Changes In Reflection Amplitude Research Articles

An Interdigital Electrode Probe for Detection, Localization and Evaluation of Surface Notch-Type Damage in Metals.

Available microwave notch-type damage detection sensors are typically based on monitoring frequency shift or magnitude changes. However, frequency shift testing needs sweep-frequency data that make scanning detection becomes difficult and time-consuming. This work presents a microwave near-field nondestructive testing sensor for detecting sub-millimeter notch-type damage detection in metallic surfaces. The sensor is loaded with an interdigital electrode element in an open-ended coaxial. It is simple to fabricate and inexpensive, as it is etched on the RC4003 patch by using printed circuit board technology. The detection is achieved by monitoring changes in reflection amplitude, which is caused by perturbing the electromagnetic field around the interdigital structure. The proposed sensor was tested on a metallic plate with different defects, and the experimental results indicated that the interdigital electrode probe can determine the orientation, localization and dimension of surface notch-type damage.

Modeling 3D time-lapse seismic response induced by CO2 by integrating borehole and 3D seismic data – A case study at the Ketzin pilot site, Germany

In order to simulate the 3D time-lapse seismic response from real data in a consistent manner detailed 3D property models at the Ketzin pilot site were constructed by robustly integrating borehole and 3D reflection seismic data. The spatial CO2 distribution and the detailed CO2 density in the reservoir were derived from dynamic flow simulations that had been history-matched to the site monitoring data. Changes in velocity and density after CO2 injection were estimated utilizing the CO2 saturation distributions at two repeat times and fluid substitution models. 4D seismic data were generated by convolving the property models at different times with an extracted wavelet.Time-lapse analysis was performed to qualitatively and quantitatively investigate the changes in reflection amplitude after CO2 injection. Comparison between the synthetic and real data at the corresponding time indicates that the 3D property models were built successfully and model the 3D time-lapse seismic response induced by CO2 injection. A synthetic experiment with two different source wavelets was implemented to investigate the impact of source non-repeatability on the seismic amplitude anomaly. Analysis shows that the same or similar sources should be used in time-lapse seismic monitoring to minimize the impact of source non-repeatability on the monitoring.

Monitoring CO2 Storage at Cranfield, Mississippi with Time-Lapse Offset VSP – Using Integration and Modeling to Reduce Uncertainty

Abstract A time-lapse Offset Vertical Seismic Profile (OVSP) data set was acquired as part of a subsurface monitoring program for geologic sequestration of CO2. The storage site at Cranfield, near Natchez, Mississippi, is part of a detailed area study (DAS) site for geologic carbon sequestration operated by the U.S. Dept. of Energy's Southeast Regional Carbon Sequestration Partnership (SECARB). The DAS site includes three boreholes, an injection well and two monitoring wells. The project team selected the DAS site to examine CO2 sequestration multiphase fluid flow and pressure at the interwell scale in a brine reservoir. The time-lapse (TL) OVSP was part of an integrated monitoring program that included well logs, crosswell seismic, electrical resistance tomography and 4D surface seismic. The goals of the OVSP were to detect the CO2 induced change in seismic response, give information about the spatial distribution of CO2 near the injection well and to help tie the high-resolution borehole monitoring to the 4D surface data. The VSP data were acquired in well CFU 31-F1, which is the ∼3200 m deep CO2 injection well at the DAS site. A preinjection survey was recorded in late 2009 with injection beginning in December 2009, and a post injection survey was conducted in Nov 2010 following injection of about 250 kT of CO2. The sensor array for both surveys was a 50-level, 3-component, Sercel MaxiWave system with 15 m (49 ft) spacing between levels. The source for both surveys was an accelerated weight drop, with different source trucks used for the two surveys. Consistent time-lapse processing was applied to both data sets. Time-lapse processing generated difference corridor stacks to investigate CO2 induced reflection amplitude changes from each source point. Corridor stacks were used for amplitude analysis to maximize the signal-to-noise ratio (S/N) for each shot point. Spatial variation in reflectivity (used to ‘map’ the plume) was similar in magnitude to the corridor stacks but, due to relatively lower S/N, the results were less consistent and more sensitive to processing and therefore are not presented. We examined the overall time-lapse repeatability of the OVSP data using three methods, the NRMS and Predictability (Pred) measures of Kragh and Christie (2002) and the signal-to-distortion ratio (SDR) method of Cantillo (2011) . Because time-lapse noise was comparable to the observed change, multiple methods were used to analyze data reliability. The reflections from the top and base reservoir were identified on the corridor stacks by correlation with a synthetic response generated from the well logs. A consistent change in the corridor stack amplitudes from pre- to post-CO2 injection was found for both the top and base reservoir reflections on all ten shot locations analyzed. In addition to the well-log synthetic response, a finite-difference elastic wave propagation model was built based on rock/fluid properties obtained from well logs, with CO2 induced changes guided by time-lapse crosswell seismic tomography ( Ajo-Franklin et al., 2013 ) acquired at the DAS site. Time-lapse seismic tomography indicated that two reservoir zones were affected by the flood. The modeling established that interpretation of the VSP trough and peak event amplitudes as reflectivity from the top and bottom of reservoir is appropriate even with possible tuning effects. Importantly, this top/base change gives confidence in an interpretation that these changes arise from within the reservoir, not from bounding lithology. The modeled time-lapse change and the observed field data change from 10 shotpoints are in agreement for both magnitude and polarity of amplitude change for top and base of reservoir. Therefore, we conclude the stored CO2 has been successfully detected and, furthermore, the observed seismic reflection change can be applied to Cranfield's 4D surface seismic for spatially delineating the CO2/brine interface.

