Amplitude Blanking Research Articles

THE POSSIBILITY OF USING STRUCTURAL DAMPING IN THE DESIGN OF A PREFABRICATED TURNING CUTTER TO REDUCE THE AMPLITUDE OF SELF-OSCILLATIONS IN THE PROCESS OF METAL CUTTING

A mathematical model of four mass oscillating circuit cutting machine was developed and studied in this paper. The influence of the internal friction in the joints of structural elements on the machine tool vibration amplitude and details. Effective self-oscillation amplitude blanking tool is possible by choosing the optimal parameters of the tool body frictional connection with tool holders

Natural Gas Resource from Gas Hydrates Offshore the East Coast of Trinidad

SummaryThe global shift toward environmentally friendly fuels has led to an increase in the usage of natural gas as an energy source. Natural gas is currently being produced from conventional gas reservoirs; however, there has been increased interest in the large repositories of highly compressed natural gas sequestered in gas hydrates. Well logs, as well as 3D seismic surveys, aid in the identification of geophysical features which support the presence of natural gas hydrates. Geophysical features include bottom-simulating reflectors (BSRs), amplitude blanking, and enhanced reflections. Previous studies of the deepwater blocks offshore the eastern coast of Trinidad provided conflicting conclusions regarding the existence of gas hydrate-bearing sediments in Blocks 25A, 25B, 26, and 27. This study investigated not only the presence, but additionally the location, depth of the BSRs, and associated natural gas hydrate-bearing sediments, as well as the volumetric estimation of the hydrate-bound gas in these blocks, as well as Block 4B. Six gas hydrate-related BSRs were identified; one each in Blocks 4B, 25A, and 26, and three in Block 27. No BSRs were observed in Block 25B. Closer inspection revealed that the BSR in Block 26 is an extension of one of the BSRs in Block 27. A total gas resource in place is estimated to be 4–19 Tcf, which indicates that the eastern coast of Trinidad has the potential to provide additional natural gas reserves.

Shallow seismic characteristics and distribution of gas in lacustrine sediments at Lake Erçek, Eastern Anatolia, Turkey, from high-resolution seismic data

The high-resolution analysis of single-channel, seismic reflection data from Lake Erçek (Eastern Anatolia) revealed a wide range of shallow gas anomalies consisting of enhanced reflections, seismic chimneys, acoustic blanking/acoustic turbidity, strong reflectors, and pockmarks, including both surface and buried pockmarks. The enhanced reflections are represented by the higher amplitude reflection patterns resulting from high acoustic impedance variations. They are mostly clustered in the NW-corner of the lake. Seismic chimneys are represented by vertical and thinned columnar disturbances of amplitude blanking and mostly occurred in deep basinal and faulted sections in the West and East of the lake. Some seismic chimneys, occurring together with pockmarks, represent vertical vent activations. Acoustic gas masking was represented by chaotic and diffuse seismic reflection patterns, including acoustic blanking and acoustic turbidity. As diffuse acoustic turbidity indicates gas-charged sediments, columnar disturbances showing acoustic blanking indicate degassing of the sediments. These features extend from SE to NW, coinciding with the deep basin morphology of the lake. A very local strong reflector was identified in the W-section of the lake, simulating the lake floor. This reflector is due to extended enhanced reflections, suggesting shallow free gas. Pockmarks observed in the lake are structurally classified into the two distinct types; surface (active) pockmarks found in the SE-part of the lake and buried (passive) pockmarks found in the NW. The former enlarge through deeper gas reservoir feedback, as the layering is impermeable, while the latter have resulted from a cessation of the reservoir feedback mechanism and/or permeable layering. In the lake, shallow gas distribution is controlled by faults, that provide the faulting-driven depositional control and earthquakes, that provide the seismicity-driven overpressure control. The shallow gas is then vertically–horizontally distributed and shaped by asymmetric depositional–stratigraphic factors. This study of Lake Erçek presents complementary information about a possible tectono-thermal origin of observed shallow gas.

