| AGU 2013 Fall Meeting, San Francisco |
| TITLE: Controls of stress and water saturation on in-situ Q for shallow (< 1 m), unconsolidated sand Abstract: NS41A-1778 |
| AUTHORS (FIRST NAME, LAST NAME): Juan Manuel Lorenzo1, James M Crane1, Jie Shen1 |
| INSTITUTIONS (ALL): 1. Geology and Geophysics, Louisiana State University, Baton Rouge, LA, United States. |
| ABSTRACT BODY:
Field investigations into the simultaneous effects of water saturation
and stress on seismic attenuation are scarce. However seismic
attenuation may be used to place constraints on water saturation with
depth, at least for homogenous, porous media. We use a publicly
available seismic dataset [Lorenzo et al., 2013] collected in a mid-size
sand tank (~6 x 9 x 0.44 m) both for open evaluation of these
relationships. In the presence of large Q gradients the assumed
equivalency of Q between the raypaths of the reference and measured
signals can lead to false Q estimates (e.g., < 0). We employ a
modified spectral ratio method to estimate in-situ Q so that the average
Q along the measured and reference ray paths differ. We estimate local Q values from average raypath Q values, penetration depths, and travel times. Local Q values (Qint) increase the most with depth (dQ/dz = 43 m-1) and stress (dQ/dσ = 0.0025/Pa) in dry sand and the least in partially saturated sand (dQ/dz = 10 m-1 and dQ/dσ = 0.0013/Pa) where attenuation created by local fluid flow reaches a maximum. Anomalous Q deviations outside this range can be explained by a divergence in effective stress, attenuation mechanism, or lithology. |
| INDEX TERMS: 7200 SEISMOLOGY, 1835 HYDROLOGY Hydrogeophysics, 1866 HYDROLOGY Soil moisture, 7299 SEISMOLOGY General or miscellaneous. |
Session: Developments and Challenges in Geophysical Monitoring ofDams and Levees II
March 19, Tuesday.
SAGEEP 2013 (Symposium onthe Application of Geophysics to Engineering and Environmental Problems)
Denver CO., March 17-22
Seismic velocity prediction in shallow (< 30m) partially-saturated,
unconsolidated sediments using effective medium theory
James Crane, Juan Lorenzo, Jie Shen and Chris White
Granular contact and fluid substitution theories estimate
elastic properties of deep, unconsolidated sediments, but fail to accurately predict
their elastic properties in the near-surface. The major difference between deep and shallow
unconsolidated sediments is the stress at the grain contacts, mostly attributed
to net overburden stress at large depths.
In near-surface sediments, physiochemical and capillary forces can be
several orders of magnitude higher than net overburden stress. We propose a constitutive elastic model which
predicts seismic velocities by incorporating physiochemical and capillary
forces into Hertz-Mindlin and Biot-Gassman theories. Traditional Hertz-Mindlin theory, which uses
hydrostatic pressure to model stress at grain contacts, is modified to include
an updated definition of total effective stress and estimates the elastic
moduli of the sediment matrix. Biot-Gassman
theory, which calculates the influence of pore constituents on elastic moduli,
is modified to include the effects of water saturation changes on total
effective stress. Effective elastic
moduli, derived from modified Hertz-Mindlin and Biot-Gassman theories, are then
used in the elastic wave equation to estimate seismic velocities. Water saturation, total effective stress, and
seismic velocity profiles are calculated to emphasize the influence of total
effective stress in elastic models for different shallow sediments (sand and
clay). Hertz-Mindlin and Biot-Gassman
theories must be modified because total effective stress can be greatly
influenced by physiochemical and capillary forces in near-surface sediments,
which were not previously included. As
depth increases, external stresses such as net overburden stress become the
largest component of total effective stress.