Seismic and geoelectric modeling studies of parameters controlling CO2 geostorage in saline formations

A possible CO2 geostorage in deep saline formation demands the development of a monitoring strategy using techniques of surface seismic reflection and electrical resistivity tomography (ERT) in boreholes. For this purpose, the resolution and sensitivity of both techniques are studied numerically regarding changes induced by CO2 sequestration.We calculated the effect of partially replacing brine with supercritical CO2 on seismic velocities and densities by using the Gassmann equation. Resulting seismic responses and reflection amplitude changes are quite strong in some cases. For thin CO2 layer (thickness<wavelength) the CO2 anomaly can be delimited laterally quite easier than vertically due to possible reflection overlap from the upper and lower CO2 boundaries.To maximize the ERT resolution, we applied the optimized and tripotential electrode configurations with current flows and potential measurements in different spatial orientations. We performed several comparative 2.5D modelling studies assuming different CO2 saturations, plume scenarios, burial depths, electrode configurations and aspect ratios. The results generally reveal the capability of the techniques to map the storage targets (CO2 plumes, saline host formation and impermeable caprock) with different degrees of resolution, smearing and artefacts. The results show the resolution superiority of the optimized arrays over the other arrays.The highest sensitivity to CO2 saturation changes is found for low saturation case in the seismic and for intermediate and high saturation in the geoelectric case. Therefore, a combination of both methods may be necessary for sn accurate determination of CO2 saturation in situ.A supercritical CO2 plume can be resolved as long as its dimensions are larger than the applied electrode spacing and seismic wavelength, respectively. Thinner layers are problematic, particularly in the ERT technique due to its smearing nature. In the North German Basin strongly the salinity increases strongly with depth, and thus causes a dramatical decrease in the resistivity contrasts and anomalies. The depth effect on the seismic resolution is two-fold: on one hand, the contrast in velocity and density between brine and CO2 increases with depth and enhances the reflection strength accordingly; on the other hand attenuation leads to an increase of wavelength causing a resolution decline.The consideration of seismic results in the inversion of ERT data strongly improves the resolution of the resulting tomograms and the CO2 estimation. It also enables reasonable monitoring (even at aspect ratio<1) of CO2 migrations in large areas and thus reduces the costs of the expensive monitoring wells.