Detection of gas chimney and its linkage with deep-seated reservoir in poseidon, NW shelf, Australia from 3D seismic data using multi-attribute analysis and artificial neural network approach

Accurate delineation of hydrocarbon seepage has significant implications in accentuating hydrocarbon migration pathways and assessing seal integrity thereby alleviating drilling hazards. Knowledge of migration pathways and understanding about the source/reservoir plays a vital role in successful evaluation of hydrocarbon seepage. Many researchers have reported events of gas leakage in Poseidon area; however, it has never been investigated in detail to confirm the origin of the gas leakage and their migration pathways. In this study, an attempt has been made to decipher the relationship between shallow gas migration expressions such as pockmarks, mud-volcanos, direct hydrocarbon indicators (DHIs), push-downs, amplitude blanking, with the reservoir present in the study area. Adopted approach includes development of a chimney probability cube (CPC), in which extracted seismic attributes are optimally combined using a non-linear multi-layer perceptron (MLP) network. During the training and testing phase of the 3-layer MLP network, 0.3–0.25 normalized RMS error and <5% of misclassification is achieved. The processed cube is further validated using spectral decomposition approach, and a cross-plot of Lambda-Rho (petrophysical parameters) versus obtained chimney probability values. The CPC facilitates an enhanced 3D visualization of gas flow channels and deciphers its association with an existing deep-seated reservoir in Plover Formation. The results of this study will provide significant input for hydrocarbon prospect evaluation as well as for making informed drilling decisions in the study area.

On the Performance of Blanking Nonlinearity in Real-Valued OFDM-Based PLC

To mitigate the impulsive noise in orthogonal frequency-division multiplexing (OFDM)-based power-line communications (PLC), a blanker can be employed at the receiver. In the literature, the performance of the blanker has been discussed and analyzed thoroughly. However, the analysis was limited only to passband OFDM PLC systems with complex-valued baseband representation. This paper aims to derive a mathematical expression of the output signal-to-noise ratio (SNR) of a blanker in baseband OFDM-based PLC, which are real-valued and used in some PLC standards. We also analyze the performance of separate real/imaginary blanking, instead of the traditional amplitude blanking, in passband OFDM-based PLC systems. It will be shown that the performance of amplitude blanking is similar to separate real/imaginary blanking in high signal-to-impulsivenoise ratio (SINR) values. However, when the SINR is low, the performance of amplitude blanking is better than the separate real/imaginary blanking. The difference is about 1 dB for SINR = -10 dB and 1.5 dB for SINR = -30 dB when signalto-background-noise ratio, SBNR = 25 dB and impulsive noise probability p = 0.01.

Variations in pockmark composition at the Vestnesa Ridge: Insights from marine controlled source electromagnetic and seismic data

AbstractThe Vestnesa Ridge marks the northern boundary of a known submarine gas hydrate province in the west Svalbard margin. Several seafloor pockmarks at the eastern segment of the ridge are sites of active methane venting. Until recently, seismic reflection data were the main tool for imaging beneath the ridge. Coincident controlled source electromagnetic (CSEM), high‐resolution two‐dimensional (2‐D) airgun, sweep frequency SYSIF, and three‐dimensional (3‐D) p‐cable seismic reflection data were acquired at the south‐eastern part of the ridge between 2011 and 2013. The CSEM and seismic data contain profiles across and along the ridge, passing several active and inactive pockmarks. Joint interpretation of resistivity models obtained from CSEM and seismic reflection data provides new information regarding the fluid composition beneath the pockmarks. There is considerable variation in transverse resistance and seismic reflection characteristics of the gas hydrate stability zone (GHSZ) between the ridge flanks and chimneys beneath pockmarks. Layered seismic reflectors on the flanks are associated with around 300 Ωm2 transverse resistance, whereas the seismic reflectors within the chimneys exhibit amplitude blanking and chaotic patterns. The transverse resistance of the GHSZ within the chimneys vary between 400 and 1200 Ωm2. Variance attributes obtained from the 3‐D p‐cable data also highlight faults and chimneys, which coincide with the resistivity anomalies. Based on the joint data interpretation, widespread gas hydrate presence is likely at the ridge, with both hydrates and free gas contained within the faults and chimneys. However, at the active chimneys the effect of gas likely dominates the resistive anomalies.

