Measuring gas bubbles in marine sediments

A collaboration between three centres:

ISVR: Institute of Sound and Vibration Research, University of Southampton, Highfield, Southampton SO17 1BJ, UK

NOC: National Oceanography Centre, University of Southampton, European Way, Southampton SO14 3ZH, UK

NPL: National Physical Laboratory, Teddington, Middlesex TW11 0LW,UK

This project used acoustical sensors and CT scanning to investigate gas bubbles in sediment. It was funded by the Engineering and Physical Sciences Research Council (EPSRC Grant number EP/D000580/1) and consisted of three subprojects: (i) Development of theory for the acoustic scattering and attenuation by bubbles in sediment; (ii) Methods for calibrating acoustical sources and sensors in sediment; (iii) lab and field measurements of gas bubbles in sediment.

^{Principle Investigator: T.G. Leighton}
Subproject (i): Development of theory for the acoustic scattering and attenuation by bubbles in sediment	Subproject (ii): Methods for calibrating acoustical sources and sensors in sediment	Subproject (iii): Lab and field measurements of gas bubbles in sediment
TG Leighton (ISVR)	SP Robinson, PD Theobald, G Hayman (NPL)	TG Leighton, A Mantouka, P Fox, PR White (ISVR)
A Mantouka (ISVR)	VF Humphrey, TG Leighton, LS Wang (ISVR)	GBN Robb, JK Dix, AI Best (NOC)
AI Best (NOC)	GBN Robb, JK Dix, AI Best (NOC)

Why do we need three subprojects? Because the objective is to use the scattering and attenuation of sound to measure gas bubbles in sediment (project (iii)), but we cannot make those measurements unless the acoustical sensors are calibrated for use in sediment (project (ii)), nor can we use those measurements to estimate the number, shapes and sizes of bubbles present unless we have a good theory for how bubbles scatter and attenuation sound in sediment (subproject (i)). Details of the three projects are given in the Table above.

Why measure gas bubbles in sediment?

Methane gas trapped in marine sediment is important for consideration of global climate;
Bubbles in marine sediment make it less stable for civil engineering projects, such as the construction of barrages, dams, offshore wind turbines, pipelines, oil installations etc.;
Gas bubbles can be useful indicators of resources for the petrochemical industries, but also present a significant 'blow-out' hazard if the drill accidentally penetrates a high pressure gas pocket.

What did we do?

Subproject (i): The full theory from subproject (i) will be written up shortly for a journal (and so cannot be pre-published here). However the starting stages of the development of the theory can be downloaded by clicking on the following 2 papers:

Leighton, T.G. Theory for acoustic propagation in marine sediment containing gas bubbles which may pulsate in a non-stationary nonlinear manner, Geophysical Research Letters, 34, 2007, L17607

Leighton, T.G. The Rayleigh-Plesset equation in terms of volume with explicit shear losses, Ultrasonics, 48(2), 2008, 85-90

Subproject (ii): The calibration exercise of subproject (ii) is described in the following downloadable paper:

Robb, G.B.N., Robinson, S.P., Theobald, P.D., Hayman, G., Humphrey, V.F., Leighton, T.G., Wang, L.S., Dix, J.K. and Best, A.I. Absolute calibration of hydrophones immersed in sandy sediment, Journal of the Acoustical Society of America, 125(5), 2009, 2918-2927

Subproject (iii): In subproject (iii) we began by finding a quick method by which remotely-obtained sub-bottom profiles can be quickly and easily (but quantitatively) interpretted in terms of the void fraction of gas present (the proportion of the material which is free gas). Details of this technique can be found in the following download:

Leighton, T.G. and Robb, G.B.N. Preliminary mapping of void fractions and sound speeds in gassy marine sediments from subbottom profiles, Journal of the Acoustical Society of America, 124(5), 2008, EL313-EL320

The technique works by looking at layers below the seabed which might reasonably be expected to be horizontal, but which appear to dip in the figure. Consider the lines marked in green in figure 1, which appear to slope downwards towards the blind spot within the green box which is below an intense pocket of gas bubbles. If we say it is reasonable to assume they should have remained horizontal (as shown by the red lines), then the apparent dipping of these layers is an artefact caused by the reduction in sound speed caused by the bubbles. By applying this principle near the gassy pocket. to the lines A1-G1 and A2-G2 at the right of the figure, we obtain estimates for the sound speed and the void fraction at these locations (the above paper gives details of the technique).