Overall performance evaluation using an equivalent circuit model for radio-frequency single-electron transistors

Charge sensitivities of a radio-frequency single-electron transistor (RF-SET) by using amplitude (AD) and phase-shift detection (PSD) of the reflected RF signals were experimentally studied. It was found that AD is most sensitive at the resonant frequency while PSD is most sensitive at a frequency slightly off the resonance. The best PSD sensitivity is better than the best AD one when the quality factor of a tank circuit is higher than 10; the higher Q-value is, the superior PSD. The maximal change in reflection amplitude and phase-shift were found proportional to the SET conductance change. The above experimental findings were confirmed by the calculation based on an equivalent circuit model.

Visualizing Unsaturated Flow Phenomena Using High‐Resolution Reflection Ground Penetrating Radar

Unsaturated flow phenomena, such as unstable wetting fronts and preferential flow, cannot be investigated using small‐scale sampling. Dye tracer experiments can help visualize the dynamics of water flow but are destructive and therefore irreproducible. We investigated the applicability of high‐resolution ground penetrating radar (GPR) for nondestructive visualization of unsaturated flow patterns arising from a forced infiltration experiment. Synthetic studies using a reflection GPR two‐dimensional finite difference time domain modeling code indicated that differences in water content caused by preferential flow and fingering could be resolved. Moisture content contrasts down to approximately 2.5% within the top 2 m were detectable, but with increasing degrees of heterogeneity in the subsurface it becomes difficult to distinguish these moisture content changes. We conducted a field experiment in which 100 mm (900 L) of Brilliant Blue dyed water was infiltrated across a 3‐ by 3‐m area in relatively homogenous and undisturbed sandy alluvial sediments. Reflection GPR data were collected before and after infiltration. Dye‐staining patterns, revealed by excavating a 2‐m‐deep trench through the infiltration area, were compared with changes in the GPR data. Reflection amplitude changes as well as reflection delay revealed significant differences within the dye‐stained area. The GPR data provided information about the unsaturated flow below the extent of the dye staining, and the results of the synthetic GPR modeling, as well as the observed changes in the real GPR data set, underline the potential of reflection GPR as a nondestructive method to map unsaturated flow phenomena.

Pressure-dependent seismic reflection amplitude changes in crystalline crust: lessons learned at the Continental Deep Drilling Site (KTB)

SUMMARY We conducted an active seismic experiment aimed at measuring changes in seismic reflection amplitudes as a consequence of fresh water injection and corresponding pressure changes at the German Continental Deep Drilling site (KTB). The injection took place at the bottom of the 4-km-deep pilot borehole in the SE2 fault zone in crystalline rock units between the springs of 2004 and 2005. Prior to the experiment, theoretical calculations indicated a possible increase in the compressional wave reflection coefficient as a result of an injection-induced reduction of the seismic velocities within the fault zone. Despite good repeatability of the emitted source signals, the experiment suffered from missing the clear reflection signals expected from the fault zone with regard to seismic data from past experiments. Applying various data-processing steps did not enhance the signals enough to obtain clear reflections or even pressure-dependent reflection amplitude changes. The signal-to-noise ratio remains smaller than the effects under observation. Provided that reflections are present in the data, the error bar of the recorded signals is of the order of 100 per cent. Therefore, we conclude that the experiment was not successful in seismically measuring pressure variations. However, important lessons for land seismic time-lapse measurements in crystalline environments have been learned: (i) The source should be capable of emitting frequencies below 30 Hz. (ii) The detector array setup proved to be partly questionable because in a scattering environment like the crystalline rocks at the KTB site, the incidence of a plane wave precondition might be violated for high-frequency signals. (iii) Near-surface variations of elastic properties likely influence seismic monitoring. (iv) Using a step function, that is a first-order pressure discontinuity, to model the subsurface pressure build-up, is very likely too simple an approach.

Shallow seismic AVO variations related to partial water saturation during a pumping test

High‐resolution shallow seismic reflection experiments were conducted during and after a pumping test of an agricultural irrigation well to image the cone of depression. Although variations in the reflection time from the top of the saturated zone were not observed, amplitude‐versus‐offset (AVO) analysis revealed changes in reflection amplitude responses that correlate temporally and spatially to expected changes to the partially saturated zone induced by the pumping and recovery of the aquifer. The AVO responses exhibit dependence on aquifer drawdown and recovery cycles and the distance from the pumping well. We propose that near‐surface soil heterogeneity and relatively rapid changes in the water table elevation during irrigation cycles caused a thickening of the partially saturated zone above the water table, which resulted in detectable changes in seismic reflection amplitudes. This study offers insights about the response of shallow seismic reflections to changes in subsurface water saturation and the potential application of seismic techniques to hydrogeophysical problems.