Seismic evidence of shallow gas from Lake Van, eastern Turkey

Analysis of multi-channel seismic reflection and chirp data from Lake Van (eastern Turkey) reveals various shallow gas indicators including seismic chimneys, enhanced reflections, bright spots, mud volcanoes, pockmarks, and acoustic blanking. The enhanced reflections, suggesting the presence of free gas, are most dominant and observed at more than 200 locations. They are characterized by very-high amplitude reflections and occur in both deep and shallow sedimentary sections. Some enhanced reflections are accompanied by very subtle seafloor expressions such as mounds, which may suggest active venting activity. Seismic chimneys or columnar zones of amplitude blanking have been observed in much of the surveyed area. Seismic chimneys in the study area cannot be associated with any known faults that would act as migration pathways for deep fluids. This suggests that the observed structures in Lake Van sediments allow the preferential emission of gases which might be for a large share of biogenic origin. The acoustic blanking, characterized by transparent or chaotic seismic facies, is seen in the eastern part of the lake. The lakeward edge of the acoustic blanking largely coincides with the 100 m water depth contour, indicating that (past) changes of the hydrostatic pressure may be responsible for the distribution of these anomalies. Mound-like features, interpreted as mud volcanoes, occur in a few locations. The presence of these features may suggest active gas emission. Very strong amplitude anomalies or bright spots with negative polarity, indicating gas-charged zones, are also seen in a number of locations. Pockmarks are observed only in the northeastern part of the study area. The scarce occurrence of pockmarks in the study area might be ascribed to a higher permeability of the lake sediments or to the absence of the substrate/reservoir providing the critical mass of gases necessary to produce such features. Turbidites, tephra layers, and deltaic deposits have the potential to provide ideal conditions to allow the sediments to act as a gas reservoir.

Imaging permafrost velocity structure using high resolution 3D seismic tomography

A 3D seismic survey (Mallik 3D), covering [Formula: see text] in the Mackenzie Delta area of Canada’s north, was conducted by industry in 2002. Numerous lakes and marine inundation create a complex near-surface structure in the permafrost terrain. Much of the near subsurface remains frozen but significant melt zones exist particularly from perennially unfrozen water bodies. This results in an irregular distribution of permafrost ice creating a complex pattern of low and high frequency near-surface velocity variations which induce significant traveltime distortions in surface seismic data. A high resolution 3D traveltime tomography study was employed to map the permafrost velocity structure utilizing first-arrival traveltimes picked from 3D seismic shot records. Approximately 900,000 traveltime picks from 3167 shots were used in the inversion. Tomographic inversion of the first-arrival traveltimes resulted in a smooth velocity model for the upper 200 m of the subsurface. Ray coverage in the model is excellent down to 200 m providing effective control for estimating velocities through tomographic inversion. Resolution tests conducted through horizontal and vertical checkerboard tests confirm the robustness of the velocity model in detailing small scale velocity variations. Well velocities were used to validate tomographic velocities. The tomographic velocities do not show systematic correlation with well velocities. The velocity model clearly images the permafrost velocity structure in lateral and vertical directions. It is inferred from the velocity model that the permafrost structure in the near subsurface is discontinuous. Extensions of surface water bodies in depth, characterized by low P-wave velocities, are well imaged by the velocity model. Deep lakes with unfrozen water, inferred from the tomographic velocity model, correlate with areas of strong amplitude blanking and frequency attenuation observed in processed reflection seismic stack sections.