Figure 1. A sub-bottom profile (taken by J. S. Lenham, J. K. Dix, and J. Bull of the NOC) and the calculated sound speed and void fraction in the sediment (click on Leighton, T.G. and Robb, G.B.N. (2008) for details).

Whilst useful, this is only a look-see technique that can be applied to historical sub-bottom profiles, or used for the quick survey of an area.

We wished to follow it up with more detailed and reliable techniques.

We used three ultrasonic transducers to transmit sound at two frequencies at the bubble - see figure 2, below. On the left (figure 2(a), you see a schematic of the three transducers insonifying bubbles in the grey mud. The one on the surface of the mud (the 'pump' transducer) sends out low frequency acoustic waves, whilst the buried 'imagining' frequency source send out high frequency waves. The two sound fields scatter off the bubbles are are detected by the buried imaging frequency receiver. The nonlinear scattering from those bubbles which are nearly spherical generates sum- and difference-frequencies, and we use these to count the number of near-spherical bubbles in the sediment.


(a)	(b)
Figure 2. (a) Schematic of 3 transducers insonifying bubbles in mud. (b). Photograph of the equipment of Figure 2(a) as shown in the lab, with the three acoustical axes shown as yellow dashed lines

However there are non-spherical bubbles in marine sediment, such as slabs of gas, or gas-filled cracks, as wells as shells, flora and fauna (Figure 4). If sound is propagated into a gassy sediment and one measures the attenuation, then it will be greater than the attenuation caused by the spherical bubbles alone.

(a)

(b)

Figure 4. (a) CT scan cross-section of a core of intertidal sediment.

(b) A Core from one of our inter-tidal site.

This could well explain the great variation seen in the measured attenuation in gassy sediments taken over the last 50 years.

Therefore we also measure the attenuation of the sound generated by the pump transducer as it passes over an array of hydrophones placed on the acoustic axis of the pump source (the hydrophones are off to the right of the photograph shown in Figure 2(b). The combined set of transducers is shown schematically in Figure 5, and as deployed in Figure 6.

Figure 5. Schematic of all three transducers, and the hydrophone array, in the sediment.

Figure 6. The rig deployed.

We compare the measured attenuation with the attenuation which will be caused by the spherical bubbles (measured using the 2-frequency technique described above) and hence we have found:

[1] the population of spherical bubbles in sediment;

[2] the contribution to attenuation caused by those spherical bubbles;

[3] the contribution to attenuation caused by other entities (e.g. slabs of gas, shells, flora and fauna etc.);

[4] How big the error would have been if we had interpreted the measured attenuation in terms of spherical bubbles only.

Further reading

Details of the findings are being written up for journal publication and so cannot as yet be placed on this site (downloads will be placed here when available). In the meantime the following papers list aspects of the study recorded so far.

Leighton, T.G. Theory for acoustic propagation in marine sediment containing gas bubbles which may pulsate in a non-stationary nonlinear manner, Geophysical Research Letters, 34, 2007, L17607

Leighton, T.G., Mantouka, A., White, P.R. and Klusek, Z. Towards field measurements of populations of methane gas bubbles in marine sediment: An inversion method required for interpreting two-frequency isonification data from sediment containing gas bubbles, Hydroacoustics, 11, 2008, 203-224

Leighton, T.G. The Rayleigh-Plesset equation in terms of volume with explicit shear losses, Ultrasonics, 48(2), 2008, 85-90

Leighton, T.G. Theory for acoustic propagation in solid containing gas bubbles, with applications to marine sediment and tissue, ISVR Technical Report 318, , University of Southampton, 2007, 26pp

Leighton, T.G. A method for estimating sound speed and the void fraction of bubbles from sub-bottom sonar images of gassy seabeds, ISVR Technical Report 320, , University of Southampton, 2007, 30pp

Robb, G.B.N., Leighton, T.G., Humphrey, V.F., Best, A.I., Dix, J.K. and Klusek, Z. Investigating acoustic propagation in gassy marine sediments using a bubbly gel mimic, ISVR Technical Report 315, , University of Southampton, 2007, 20pp

Mantouka, A., Leighton, T.G., Best, A.I., Dix, J.K and White, P.R. Inferring gas of intertidal sediments from attenuation and scattering measurements, Proceedings of the Third International Conference on Underwater Acoustic Measurements, Technologies and Results, Nafplion, Greece, 21-26 June 2009, , 2009, 733-73

