In situ measurements of P-wave attenuation in the methane hydrate- and gas-bearing sediments of the Blake Ridge

2000 ◽  
Author(s):  
W.T. Wood ◽  
W.S. Holbrook ◽  
H. Hoskins
Keyword(s):  
Methane Hydrate ◽  
Wave Attenuation ◽  
In Situ Measurements ◽  
P Wave ◽  
Blake Ridge ◽  
Gas Bearing

Estimation of free gas saturation from seismic reflection surveys by the genetic algorithm inversion of a P-wave attenuation model

Geophysics ◽  
2012 ◽  
Vol 77 (4) ◽  
pp. R175-R187 ◽  
Author(s):  
Eugene C. Morgan ◽  
Maarten Vanneste ◽  
Isabelle Lecomte ◽  
Laurie G. Baise ◽  
Oddvar Longva ◽  
...  
Keyword(s):  
Genetic Algorithm ◽  
Seismic Reflection ◽  
Wave Attenuation ◽  
In Situ Measurements ◽  
P Wave ◽  
Ratio Method ◽  
Attenuation Model ◽  
Free Gas ◽  
Gas Saturation

Many previously proposed methods of estimating free gas saturation from seismic survey data rely on calibration to invasively collected, in situ measurements. Typically, such in situ measurements are used to parameterize or calibrate rock-physics models, which can then be applied to seismic data to achieve saturation estimates. We tested a technique for achieving estimates of the spatial distribution of gas saturation solely from shipboard seismic surveys. We estimated the quality factor from seismic reflection surveys using the spectral ratio method, and then inverted a mesoscopic-scale P-wave attenuation model to find the parameters that matched the modeled attenuation to our estimates of observed attenuation within the range of seismic frequencies. By using a genetic algorithm for this inversion, we not only searched efficiently for a global solution to the nonlinear set of equations that compose the model, but also constrain the search to a relatively broad set of realistic parameter values. Thus, our estimates do not rely on in situ measurements of these parameters, but on distributions of their possible values, many of which may be referenced from literature. We first tested this method at Blake Ridge, offshore North and South Carolina, where an approximately 400-m-deep gas-saturated zone underlies a field of methane hydrates. The extensive field work and subsequent studies at this site make it ideal for validating our method. We also demonstrated the applicability of our method to shallower deposits by presenting results from Finneidfjord, Norway, where the inversion of the P-wave attenuation model recognizes very small gas saturations.

P-wave attenuation measurements from laboratory resonance and sonic waveform data

Geophysics ◽  
1989 ◽  
Vol 54 (1) ◽  
pp. 76-81 ◽  
Author(s):  
D. Goldberg ◽  
B. Zinszner
Keyword(s):  
Wave Attenuation ◽  
Compressional Wave ◽  
P Wave ◽  
Laboratory Measurements ◽  
Frequency Range ◽  
Waveform Data ◽  
Sonic Log ◽  
In Situ Conditions ◽  
The Mean

We computed compressional‐wave velocity [Formula: see text] and attenuation [Formula: see text] from sonic log waveforms recorded in a cored, 30 m thick, dolostone reservoir; using cores from the same reservoir, laboratory measurements of [Formula: see text] and [Formula: see text] were also obtained. We used a resonant bar technique to measure extensional and shear‐wave velocities and attenuations in the laboratory, so that the same frequency range as used in sonic logging (5–25 kHz) was studied. Having the same frequency range avoids frequency‐dependent differences between the laboratory and in‐situ measurements. Compressional‐wave attenuations at 0 MPa confining pressure, calculated on 30 samples, gave average [Formula: see text] values of 17. Experimental and geometrical errors were estimated to be about 5 percent. Measurements at elevated effective pressures up to 30 MPa on selected dolostone samples in a homogeneous interval showed mean [Formula: see text] and [Formula: see text] to be approximately equal and consistently greater than 125. At effective stress of 20 MPa and at room temperature, the mean [Formula: see text] over the dolostone interval was 87, a minimum estimate for the approximate in‐situ conditions. We computed compressional‐wave attenuation from sonic log waveforms in the 12.5–25 kHz frequency band using the slope of the spectral ratio of waveforms recorded 0.914 m and 1.524 m from the source. Average [Formula: see text] over the interval was 13.5, and the mean error between this value and the 95 percent confidence interval of the slope was 15.9 percent. The laboratory measurements of [Formula: see text] under elevated pressure conditions were more than five times greater than the mean in‐situ values. This comparison shows that additional extrinsic losses in the log‐derived measurement of [Formula: see text], such as scattering from fine layers and vugs or mode conversion to shear energy dissipating radially from the borehole, dominate the apparent attenuation.