High-resolution P-Cable 3D seismic imaging of gas chimney structures in gas hydrated sediments of an Arctic sediment drift

The newly developed P-Cable 3D seismic system allows for high-resolution seismic imaging to characterize upper geosphere geological features focusing on geofluid expressions (gas chimneys), shallow gas and gas hydrate reservoirs. Seismic imaging of a geofluid system of an Arctic sediment drift at the Vestnesa Ridge, offshore western Svalbard, provides significantly improved details of internal chimney structures from the seafloor to ∼500 m bsf (below seafloor). The chimneys connect to pockmarks at the seafloor and indicate focused fluid flow through gas hydrated sediments. The pockmarks are not buried and align at the ridge-crest pointing to recent, topography-controlled fluid discharge. Chimneys are fuelled by sources beneath the base of gas hydrate stability zone (GHSZ) that is evident at ∼160–170 m bsf as indicated by a bottom-simulating reflector (BSR). Conduit centres that are not vertically straight but shift laterally by up to 200 m as well as discontinuous internal chimney reflections indicate heterogeneous hydraulic fracturing of the sediments. Episodically active, pressure-driven focused fluid flow could explain the hydro-fracturing processes that control the plumbing system and lead to extensive pockmark formation at crest of the Vestnesa Ridge. High-amplitude anomalies in the upper 50 m of the chimney structures suggest formations of near-surface gas hydrates and/or authigenic carbonate precipitation. Acoustic anomalies, expressed as high amplitudes and amplitude blanking, are irregularly distributed throughout the deeper parts of the chimneys and provide evidence for the variability of hydrate and/or carbonate formation in space and time.

Focussed fluid flow on the Hikurangi Margin, New Zealand — Evidence from possible local upwarping of the base of gas hydrate stability

The southern Hikurangi Subduction Margin is characterized by significant accretion with predicted high rates of fluid expulsion. Bottom simulating reflections (BSRs) are widespread on this margin, predominantly occurring beneath thrust ridges. We present seismic data across the Porangahau Ridge on the outer accretionary wedge. The data show high-amplitude reflections above the regional BSR level. Based on polarity and reflection strength, we interpret these reflections as being caused by free gas. We propose that the presence of gas above the regional level of BSRs indicates local upwarping of the base of gas hydrate stability caused by advective heatflow from upward migrating fluids, although we cannot entirely rule out alternative processes. Simplified modelling of the increase of the thermal gradient associated with fluid flow suggests that funnelling of upward migrating fluids beneath low-permeability slope basins into the Porangahau Ridge would not lead to the pronounced thermal anomaly inferred from upwarping of the base of gas hydrate stability. Focussing of fluid flow is predicted to take place deep in the accretionary wedge and/or the underthrust sediments. Above the high-amplitude reflections, sediment reflectivity is low. A lack of lateral continuity of reflections suggests that reflectivity is lost because of a destruction of sediment layering from deformation rather than gas-hydrate-related amplitude blanking. Structural permeability from fracturing of sediments during deformation may facilitate fluid expulsion on the ridge. A gap in the BSR in the southern part of the study area may be caused by a loss of gas during fluid expulsion. We speculate that gaps in otherwise continuous BSRs that are observed beneath some thrusts on the Hikurangi Margin may be characteristic of other locations experiencing focussed fluid expulsion.

Amplitude and frequency anomalies in regional 3D seismic data surrounding the Mallik 5L-38 research site, Mackenzie Delta, Northwest Territories, Canada