Time-lapse crosswell seismic and VSP monitoring of injected CO2 in a brine aquifer

Seismic surveys successfully imaged a small scale CO2 injection (1,600 ton) conducted in a brine aquifer of the Frio Formation near Houston, Texas. These time-lapse borehole seismic surveys, crosswell and vertical seismic profile (VSP), were acquired to monitor the CO2 distribution using two boreholes (the new injection well and a pre-existing well used for monitoring) which are 30 m apart at a depth of 1,500 m. The crosswell survey provided a high-resolution image of the CO2 distribution between the wells via tomographic imaging of the P-wave velocity decrease (up to 500 m/s). The simultaneously acquired S-wave tomography showed little change in S-wave velocity, as expected for fluid substitution. A rock physics model was used to estimate CO2 saturations of 10–20% from the P-wave velocity change. The VSP survey resolved a large (∼70%) change in reflection amplitude for the Frio horizon. This CO2 induced reflection amplitude change allowed estimation of the CO2 extent beyond the monitor well and on three azimuths. The VSP result is compared with numerical modeling of CO2 saturations and is seismically modeled using the velocity change estimated in the crosswell survey.

Seabed characterization through a range of high-resolution acoustic systems – a case study offshore Oman

This study uses three acoustic instruments (different in their operating frequencies, 13, 3.5, and 6–10 kHz, and deployment type, hull-mounted, surface-towed and deep-towed) to investigate and characterize the acoustic response of seafloor NE of Oman in a frequency-independent manner. High-resolution control was achieved by having selected areas of our acoustic transects ground-truthed by sampling and/or sea-floor photography. On the regional scale, the greatest degree of change in backscatter amplitude was correlated with major changes of seabed morphology and lithology. However, small-scale roughness had the biggest effect on amplitude on the local scale, i.e. within each area of specific seafloor type. The study also shows that seafloor reflection amplitude changes are far more easily detected by deep-towed instrument than by surface-towed or hull-mounted systems. Whilst there are significant changes in bioturbation types and density along the transects, the suite of instruments deployed was not able to pick up the effect of the bioturbation on acoustic signals.

Lithology and fluid prediction from amplitude versus offset (AVO) seismic data

AbstractSeismic reflection data as used in the oil industry is acquired and processed as multitrace data with source‐receiver offsets from a few hundred metres (short offset) to several kilometres (long offset). This set of data is referred to as ‘pre‐stack’. The traces are processed by velocity analysis, migration and stacking to yield a data volume of traces with ‘zero‐offset’. The signal‐to‐noise enhancement resulting from this approach is very significant. However, reflection amplitude changes in the pre‐stack domain may also be analysed to yield enhanced rock physics parameter estimates. Pre‐stack seismic data is widely used to predict lithology, reservoir quality and fluid distribution in exploration and production studies. Amplitude versus offset (AVO) data, especially anomalous signals, have been used for decades as indicators of hydrocarbon saturation and favourable reservoir development. Recently, enhanced quantification of these types of measurement, using seismic inversion techniques in the pre‐stack domain, have significantly enhanced the utility of such measurements. Using these techniques, for example, probability of the occurrence of hydrocarbons throughout the seismic data can be estimated, and as a consequence the many pre‐stack volumes acquired in a three‐dimensional (3D) can be survey, reduced to a single, more interpretable volume. The possibilities of 4D time lapse observation extend the measurements to changes in fluid content (and pressure) with time, and with obvious benefits in establishing the accuracy of dynamic reservoir models and improvements in field development planning. As an illustration, recent results from the Nelson Field (UK North Sea), are presented where we show the method by which probability volumes for oil sands may be calculated. The oil–sand probability volumes for three 3D seismic datasets acquired in 1990, 1997 and 2000 are compared and production effects in these data are demonstrated.