scholarly journals In situ measurements of thermal diffusivity in sediments of the methane-rich zone of Cascadia Margin, NE Pacific Ocean

Elem Sci Anth ◽  
2015 ◽  
Vol 3 ◽  
Author(s):  
Kira Homola ◽  
H. Paul Johnson ◽  
Casey Hearn
Keyword(s):  
Thermal Diffusivity ◽  
Pacific Ocean ◽  
Methane Hydrate ◽  
Bottom Water ◽  
Global Scale ◽  
In Situ Measurements ◽  
Bottom Water Temperature ◽  
Cascadia Margin ◽  
Ne Pacific

Abstract Thermal diffusivity (TD) is a measure of the temperature response of a material to external thermal forcing. In this study, TD values for marine sediments were determined in situ at two locations on the Cascadia Margin using an instrumented sediment probe deployed by a remotely operated vehicle. TD measurements in this area of the NE Pacific Ocean are important for characterizing the upslope edge of the methane hydrate stability zone, which is the climate-sensitive boundary of a global-scale carbon reservoir. The probe was deployed on the Cascadia Margin at water depths of 552 and 1049 m for a total of 6 days at each site. The instrumented probe consisted of four thermistors aligned vertically, one sensor exposed to the bottom water and one each at 5, 10, and 15 cm within the sediment. Results from each deployment were analyzed using a thermal conduction model applying a range of TD values to obtain the best fit with the experimental data. TD values corresponding to the lowest standard deviations from the numerical model runs were selected as the best approximations. Overall TDs of Cascadia Margin sediments of 4.33 and 1.15 × 10–7 m2 s–1 were calculated for the two deployments. These values, the first of their kind to be determined from in situ measurements on a methane hydrate-rich continental margin, are expected to be useful in the development of models of bottom-water temperature increases and their implications on a global scale.

Direct seismic detection of methane hydrate on the Blake Ridge

Geophysics ◽  
2003 ◽  
Vol 68 (1) ◽  
pp. 92-100 ◽  
Author(s):  
Matthew J. Hornbach ◽  
W. Steven Holbrook ◽  
Andrew R. Gorman ◽  
Kara L. Hackwith ◽  
Daniel Lizarralde ◽  
...  
Keyword(s):  
Methane Hydrate ◽  
Thick Layer ◽  
P Wave ◽  
Upward Migration ◽  
Seismic Reflection Data ◽  
Bottom Simulating Reflector ◽  
Reflection Data ◽  
Bright Spots ◽  
Seismic Detection ◽  
Blake Ridge

Seismic detection of methane hydrate often relies on indirect or equivocal methods. New multichannel seismic reflection data from the Blake Ridge, located approximately 450 km east of Savannah, Georgia, show three direct seismic indicators of methane hydrate: (1) a paleo bottom‐simulating reflector (BSR) formed when methane gas froze into methane hydrate on the eroding eastern flank of the Blake Ridge, (2) a lens of reduced amplitudes and high P‐wave velocities found between the paleo‐BSR and BSR, and (3) bright spots within the hydrate stability zone that represent discrete layers of concentrated hydrate formed by upward migration of gas. Velocities within the lens (∼1910 m/s) are significantly higher than velocities in immediately adjacent strata (1820 and 1849 m/s). Conservative estimates show that the hydrate lens contains at least 13% bulk methane hydrate within a 2‐km3 volume, yielding 3.2 × 1010kg [1.5 TCF (4.2 × 1010 m3] of methane. Low seismic amplitudes coupled with high interval velocities within the lens offer evidence for possible methane hydrate “blanking.” Hydrate bright spots yield velocities as high as 2100 m/s, with bulk hydrate concentrations predicted as high as 42% in an approximately 15‐m thick layer. Our results show that, under certain circumstances, hydrate in marine sediments can be directly detected in seismic reflections but that quantification of hydrate concentrations requires accurate velocity information.