Amplitude and frequency anomalies associated with lakes and drainage systems were observed in a 3D seismic data set acquired in the Mallik area, Mackenzie Delta, Northwest Territories, Canada. The site is characterized by large gas hydrate deposits inferred from well-log analyses and coring. Regional interpretation of the gas hydrate occurrences is mainly based on seismic amplitude anomalies, such as brightening or blanking of seismic energy. Thus, the scope of this research is to understand the nature of the amplitude behavior in the seismic data. We have therefore analyzed the 3D seismic data to define areas with amplitude reduction due to contamination from lakes and channels and to distinguish them from areas where amplitude blanking may be a geologic signal. We have used the spectral ratio method to define attenuation (Q) over different areas in the 3D volume and subsequently applied Q-compensation to attenuate lateral variations ofdispersive absorption. Underneath larger lakes, seismic amplitude is reduced and the frequency content is reduced to [Formula: see text], which is half the original bandwidth. Traces with source-receiver pairs located inside of lakes show an attenuation factor Q of [Formula: see text], approximately half of that obtained for source-receiver pairs situated on deep, continuous permafrost outside of lakes. Deeper reflections occasionally identified underneath lakes show low-velocity-related pull-down. The vertical extent of the washout zones is enhanced by acquisition with limited offsets and from processing parameters such as harsh mute functions to reduce noise from surface waves. The strong attenuation and seismic pull-down may indicate the presence of unfrozen water in deeper lakes and unfrozen pore water within the sediments underlying the lakes. Thus, the blanking underneath lakes is not necessarily related to gas migration or other in situ changes in physical properties potentially associated with the presence of gas hydrate.

Evidence of soft sediment deformation, fluid escape, sediment failure and regional weak layers within the late Quaternary mud deposits of the Adriatic Sea

The late Quaternary sediments of the Adriatic Sea show clear evidence for fluid flow and shallow subsurface sediment mobilization. Amplitude blanking in high-resolution seismic data and limited sediment core information document that free gas is diffused at very shallow stratigraphic levels; pockmarks and blow-out pipes indicate expulsion of overpressured fluid; possible polygonal fault systems denote areas of sediment contraction and dewatering. Core data and VHR seismic profiles suggest that very high accumulation rates of homogeneous and fine-grained deposits result in fluid accumulation (high pore water pressure) within low-permeability sediments. The fluid flow processes that are active under such regime are still poorly understood. Enhanced accumulation rates during particular intervals can potentially result in trapping of fluids leading to increased pore pressure and decreased shear strength. Seismicity is high in the study area, and recurrent earthquakes or tsunamis provide an effective mechanism leading to the cyclic loading of sediment, favoring mobilization of gas-charged sediments. This paper provides an assessment of the shallow deformation styles, possible fluid flow processes and subsurface sediment mobilization features in the Quaternary muddy succession of the Adriatic shelf and slope. This succession includes forced regression deposits originated during repeated eustatic falls, lowstand prograding deposits formed during the Last Glacial Maximum (LGM), backstepping late Quaternary transgressive deposits and the late Holocene highstand progradational mud wedge. In each of these units, a variety of sediment deformation patterns are confined to well-defined stratigraphic intervals, typically above regionally extensive downlap surfaces that likely acted as detachment surfaces and left unaffected the succession beneath them. It is suggested that intervals of rapid progradation (resulting in rates of deposition in the order of 30 m in less than 10 kyr) generate downlapping deposits that are more prone to failure if perturbed soon after a critical thickness has accumulated and internal overpressures, likely related to the rapid rate of deposition, have not yet dissipated. Among the broad variety of sediment deformations identified, the late Holocene mud wedge shows a high degree of complexity in internal geometry just above a regional downlap surface (the maximum flooding surface [mfs]). According to geochronological determinations and tephra correlation, the mfs marks also an interval of condensed deposition between ca. 6 and 3.5 cal. kyr BP. The progradational wedge deposited above the mfs during the last ca. 3.5 cal. kyr, with sediment accumulation rates up to as much as 10 cm yr −1, likely hindering a rapid dissipation of interstitial fluids. A suite of deformation features occurs above the mfs regional downlap surface affecting this fast-depositing progradational unit. The picture is further complicated by variable deposition on preferred sides of seafloor irregularities caused by sediment deformation, indicating that bottom currents interact with the complex topography created by sediment deformation. These complex features (inferred to be genetically linked) nowhere affect the deposits underlying their basal downlap surface, a character shared also by the other reported cases of sediment deformations within older rapidly deposited units in the Adriatic.