THE SEISMIC REFLECTION RESPONSE TO CHANGES IN PRESSURE

Hydrocarbons are often interpreted using seismic attributes, the most common being high anomalous amplitudes or bright spots. Seismic data processing and interpretation is involved in enhancing such anomalies, thereby improving the interpretation for both optimised drilling and volumetric assessment. However, not all bright spots are hydrocarbons since there have been occasions when an economically drillable bright spot unexpectedly produces a dry hole. This paper discusses the first laboratory-based experiments aimed at understanding seismic reflection effects caused by changes in pressure. Water-saturated unconsolidated sand in an unconfining container was pressured from room pressure to over 2 MPa in step manner and then reduced back to room pressure. After each step in application of overburden pressure change, a zero offset 3D seismic survey was performed using ultrasonic transducers. This approach was repeated after the injection of air, to observe the seismic response to gas under changing pressure.The velocity of sand increased as a function of pressure until a value of approximately 1 MPa was attained, when fracturing became apparent. Upon pressure reduction below 2MPa, fracture healing became apparent; there were also rapid changes in reflection amplitudes due to changes in pressure. It is under high pressure that preferential alignment of parallel grains at specific distances relative to the seismic wavelength may result in ‘seismic tuning’, and apparent bright spots. It was noted that a pressure increase caused anti-clockwise rotation of the AVO fluid line; this may provide an indication of whether a change in seismic character is a result of a pressure or gas reduction. Some apparent bright spots within sand/shale sequences may also result from seismic tuning effects developed at specific seismic wavelengths, rather than from the presence of hydrocarbons; the sense of rotation of the fluid line may also help to differentiate between a pressure change or gas reduction.

Estimation of amounts of gas hydrate in marine sediments using amplitude reduction of seismic reflections

Gas hydrates (solid, crystalline water-gas mixtures) exist in sediments just below the seafloor. In seismic profiles, hydrate cementation creates zones of increased velocity and reduced amplitudes of stratal reflections (blanking). By using sediment velocities (estimated by an inversion method), known sediment porosity, and pure hydrate velocity, the amount of hydrate in the highest velocity and most intensively blanked sediments off the southeastern U.S. is calculated; this represents maximum hydrate cementation. To create seismic models of the range of possible blanking effects, ordinary, nonhydrated sediments across a reflecting boundary (caused by a porosity change) are “replaced” with this maximum-hydrate end member in various proportions. Three classes of blanking are defined; class boundaries represent a change in reflection amplitude by a factor of 2, and the classes are relatable to the amounts of hydrate in bulk sediment. In order to estimate the amount of hydrate, these classes are mapped in a grid of reflection profiles processed to preserve relative amplitude, and the first semiquantitative estimate of gas hydrates in deep-sea sediments is produced. [Work supported by DOE.]

The effects of transverse isotropy on reflection amplitude versus offset

Studies have shown that elastic properties of materials such as shale and chalk are anisotropic. With the increasing emphasis on extraction of lithology and fluid content from changes in reflection amplitude with shot‐to‐group offset, one needs to know the effects of anisotropy on reflectivity. Since anisotropy means that velocity depends upon the direction of propagation, this angular dependence of velocity is expected to influence reflectivity changes with offset. These effects might be particularly evident in deltaic sand‐shale sequences since measurements have shown that the P-wave velocity of shales in the horizontal direction can be 20 percent higher than the vertical P-wave velocity. To investigate this behavior, a computer program was written to find the P- and S-wave reflectivities at an interface between two transversely isotropic media with the axis of symmetry perpendicular to the interface. Models for shale‐chalk and shale‐sand P-wave reflectivities were analyzed.

