Sediment characterization by geo-acoustic inversion in a shallow water environment using standard seismic equipment

8/3/2019 Sediment characterization by geo-acoustic inversion in a shallow water environment using standard seismic equipment

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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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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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Measurement configuration:

Two seismic data sets have been acquired:

North Sea, The Netherlands

Danube River, Hungary


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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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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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Description of the data setExample of acquired data


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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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Classification approachAssumed model, expected results

For the environment andmeasurement geometryconsidered, received signals areexpected to be hardly influencedby the attenuation coefficient


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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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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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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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Description of the data set

Area


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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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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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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



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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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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Illustration of head waves


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Assumed model


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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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Results

The estimated sediment sound speedVSP: 1745 m/s


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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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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

Documents

Sediment characterization by geo-acoustic inversion in a shallow water environment using standard seismic equipment