sediment characterization by geo-acoustic inversion in a shallow water environment using standard...
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Challenge the future
DelftUniversity ofTechnology
Sediment characterization by geo-acoustic
inversion in a shallow waterenvironment using standard seismic
equipmentMirjam Snellen1, Koen Duijnmayer1, Guy G. Drijkoningen2, Dick G. Simons1
1Acoustic Remote Sensing Group, Faculty of Aerospace Engineering, Delft University of Technology,The Netherlands
2 Department of Geo-technology, Faculty of Civil Engineering and Geosciences, Delft University ofTechnology, The Netherlands
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2Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Knowledge of underwater sediment layers for example
relevant for: Dredging
Off-shore construction works
Retrieving sand for concrete production
Geology
Traditional acquisition:
Bottom sampling
Boreholes
Many samples needed!
Introduction
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3Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Introduction, continued
Acoustic remote sensing techniques for sediment classification
are of high interest:
Multi-beam and single-beam systems for the upper part of the
sediment
Low frequency seismic systems allow for retrieving information
regarding the deeper sediment layers
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4Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Introduction, continued
Measurement configuration:
Two seismic data sets have been acquired:
North Sea, The Netherlands
Danube River, Hungary
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5Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Introduction, continued
Measurement configuration:
Two seismic data sets have been acquired:
North Sea, The Netherlands
Danube River, Hungary
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6Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Description of the data setLogistics and area
Survey carried out in June
2002
Four tracks sailed
Sampling frequency of
3000 Hz
One shot per 5 seconds
Sailing speed 1-2 m/s
Distance between shots: ~
10 m
SBES for measuring the
water depth Source-receiver distance
~20 m
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7Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Description of the data setSource and Receiver characteristics
Airgun used as the acoustic source (200 Hz)
Source at 1.5 m depth
Array (53 m) with 18 hydrophone groups (12 hydrophones each)
Array kept in the middle of the water column
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8Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Description of the data setExample of acquired data
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9Sediment characterization by geo-acoustic inversion in a shallow waterenvironment using standard seismic equipment
Classification approachGeneral
Model-data match based on received signal shape
Forward modeling based on broadband normal mode modeling
Due to varying measurement geometry, inversion for bothgeometric and geo-acoustic parameters
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10Sediment characterization by geo-acoustic inversion in a shallow water
environment using standard seismic equipment
Classification approachAssumed model
Model input parameters:
Source depth
Receiver depth
Distance source - receiver
Water depth
Water sound speed
Sediment sound speed
Sediment density
Sediment attenuation
High speed virtual sub-bottom toovercome long-range approximation
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11Sediment characterization by geo-acoustic inversion in a shallow water
environment using standard seismic equipment
Classification approachAssumed model, expected results
For the environment andmeasurement geometryconsidered, received signals areexpected to be hardly influencedby the attenuation coefficient
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12Sediment characterization by geo-acoustic inversion in a shallow water
environment using standard seismic equipment
Classification approachThe optimization approach
The search bounds:
Energy function is based on the correlationbetween modeled and measured matched
filtered signals
Differential evolution used for theoptimization
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13Sediment characterization by geo-acoustic inversion in a shallow water
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Results
Track 7, 45
Results for tracks 7 and 45 similar
Geometric parameters agree with known values.
Estimates for the geo-acoustic parameters (track 7):
In agreement with expected value. Precision can beincreased through the use of a higher sampling rate
1600-1650 m/s up to 3500 m, no reliable estimatesfrom 3500-6000 m
Almost random, as expected
Lower than expected based on the sound speedestimates. At least partly due to neglecting shearwaves
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Results
Track 7, 45, continued
Geometry causes interference of arrivals Uncertainty about the exact source pulse
prevents exact reconstruction of the receivedsignal
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15Sediment characterization by geo-acoustic inversion in a shallow water
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Results
Track 70
Again the geometric parameters in agreementwith known measurement configuration Estimates for the geo-acoustic parameters stablealong the track From ~900 m on, sound speed estimates at twodistinct values
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Results
Track 70, dual layer inversions
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Results
Validation
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18Sediment characterization by geo-acoustic inversion in a shallow water
environment using standard seismic equipment
Measurement configuration:
Two seismic data sets have been acquired:
North Sea, The Netherlands
Danube River, Hungary
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19Sediment characterization by geo-acoustic inversion in a shallow water
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Description of the data set
Area
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20Sediment characterization by geo-acoustic inversion in a shallow water
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Description of the data set
Logistics
Survey carried out in October 2008
Source and receiver array spaced 30 m apart
Sample frequencies of 2000 and 8000 Hz
All tracks were sailed in upstream direction (sailing
speed of 1 m/s)
One shot every 4.5 sec (distance between shots ~ 4 m)
GPS receivers at mounted close to source and receiver
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21Sediment characterization by geo-acoustic inversion in a shallow water
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Description of the data set
Source and Receiver characteristics
Airgun used as the acoustic source (150 Hz)
Source placed at 1.3 and 2.0 m depth
Reference hydrophone used for measuring the emitted signal
For tracks 1 and 4: Array with 24 hydrophone groups (4hydrophones each)
Array at the air-water interface
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Description of the data set
Example of acquired data
Surface waves(propagation speedof 200 -300 m/s)
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Sediment characterization by geo-acoustic inversion in a shallow water
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Description of the data set
Ground truth
Vertical seismic profile (VSP) data
12 hydrophones
82.5 m deep borehole
Source at 20 m from borehole
Analysis of VSP data: 1745 m/s
sediment sound speed
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Sediment characterization by geo-acoustic inversion in a shallow water
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Classification approach
General
Uncertainties in the source pulse, combined with the shallow
water high sediment sound speed environment preventedclassification conform the approach taken for the North Sea data:
Modelling of the complete echo shape
Searching for all unknown geo-acoustic and geometric parameters by
maximizing the model-data agreement
Alternative approach for classification based on the head waves
(travel time tomography)
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Sediment characterization by geo-acoustic inversion in a shallow water
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Classification approach
Illustration of head waves
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Sediment characterization by geo-acoustic inversion in a shallow water
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Classification approach
Assumed model
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Sediment characterization by geo-acoustic inversion in a shallow water
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Classification approach
Cost function and optimization approach
Cost function:
Source depth, water depth and water sound speed set to: 1.3 m,
3.5 m, and 1448 m/s
Estimates for source-receiver range constrained by GPS
measurements
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Sediment characterization by geo-acoustic inversion in a shallow water
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Results
The estimated sediment sound speedVSP: 1745 m/s
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Sediment characterization by geo-acoustic inversion in a shallow water
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Conclusions
Methods developed that allow for sediment classification based
on standard seismic measurements
The first approach was successfully applied to the North Seadataset
The geometry and source pulse need to be known very well for
precise and accurate results
Because of neglecting elastic waves, an effective density is invertedfor
A second sediment layer is present at the end of track 70
The measurement geometry is such that the signals contain almost
no information of the sediment attenuation coefficient
Additional groundtruth needed
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Sediment characterization by geo-acoustic inversion in a shallow water
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Conclusions, continued
The second approach is based on inversion of data taken with a
seismic measurement configuration Sound speed estimate in agreement with VSP derived sound speed
A sufficiently large distance between source and receiver is required
Combination with bathymetry information