| 2012 Fall Meeting Search Results | Cite abstracts as Author(s) (2012), Title, Abstract xxxxx-xxxx presented at 2012 Fall Meeting, AGU, San Francisco, Calif., 3-7 Dec. |
| CONTROL ID: 1498308 |
| TITLE: Free online seismic data from a sand tank experiment |
| AUTHORS (FIRST NAME, LAST NAME): Juan Manuel Lorenzo1, David Smolkin1, Christopher White2, Shannon Chollett2, Ting Sun2 |
| INSTITUTIONS (ALL): 1. Dept Geology & Geophysics, Louisiana State Univ, Baton Rouge, LA, United States. 2. Dept. Petroleum Engineering, Louisiana State Univ, Baton Rouge, LA, United States. |
| ABSTRACT BODY: Theoretical fluid
flow models are used regularly to predict and analyze porous media flow
but require verification against natural systems. Seismic monitoring in
a controlled laboratory setting at a nominal scale of 1:1000 in the
acoustic frequency range can help improve fluid flow models as well as
elasto-granular models for uncompacted, saturated-unsaturated soils. A mid-scale sand tank allows for many highly repeatable, yet flexible, experimental configurations with different material compositions and pump rates while still capturing phenomena such as patchy saturation, flow fingering, or layering. The tank (~6 x 9 x 0.44 m) contains a heterogeneous sand pack (1.52-1.7 phi). In a set of eight experiments the water table is raised inside the sand body at increments of ~0.05 m. Seismic events (vertical component) are recorded by a pseudo-walkaway 64-channel accelerometer array (20 Hz-20 kHz), at 78 kS/s, in 100- scan stacks so as to optimize signal-to-noise. Three screened well sites monitor water depth (+/- 3 mm) inside the sand body. Data sets comprise SEG-Y formatted files (seismic) and are publicly downloadable from the internet (http://github.com/cageo/Lorenzo-2012), in order to allow comparisons of different seismic and fluid flow analyses. The capillary fringe does not appear to completely saturate, as expected, because the interpreted compressional-wave velocity values remain so low (< 210 m/s). Even the highest water levels there is no large seismic impedance contrast across the top of the water table to generate a clear reflector. Preliminary results indicate an immediate need for several additional experiments whose data sets will be added to the online database. Future benchmark data sets will grow with a control data set to show conditions in the sand body before water levels rise, and a surface 3D data set. In later experiments, buried sensors will help reduce seismic attenuation effects and in-situ saturation sensors will provide calibration values. |
| KEYWORDS: [1835] HYDROLOGY / Hydrogeophysics, [1866] HYDROLOGY / Soil moisture, [7203] SEISMOLOGY / Body waves, [7294] SEISMOLOGY / Seismic instruments and networks. |
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| 2011 Fall Meeting Search Results |
Cite abstracts as Author(s) (2011), Title, Abstract xxxxx-xxxx presented at 2011 Fall Meeting, AGU, San Francisco, Calif., 5-9 Dec. |
: S43A-2225 Poster
TI: Scaling Analysis and Seismic Resolvability of Transition Zone in an Air-Water System
AU: *Sun, T
EM: tsun2@tigers.lsu.edu
AF: Petroleum Engineering Dep., Louisiana State University, Baton Rouge, LA, USA
AU: White, C
EM: cdwhite@lsu.edu
AF: Petroleum Engineering Dep., Louisiana State University, Baton Rouge, LA, USA
AU: Lorenzo, J M
EM: gllore@lsu.edu
AF: Petroleum Engineering Dep., Louisiana State University, Baton Rouge, LA, USA
AU: Shuai, Y
EM: yshuai2@tigers.lsu.edu
AF: Petroleum Engineering Dep., Louisiana State University, Baton Rouge, LA, USA
AU: Chollett, S
EM: scholl1@tigers.lsu.edu
AF: Petroleum Engineering Dep., Louisiana State University, Baton Rouge, LA, USA
AB:
Unsaturated flow in aquifers commonly includes a transition zone caused
by capillary forces. Saturation varies within this zone, affecting many
properties of interest, including effective permeability and acoustic
velocities. Moreover, in systems with a rising or falling water table,
the shape of the transition zone varies. This problem is of interest
because it affects water deliverability and supply, and is analogous to
segregated flow in oil and gas reservoirs. If transition zone shape is
understood, it can be used to predict saturations and acoustic
velocities, and our ability of using seismic methods to resolve water
table locations in aquifers will be improved. The inspectional analysis
isolates the effects in terms of a capillary and gravity number. We show
that transition zone thickness scales on these using numerical
simulation. Since Carmen Kozeny equation is used to convert the pore
scale to the reservoir scale, the numerical simulations will run with
different grain sizes and injection or production rates are run and.
Water distribution and transition zone thickness are recorded at
different time steps. Based on that, we investigated how the thickness
of transition zone changes with different capillary number and gravity
number. The Hertz-Mindlin method is applied here to calculate acoustic
properties from the numerical simulation results, then raytracing
simulation runs and the seismic resolvability of transition zone is
analyzed. The scaling analysis results show that dimensionless thickness
of transition zone has a linear relationship with capillary number when
gravity number are the same. Therefore, the transition zone thickness
can be predicted in large scale aquifers. From seismic analysis results,
we found the transition zone thickness and the water table location are
the two main factors that affect the refracted signals. With realistic
models for permeability and capillary pressure, we investigate the
seismic resolvability of the air-water transition for two cases: a
meter-scale experiment and a 100-meter thick aquifer. The results show
that if the transition zone thickness and refracted signal are known,
the water table location can be predicted.
DE: [0520] COMPUTATIONAL GEOPHYSICS / Data analysis: algorithms and implementation
DE: [0935] EXPLORATION GEOPHYSICS / Seismic methods
DE: [5102] PHYSICAL PROPERTIES OF ROCKS / Acoustic properties
SC: Seismology (S)
MN: 2011 Fall Meeting
| 2010
Meeting of the Americas Search Results |
Cite
abstracts as Author(s) (2010), Title, Eos Trans. AGU, 91(26), Meet. Am. Suppl., Abstract xxxxx-xx |
HR:
0800h |