoB22605 (MeBo-17) represent the younger (upper) levee sequence of the channel, that has been eroded at drill site GeoB22603 (MeBo-16). Splicing seismic data across the channel from the East (MeBo-16) to the West (MeBo-17) demonstrates the continuation of reflectors underneath the channel. The upper ~50 m below seafloor (mbsf) at site MeBo-16 do not stratigraphically belong to the same sequence of the (deeper) levee-deposits. Above the marked erosional unconformity, sediments at Site MeBo-16 are likely derived by a mixture of repeated slump-events (identified as seismically transparent units) interbedded with hemi-pelagic sedimentation. Similarly, sediments within the upper ~20 mbsf at Site MeBo-17 are not stratigraphically the same levee-deposits, but are derived by a mixture of slump-events (also seen in the marked seafloor amphitheatre architecture of a large failure complex extending further upslope) and hemi-pelagic sedimentation. All observations combined show that the seismically observed stratigraphic pattern represents a reflectivity sequence mostly driven by variations in density (porosity) and correspondingly by changes in P-wave velocity and electrical resistivity. Observations from the geophysical log- and core, as well as geochemical data, show no evidence for the presence of any significant gas hydrates within the drilled/cored sequences.

(ChinaVis 2019) uncertainty visualization in stratigraphic correlation based on multi-source data fusion

As a most important step in geological interpretation, stratigraphic correlation plays important roles in reservoir estimation and geologic modeling. A variety of datasets are used for stratigraphic correlation, such as well-logging data and seismic data, which are collected by different kinds of sensors. However, much uncertainty will be generated in the traditional course of stratigraphic correlation, because the complex underground geological structures cannot be comprehensively depicted by single dataset. Therefore, in this paper, we propose a visualization system to present and reduce the uncertainty in stratigraphic correlation based on the fusion analysis of multi-source datasets. First, a synthetic seismogram is modeled for each drilling well and a traditional time-depth conversion is conducted to match the seismic data and logging data. Then, an uncertainty model is proposed to quantify the depth difference between seismic horizons and stratigraphic structures extracted from different datasets. Furthermore, a set of visual designs are integrated into an uncertainty visualization system, enabling users to conduct intuitive uncertainty exploration and supervised optimization of stratigraphic correlation. Case studies based on real-world datasets and interviews with domain experts have demonstrated the effectiveness of our system in analyzing the uncertainty of stratigraphic correlation and refining the results of geological interpretation.

An Interactive Integrated Interpretation of GPR and Rayleigh Wave Data Based on the Genetic Algorithm

Ground-penetrating radar (GPR) and the seismic surface wave method are two geophysical techniques commonly used in near-surface surveys up to a depth of tens of meters. GPR can be utilized to clearly distinguish lithologic interfaces, while the seismic surface wave method (hereby referred to as the Rayleigh wave) is employed for its high sensitivity to the shear (S) wave velocity. However, the propagation velocity of the electromagnetic (EM) wave which is both initiated and tracked by GPR is difficult to determine within the stratum. Unfortunately, this can affect the time–depth conversion and interpretation of the radargram. Moreover, both the horizontal resolution and the detectability of the properties of the shallow stratum are limited by the seismic geometry of the Rayleigh wave data. It is important to note that the non-uniqueness of geophysical inversion problem also generates additional biases. To overcome the problems mentioned above, we put forward an interactive integrated geophysical system that utilizes the thickness as a bridge to connect GPR and the Rayleigh wave data in the data processing and inversion. In this study, we employed the velocity of a radagram and the Rayleigh wave dispersion curve inversion based on a genetic algorithm to reconstruct the near-surface distribution. Then, we set the thickness derived from the GPR data as limit of the Rayleigh wave dispersion curve inversion. Hence, we applied the constrained inversion result to determine the most accurate EM wave velocity. As expected, both the constrained inversion of the Rayleigh wave dispersion curve and the velocity correction of the radargram performed better than did the geophysical method alone, both in the numerical models and in the field. Finally, we found the interactive integrated geophysical system to be more conducive for geological interpretation in near-surface surveys.

3-D seismic exploration across the deep geothermal research platform Gro\xdf Sch\xf6nebeck north of Berlin/Germany

The North German Basin is one of the three major type localities in Germany for deep geothermal energy. Here, the pore space is the dominant parameter, in contrast to fractures (Rhine Graben) and karst (Molasse Basin). To further develop the geothermal research platform Groß Schönebeck located in the Northeast German Basin, a subset of the North German Basin, we investigated the geological structure and the existence of possible fault systems in the subsurface. For this purpose, we carried out a high-resolution 3-D reflection seismic survey at the location to overcome methodical restrictions of the few 2-D seismic profiles that cross the area of interest, such as spatial focusing effects and fault imaging. The survey area extends 8 km × 8 km at the surface and focusses down to reservoir depths at 4 km. With four vibrators as source (12–96 Hz sweep, 12 s duration), we used a source line spacing of 700 m and a receiver line spacing of 400 m with both 50 m source and geophone spacing. Data processing encompassed CRS (Common Reflection Surface) stacking, post-stack time migration and depth conversion. We observed a smooth doming of the Zechstein salt from 0.6 to 1 km thickness above the continuous top of the Rotliegend Group at around 4 km depth, and the Mesozoic horizons above appear as mainly continuous reflection surfaces with gentle undulations and occasional normal faulting. We highly resolved the supra-salt sequences in the study area for the first time, which allowed us image an almost complete suite of reflectors mapped in other parts of the Northeast German Basin. However, less resolved, lower frequency images are encountered deeper than ~ 4 km. Two important factors for further field development are that we do not observe an apparent influence of crustal-scale faults, which were expected from former conceptual models for the region, and that at the current status of work, the reservoir does not show a fracture-dominated character.