Seafloor Acoustic Properties:

Rocks, Sediments, Hydrates and Gas

Angus I. Best

National Oceanography Centre

University of Southampton Waterfront Campus

European Way

Southampton SO14 3ZH

United Kingdom

angus.best@noc.ac.uk

Outline

•Introduction

• Dynamic & complex seafloor

• Velocity, attenuation, frequency & sediment texture studies

•Laboratory studies:

• Ultrasonic pulse-echo

• Resonant column

• Acoustic pulse tube

•Phenomena:

• Frequency-dependent seismic anisotropy

• Methane hydrates

• Gassy sediments

•Summary

20 mins

Complex Seafloor Topography & Sound Propagation

Seafloor interactions

Dynamic Seafloor Environment

Canyons, sediment flows, landslides, tsunamis,

vents, gas seeps

Hydrothermal vents Methane gas

& hydrate

Tsunami

Earthquake

triggers

Sediment acoustics: velocity, attenuation, frequency

Best et al. (2001) JASA

Seafloor

sample

studies in lab

In situ seafloor acoustic

experiments

In situ In situ

Laboratory

Laboratory

Relating Acoustic & Sediment Textural Parameters

20

25

30

35

40

45

50

55

60

65

0 10 20 30 40 50 60 70 80 90 100

Mud content (% by mass)

Po

rosit

y (

%)

Hormuz dataModel AModel BModel C

1400

1500

1600

1700

1800

1900

2000

2100

20 30 40 50 60 70 80 90 100

Porosity (%)

P-w

ave v

elo

cit

y,

Vp

(m

/s)

Hormuz data

Lower Florida Keys

GEOSEIS Carbonates

GEOSEIS siliciclastics

Clay particles

Sand particles

Best et al.

Seismic Anisotropy

Seismic anisotropy is the variation of seismic properties

with direction in the Earth.

Armstrong et al. (1994) •Preferential grain alignment

•Bedding

•Aligned fractures

Bedding related Fracture related

Ultrasonic Pulse-echo Rig (S-wave splitting)

McCann & Sothcott (1992); Best (1992) Vp, Vs ± 0.3%; Qp-1, Qs-1 ± 10%

Best et al (2007) GP

Rotatable S-wave transducer

Ultrasonic S-wave splitting & fracture density

Δt = 17.58 µs (fast) Shear wave splitting (SWS)

SWS = 100 * (S1 – S2) / S1

SWS Fracture density

http://www.bgs.ac.uk/research/energy/seismicTechniques.html

Fracture density, ε = Na3/V

Theory of frequency-

dependent seismic anisotropy

(Chapman 2003)

Synthetic reservoir rocks with aligned fractures

45°sample Aligned penny-shaped fractures

Natural samples – too much variability

between separate core plugs for lab

studies…

Also, how control fracture geometry?

Solution – prepare synthetic samples

Fractured sandstone results – fluid viscosity & saturation

Tillotson et al (2014) GP P

-wave a

nis

otr

opy,

ε

P-wave anisotropy parameter ε:

observations & model at 500 kHz

White’s model +/- squirt flow theory for

f-dependent anisotropy

Amalokwu et al (2015b)

100%

Gas 100%

Water

Seafloor gassy sediment acoustics

Chirp 2 - 8 kHz

Gas bubble morphology controlled

by sediment texture

Best et al (2003), Leighton (2007), …

Hydrate saturation & morphology studies

Mud

Mud

Hydrate

Sandgrain

Linked to sediment type

& reservoir properties

Seafloor Methane Hydrate Studies

Marin-Moreno et al. (2017, JGR); Best et al. (2013, EPSL); Best, Priest & Clayton (2010, SEG Book); Priest, Best & Clayton (2005, JGR); Priest, Best & Clayton (2006, GJI)

Resonant column

< 200 Hz Acoustic Pulse Tube

< 1 – 10 kHz

Joint ultrasound/ERT

Frequency dependence & hydrate morphology studies

New theory of frequency-dependent velocity &

attenuation in seafloor hydrates

HBES Model – predicts elevated attenuation & velocity dispersion in seismic & sonic bands due to

squirt flow caused by gas/water fluid inclusions in hydrate grains

[based on HEG model of Best et al. (2013)]

Marin-Moreno et al. (2017) JGR

Synchrotron time-lapse (4D) X-ray CT

45h 10 mins

23h 30 mins

Sahoo et al. (2018) G-cubed

Summary

Seafloor is a complex & dynamic environment

• Acoustic interaction far from understood

Seismic (acoustic) anisotropy is important for fracture characterisation in rocks & sediments

• Shallow seafloor studies (fluid flow, gas escape chimneys, ...)

Gassy sediment studies

• Frequency dependence, gas bubble morphology, sediment type…

Seafloor methane hydrate studies

• Frequency dependence, hydrate/gas bubble morphology…

New acoustics theory can be used to better interpret seafloor seismo-acoustic data:

• Resource assessment (hydrates, deep sea minerals, CO2 storage,…)

• Geohazard assessment (gassy sediments, slope stability,…)

• Geotechnical surveys (wind farms, cable routes, …)

• Naval defence (sonar performance, MCM, amphibious warfare, …)

• Ocean acoustics (environmental monitoring)