A Preliminary Study of Gas Hydrates in Dongsha Region North of South China Sea

AbstractSeismic, sonic logging and geothermal data are analyzed to show the distribution of gas hydrates in the Dongsha region north of the South China Sea. Bottom simulating reflectors (BSRs) and amplitude blanking zones are revealed in seismic profiles. A high velocity interval and a velocity reverse are distinguished by sonic logging.The BSR depth is compared with thickness of gas hydrate stability zone from geothermal gradient data. The BSR at ODP sites 1144 and 1148 occur at 654 and 475 meters below sea floor respectively. The top of the amplitude blanking zone may be related to gas hydrate bearing strate in a region adjacent to ODP site 1144, at about 450 meters below the sea floor.

Amplitude blanking related to the pore-filling of gas hydrate in sediments

Seismic indicators of gas-hydrate-bearing sediments include elevated interval velocities and amplitude reduction of seismic reflections owing to the presence of gas hydrate in the sediment’s pore spaces. However, large amplitude blanking with relatively low interval velocities observed at the Blake Ridge has been enigmatic because realistic seismic models were absent to explain the observation. This study proposes models in which the gas hydrate concentrations vary in proportion to the porosity. Where gas hydrate concentrations are greater in more porous media, a significant amplitude blanking can be achieved with relatively low interval velocity. Depending on the amount of gas hydrate concentration in the pore space, reflection amplitudes from hydrate-bearing sediments can be much less, less or greater than those from corresponding non-hydrate-bearing sediments.

Multi-frequency seismic study of gas hydrate-bearing sediments in Lake Baikal, Siberia

Abstract In this paper we present and discuss the frequency-dependent behaviour of the acoustic characteristics of methane hydrate-bearing sediments in Lake Baikal, Siberia. Five different types of seismic sources (airgun-array, two types of single airguns, watergun and sparker) are used, encompassing a frequency bandwidth from 10 up to 1000 Hz. On low-frequency airgun-array data, the base of the hydrate stability zone (HSZ) is observed as a high-amplitude bottom-simulating reflection (BSR) with reversed polarity. The amplitude and continuity of the BSR decrease or even disappear on medium- to high-frequency data, a feature explained in terms of vertical and horizontal resolution. The increasing reflection amplitude of the BSR with increasing offset, the calculated reflection coefficient of the BSR and the occurrence of enhanced reflections below the BSR suggest the presence of free gas below the HSZ. The observation of some enhanced reflections extending above the BSR may be interpreted as an indication for free gas co-existing with hydrates within the HSZ. Amplitude blanking above the BSR is highly variable while the BSR itself appears to act as a low-pass frequency filter for medium- to high-frequency data. New single-channel airgun profiles provide the first seismic information across the Baikal Drilling Project (BDP-97) deep drilling site, at which hydrate-bearing sediments were retrieved at about 200 m above the base of the local HSZ. At the drilling site there are no seismic characteristics indicative of the presence of hydrates. Combination of the drilling and seismic information has allowed us to make a rough estimation of the volume of hydrates and carbon stored in the sediments of Lake Baikal, which lead us to conclude that the Lake Baikal gas hydrate reservoirs do not form a prospective energy resource.