Robb, G.B.N., Hayman, G., Theobald, P.D., Humphrey, V.F., Robinson, S.P., Leighton, T.G., Dix, J.K. and Best, A.I. A method for calibrating hydrophones immersed in sandy sediment, Proceedings of the 9th European Conference on Underwater Acoustics, (ECUA2008),Paris, France, 29 June - 4 July, 1150, 2008, 137-142

Hayman, G., Theobald, P., Robb, G.B.N., Robinson, S., Humphrey, V.F., Leighton, T.G., Dix, J.K. and Best, A.I. Hydrophone performance in sediment, Proceedings of the Second International Conference on Underwater Acoustic Measurements, Technologies and Results, Heraklion, Crete, Greece, 25-29 June, , 2007, 453-458

Robb, G.B.N., Leighton, T.G., Dix, J.K., Best, A.L., Humphrey, V.F. and White, P.R. Measuring bubble populations in gassy marine sediments: A review, Proceedings of the Institute of Acoustics, 28(1), 2006, 60-8

Related seabed studies:

Leighton, T.G. Guest Editorial for Applied Acoustics Special Issue on: The detection of buried marine targets, Applied Acoustics, 69(5), 2008, 385-6

Leighton, T.G. and Evans, R.C.P. The detection by sonar of difficult targets (including centimetre-scale plastic objects and optical fibres) buried in saturated sediment,Applied Acoustics, 69(5), 2008, 438-463

Gutowski, M., Bull, J.M., Dix, J.K., Henstock, T.J., Hillier, T., Leighton, T.G. and White, P.R. 3D high-resolution acoustic imaging of the sub-seabed (Also published in Applied Acoustics 69(3) as a result of publisher's error), Applied Acoustics, 69(5), 2008, 412-421

Robb, G.B.N., Best, A.I., Dix, J.K., White, P.R., Leighton, T.G., Bull, J.M. and Harris, A. The measurement of the in situ compressional wave properties of marine sediments, IEEE Journal of Oceanic Engineering, 32(2), 2007, 484-496

Robb, G.B.N., Best, A.I., Dix, J.K., Bull, J.M., Leighton, T.G. and White, P.R. The frequency dependence of compressional wave velocity and attenuation coefficient of intertidal marine sediments, Journal of the Acoustical Society of America, 120(5), 2006, 2526-37

Bull, J.M., Gutowski, M., Dix, J.K., Henstock, T.J., Hogarth, P., Leighton, T.G. and White, P.R. Design of a 3D Chirp sub-bottom imaging system, Marine Geophysical Researches, 26(2-4), 2005, 157-69

Gutowski, M., Bull, J., Dix, J.K., Henstock, T., Hogarth, P., White, P.R. and Leighton, T.G. Chirp sub-bottom profiler source signature design and field testing,Marine Geophysical Researches, 23(5-6, 2002 (c)), 2004, 481-92

Leighton, T.G. and Evans, R.C.P. Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 1: Background, ISVR Technical Report 309, , University of Southampton, 2007, 40pp

Leighton, T.G. and Evans, R.C.P. Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 2: Design and commissioning of test tank, ISVR Technical Report 310, , University of Southampton, 2007, 68pp

Evans, R.C.P. and Leighton, T.G. Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 3: Experimental investigation of acoustic penetration of saturated sediment, ISVR Technical Report 311, , University of Southampton, 2007, 43pp

Evans, R.C.P. and Leighton, T.G. Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 4: Experimental investigations into the acoustic detection of objects buried in saturated sediment, ISVR Technical Report 312, , University of Southampton, 2007, 81pp

Evans, R.C.P. and Leighton, T.G. Studies into the detection of buried objects (particularly optical fibres) in saturated sediment. Part 5: An acousto-optic detection system, ISVR Technical Report 313, , University of Southampton, 2007, 50pp

Robb, G., White, P.R., Bull, J.M., Best, A.I., Leighton, T.G. and Dix, J.K. The estimation of geoacoustic properties from broadband acoustic data, focusing on instantaneous frequency techniques, ISVR Technical Report 298, Southampton, University of Southampton, 2002, 49p

Bull, J.M., Gutowski, M., Dix, J.K., Henstock, T.J., Hogarth, P., Leighton, T.G. and White, P.R. 3D chirp sub-bottom imaging system: design and first 3D volume,Proceedings of the International Conference on Underwater Acoustic Measurements, Technologies and Results, Heraklion, Crete, 28 June-1 July 2005, II, 2005, 777-82

This page was last updated by TG Leighton,8 July 2009

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