Nature of the eastern boundary of the Rio Grande rift from COCORP surveys in the Albuquerque basin, New Mexico

Reprocessing and synthetic seismic modeling of COCORP profiles across the eastern margin of the Rio Grande rift at Abo Pass, New Mexico, suggest that synthetic and antithetic Cenozoic normal faults sole into or are truncated by a northwest dipping listric master fault which bounds the eastern side of the basin and reaches a depth of at least 10 km beneath the southern Albuquerque basin. This fault appears to project toward the surface position of the Los Pinos Cenozoic normal fault, which lies basinward of and trends parallel to a mapped Laramide duplex thrust zone. The Los Pinos fault may sole into the older thrust, reactivating this preexisting compressional structure at depth. A lateral change in reflection amplitude and frequency is conspicuous across the eastern rift margin. Analysis demonstrates that source coupling is the main cause of a much deteriorated signal below the rift basin, although complex ray paths and attenuation by the graben fill may contribute.

Seismic Lithology and Pitfalls in Seismic Porosity Detection: ABSTRACT

One common method of searching for anomalies associated with increased porosity is to integrate the stacked traces to obtain an acoustic section or, if a suitable relation between density and velocity is known, a interval velocity section. Lateral variations in the color-plotted displays can then be interpreted in terms of porosity variation associated with inferred velocity variation. Buried in this procedure is the assumption that the stacked section represents reflectivity at normal incidence. In the developing technique of seismic lithology, where variations in reflection amplitude with source receiver offset are used to infer lithologic properties, examples are accumulating that show dramatic changes in reflection amplitude with offset within common depth point gathers. Consequently, after stacking, the stacked section will be poorly correlated with normal incidence reflectivity. Recent development in lithologic analysis have led to a procedure that allows the normal incidence reflectivity to be estimated from the offset variation. The results of this procedure show greater continuity and geologic interpretability. More important, upon trace integration, the resultant impedance sections show greater stability leading to a higher confidence level in potential porosity anomalies. End_of_Article - Last_Page 842------------

Seismic Stratigraphy and Global Changes of Sea Level, Part 9: Seismic Interpretation of Clastic Depositional Facies

Depositional facies are predictable from data through an orderly approach to the interpretation of reflections. We term this approach seismic facies analysis. Seismic facies analysis procedures are discussed in detail in Part 6, Vail et al, this volume. Seismic facies types generated by sand-shale strata vary mostly as a function of water depth at the time of deposition. Therefore, a regional environmental framework of shelf, shelf margin, and basin slope and basin floor, provides a useful gross subdivision for classification of clastic facies units. Shelf environments are characterized by general parallelism of reflections. Changes in reflection amplitude, frequency, continuity, interval velocity, and broad, low-relief mounds are the principal factors in defining facies units. The shelf-margin and prograded-slope environment typically contains thick marine sediments and has water depths sufficient for the development of complex arrangements of sigmoid and oblique prograding reflection patterns. The basin slope and floor environment includes a variety of deep basin facies as well as nonprograding slope facies and facies that extend from the slope into the basin deep.

SEISMIC SIGNATURES OF SEDIMENTATION MODELS

Seismic techniques have been used mainly for structural interpretation, but mounting interest in stratigraphic applications is evident. Estimation of sand‐shale ratios from seismically derived average velocities is a recent example of a stratigraphic application. Except in the case of tall pinnacle reefs, today direct location of stratigraphic traps by reflection methods is restricted, at best, to areas of very high quality data and abundant well control. However, it may be possible to interpret some useful stratigraphic characteristics from seismic reflections, the interpretation being based upon the concept of sedimentation models. Most stratigraphic sequences are not random stacks of various lithologies. Commonly, they are well organized and have units with characteristic contacts, thicknesses, lateral extents, lateral facies changes, and vertical sequence. These orderly characteristics are summarized in sedimentation models, where the control of lithologic distribution by dominant depositional processes is emphasized. Three sedimentation models for sandstone and shale sequences are presented. For each, one example is described and converted to a synthetic reflection seismic cross‐section. These cross‐sections are each distinct in terms of reflection polarities, areal changes in reflection amplitudes, continuity of events, and lateral interval velocity changes. The simplified models, although limited in their scope, suggest that additional stratigraphic information can be gleaned from reflection seismic data. To exploit this promise, record processing techniques that emphasize recognition of reflection polarities, amplitudes, continuity, and interval velocities must be developed or improved. It is also necessary to improve our knowledge of seismic boundaries in a variety of stratigraphic sequences. Though difficult, these valuable goals appear attainable.