High-frequency seismic response during permeability reduction due to biopolymer clogging in unconsolidated porous media

Geophysics ◽  
2013 ◽  
Vol 78 (6) ◽  
pp. EN117-EN127 ◽  
Author(s):  
Tae-Hyuk Kwon ◽  
Jonathan B. Ajo-Franklin
Keyword(s):  
Porous Media ◽  
Seismic Response ◽  
Wave Velocity ◽  
Wave Attenuation ◽  
P Wave ◽  
Ultrasonic Frequency ◽  
Permeability Reduction ◽  
Frequency Range ◽  
P Wave Velocity

The accumulation of biopolymers in porous media, produced by stimulating either indigenous bacteria or artificially introduced microbes, readily blocks pore throats and can effectively reduce bulk permeability. Such a microbial clogging treatment can be used for selective plugging of permeable zones in reservoirs and is considered a potentially promising approach to enhance sweep efficiency for microbial enhanced oil recovery (MEOR). Monitoring in situ microbial growth, biopolymer formation, and permeability reduction in the reservoir is critical for successful application of this MEOR approach. We examined the feasibility of using seismic signatures (P-wave velocity and attenuation) for monitoring the in situ accumulation of insoluble biopolymers in unconsolidated sediments. Column experiments, which involved stimulating the sucrose metabolism of Leuconostoc mesenteroides and production of the biopolymer dextran, were performed while monitoring changes in permeability and seismic response using the ultrasonic pulse transmission method. We observed that L. mesenteroides produced a viscous biopolymer in sucrose-rich media. Accumulated dextran, occupying 4%–6% pore volume after [Formula: see text] days of growth, reduced permeability more than one order of magnitude. A negligible change in P-wave velocity was observed, indicating no or minimal change in compressive stiffness of the unconsolidated sediment during biopolymer formation. The amplitude of the P-wave signals decreased [Formula: see text] after [Formula: see text] days of biopolymer production; spectral ratio analysis in the 0.4–0.8-MHz band showed an approximate 30%–50% increase in P-wave attenuation ([Formula: see text]) due to biopolymer production. A flow-induced loss mechanism related to the combined grain/biopolymer structure appeared to be the most plausible mechanism for causing the observed increase in P-wave attenuation in the ultrasonic frequency range. Because permeability reduction is also closely linked to biopolymer volume, P-wave attenuation in the ultrasonic frequency range appears to be an effective indicator for monitoring in situ biopolymer accumulation and permeability reduction and could provide a useful proxy for regions with altered transport properties.

The Carbon:²³⁴Thorium ratios of sinking particles in the California Current Ecosystem 2: Examination of a thorium sorption, desorption, and particle transport model

2019 ◽  
Author(s):  
Michael Stukel ◽  
Thomas Kelly
Keyword(s):  
Organic Carbon ◽  
Water Column ◽  
Particulate Organic Carbon ◽  
Transport Model ◽  
California Current ◽  
In Situ Measurements ◽  
Power Law Distribution ◽  
Sinking Particles ◽  
Organic Carbon Concentration

Thorium-234 (234Th) is a powerful tracer of particle dynamics and the biological pump in the surface ocean; however, variability in carbon:thorium ratios of sinking particles adds substantial uncertainty to estimates of organic carbon export. We coupled a mechanistic thorium sorption and desorption model to a one-dimensional particle sinking model that uses realistic particle settling velocity spectra. The model generates estimates of 238U-234Th disequilibrium, particulate organic carbon concentration, and the C:234Th ratio of sinking particles, which are then compared to in situ measurements from quasi-Lagrangian studies conducted on six cruises in the California Current Ecosystem. Broad patterns observed in in situ measurements, including decreasing C:234Th ratios with depth and a strong correlation between sinking C:234Th and the ratio of vertically-integrated particulate organic carbon (POC) to vertically-integrated total water column 234Th, were accurately recovered by models assuming either a power law distribution of sinking speeds or a double log normal distribution of sinking speeds. Simulations suggested that the observed decrease in C:234Th with depth may be driven by preferential remineralization of carbon by particle-attached microbes. However, an alternate model structure featuring complete consumption and/or disaggregation of particles by mesozooplankton (e.g. no preferential remineralization of carbon) was also able to simulate decreasing C:234Th with depth (although the decrease was weaker), driven by 234Th adsorption onto slowly sinking particles. Model results also suggest that during bloom decays C:234Th ratios of sinking particles should be higher than expected (based on contemporaneous water column POC), because high settling velocities minimize carbon remineralization during sinking.

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