Amplitude blanking in seismic profiles from Lake Baikal

Imaging of the deepest sedimentary section in Lake Baikal using multichannel seismic profiling was hampered by amplitude blanking that is regionally extensive, is associated with water depths greater than about 900 m and occurs at sub-bottom depths of 1–2 km in association with the first water-bottom multiple. Application of a powerful multiple suppression technique improved the quality of occasional discontinuous, dipping primary reflections, but failed to substantially alter the non-reflective character of the blanking zone. Detailed analysis of amplitudes from original data and synthetic models show that the threshold for detecting primary energy in deep water of Lake Baikal occurs when the primary is about 14–20 dB less than the multiple energy. The blanking occurs because of anomalously low reflectivities of the deep sediments coupled with this 20 dB limitation in real data processing. The blanking cuts across seismic stratal boundaries, and is therefore probably unrelated to depositional lithologies. The deepest, early rift deposits, inferred to come from a mixed fluvial and lacustrine setting, do not easily explain the widespread and uniform character of the blanked deposits. More likely, blanking occurs because of processes or phenomena that physically alter the deposits, causing them to be non-reflective and/or highly attenuating. No single process explains all the observations, but a combination of diagenesis, overpressure, and the presence of dispersed free gas at sub-bottom depths of 1–2 km, offer plausible and possible conditions that contribute to blanking.

Seismic character of gas hydrates on the Southeastern U.S. continental margin

Gas hydrates are stable at relatively low temperature and high pressure conditions; thus large amounts of hydrates can exist in sediments within the upper several hundred meters below the sea floor. The existence of gas hydrates has been recognized and mapped mostly on the basis of high amplitude Bottom Simulating Reflections (BSRs) which indicate only that an acoustic contrast exists at the lower boundary of the region of gas hydrate stability. Other factors such as amplitude blanking and change in reflection characteristics in sediments where a BSR would be expected, which have not been investigated in detail, are also associated with hydrated sediments and potentially disclose more information about the nature of hydratecemented sediments and the amount of hydrate present. Our research effort has focused on a detailed analysis of multichannel seismic profiles in terms of reflection character, inferred distribution of free gas underneath the BSR, estimation of elastic parameters, and spatial variation of blanking. This study indicates that continuous-looking BSRs in seismic profiles are highly segmented in detail and that the free gas underneath the hydrated sediment probably occurs as patches of gas-filled sediment having variable thickness. We also present an elastic model for various types of sediments based on seismic inversion results. The BSR from sediments of high ratio of shear to compressional velocity, estimated as about 0.52, encased in sediments whose ratios are less than 0.35 is consistent with the interpretation of gasfilled sediments underneath hydrated sediments. This model contrasts with recent results in which the BSR is explained by increased concentrations of hydrate near the base of the hydrate stability field and no underlying free gas is required.

Method of estimating the amount of in situ gas hydrates in deep marine sediments

The bulk volume of gas hydrates in marine sediments can be estimated by measuring interval velocities and amplitude blanking of hydrated zones from true amplitude processed multichannel seismic reflection data. In general, neither velocity nor amplitude information is adequate to independently estimate hydrate concentration. A method is proposed that uses amplitude blanking calibrated by interval velocity information to quantify hydrate concentrations in the Blake Ridge area of the US Atlantic continental margin. On the Blake Ridge, blanking occurs in conjunction with relatively low interval velocities. The model that best explains this relation linearly mixes two end-member sediments: hydrated and unhydrated sediment. Hydrate concentration in the hydrate end-member can be calculated from a weighted equation that uses velocity estimated from the seismic data, known properties of the pure hydrate, and porosity inferred from a velocity-porosity relationship. Amplitude blanking can be predicted as the proportions of hydrated and unhydrated sediment change across a reflection boundary. Our analysis of a small area near DSDP 533 indicates that the amount of gas hydrates is about 6% in total volume when the interval velocity is used as a criterion and about 9.5% when amplitude information is used. This compares with a calculated value of about 8% derived from the only available measurement in DSDP 533.

Aspects of FT-ICR software—III: Chirp Excitation

Chirp excitation (a pulse fast frequency sweep) solves the problem of exciting a very wide bandwidth with limited transmitter peakpower. Analysis of th For frequency-selective excitation some technically trivial modifications of the chirp pulse are proposed: amplitude blanking, frequency jump and sign Finally, corrections are derived for the effects of delayed signal acquisition.