Lupi 2014a , Lusi mud eruption triggered by geometric focusing of seismic waves (Corrected 2013)

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Lupi 2014a

Lusi mud eruption triggered by geometric focusing of seismic waves

M. Lupi, E. H. Saenger, F. Fuchs and S. A. Miller

Nature Geoscience 6, 642–646 (2013); published online 21 July 2013; corrected after print 28 August 2014.

In our 2013 article1 numerical simulations. We were subsequently alerted to artefacts in that velocity profile, so below we present revised simulation results, based on additional data.

The seismic P-wave (Vp) and S-wave (Vs) velocity profiles measured in the BJP1 borehole (Supplementary Fig. 1) show that the Vp profile extends from a depth of about 300 m to the bottom of the section.

The S-wave and density profiles, however, were only determined  from the depth of the casing (approximately 1,100 m) to the bottom of the section.

As we mentioned previously1 more vigorously to S-wave energy, but the critical information about the S-wave mechanical impedance (Vs multiplied by density, ρ) does not exist for the first 1,100 m of this section.

Instead, we estimate an S-impedance profile above the mud layer by using the observed Vp profile and the observation that Vs in the mud layer is as low as 380 m s–1 at 1,100 m depth.

This extremely low value reinforces what  has been pointed out elsewhere2,3, that the mud layer is a low-velocity zone representative of an over-pressured and under-consolidated sedimentary horizon.

Such horizons are common throughout sedimentary basins in Southeast Asia.

We estimate Vs above the mud layer using experimental data (Supplementary Fig. 2) showing the relationship between Vs and Vp.

Although the Vp profile above the mud layer seems not to vary significantly (Supplementary Fig. 1a), a closer inspection (Supplementary Fig. 1b) shows that the Vp steadily increases just above the mud layer from about 1,500 m s–1 to about 2,000 m s–1, low effective stress4 between about 700 and 875 m depth.

The steady increase in Vp with depth, typical of a normal compacting horizon, indicates lower  fluid pressures relative to the fluid pressure in the underlying mud layer.

We assume that the top of the mud layer corresponds to the  observed drop in Vp at around 900 m depth, which is consistent with the well log data (Supplementary Fig. 3).

Using the recorded Vp constraint of 2,000 m s–1 with a Vp/Vs ratio of about 2.7 (Supplementary Fig. 2), we estimate Vs at the top boundary of the mud layer to  be about 750 m s–1 .

We assume that the 380 m s–1 Vs recorded at 1,100 m depth extends to the top of the mud layer because of the relatively constant and reduced Vp below the compacting layer (Supplementary Fig. 1b).

It should be emphasized that there is considerable  uncertainty in Vs above the mud layer, but the observed reduction in Vp with depth (after a systematic increase of velocity with depth in  the layer above) corresponds to a far greater reduction in Vs within the mud layer.

Therefore, the interface between the mud layer and the compacting layer corresponds to an impedance contrast.

This is evident in the elevated Vp/Vs ratios of about 4.5 within the mud layer (Supplementary Fig. 4), which again indicate low effective normal stress (Supplementary Fig. 2).

At low effective stress, Vp and Vs are only weakly coupled whereby Vp remains relatively constant while Vs varies depending on the pore pressure.

The effective stress depend-ence on Vp/Vs ratios occurs because Vs is solely dependent on the shear modulus while Vp is dominated by the bulk modulus.

Since shear modulus varies strongly as a function of pore pressure, small changes in pore pressure at low effective stress generate large changes in Vs, with little influence on Vp.

From the available experimental data (Supplementary Fig. 2), we can expect about a factor of two difference in Vp/Vs.

We multiply our estimated Vs profile (Fig. 1a) with the measured density profile (see Supplementary Fig. 3), using 1,800 kg m–3 where there is no data, to generate a new impedance profile (Fig. 1b).

We used this impedance profile as input for our numerical simulation, using the same input and boundary conditions as described previously1have been removed.

The results from our revised simulations (Fig. 1c) show that our estimated impedance-contrast between the low-velocity mud layer  and the compacting sediments above produces a comparable focusing effect and maximum shear strain, as we reported previously1

Notably, our two-dimensional simulations underestimate by a factor of five the additional amplification when the third dimension of  this parabolic structure is considered Our conclusions1

References

1.  Lupi, M., Saenger, E. H., Fuchs, F. & Miller, S. A. Lusi mud eruption triggered by geometric focusing of seismic waves. Nature Geosci. 6, 642–646 (2013).

2.  Istadi, B. P., Pramono, G. H., Sumintadireja, P. & Alam, S. Modeling study of growth and potential geohazard for LUSI mud volcano: East Java, Indonesia.  Mar. Petrol. Geol. 26, 1724–1739 (2009).

3.  Tanikawa, W., Sakaguchi, M., Wibowo, H. T., Shimamoto, T. & Tadai, O. Fluid transport properties and estimation of overpressure at the LUSI mud volcano, East Java Basin. Engin. Geol. 116, 73–85 (2010).

4.  Lee, M. W. Predicting S-Wave Velocities for Unconsolidated Sediments at Low Effective Pressure (USGS Scientific Investigations report 2010–5138, 2010).

5.  Davis, P. Triggered mud eruption? Nature Geosci. 6, 592–593 (2013).

Acknowledgements

We thank Maxwell Rudolph at Portland State University, USA, Mark Tingay at the University of Adelaide, Australia, Michael Manga and Chi-Yuen Wang at the University of California, Berkeley, USA, and Richard Davies at Durham University, UK, for alerting us to the errors in the original seismic velocity profile and for providing us with the additional data from the BJP1 borehole.

Figure 1 | Revised numerical simulations.

a, We estimate a Vs (red line) profile based on the measured Vp (green line) and Vs (blue line) profiles.

The model domain was discretized into 21 layers (with higher resolution for the first 2,000 m) approximated from the measured and estimated profiles  (Supplementary Figs 1 and 2).

Experimental data4 suggest that Vs varies indirectly with Vp. That is, Vp ∝ AVs, where A is a coefficient that varies  depending on the shear modulus, pore pressure and effective pressure (Supplementary Fig. 2).

Hence Vs does not always correlate positively with Vp.  The observation of Vp = 2,000 m s–1 directly above the mud layer (Supplementary Fig. 1b) implies from Supplementary Fig. 2 that Vs = 750 m s–1, while  further observations of Vp = 1,600–1,750 m s–1 in the mud layer are also consistent with the observation of Vs = 380 m s–1 and Vp /Vs = 4.5 in the mud  layer (Supplementary Fig. 4).

Therefore, we suggest there is little uncertainty in the magnitude of the impedance contrast, and small changes in these  values will not significantly affect our results because they scale with impedance contrast.

b, We use the S-wave estimates (a) to construct an S-wave impedance profile (with units kg m–2 s–1).

c, We use the S-wave impedance profile (b) in our numerical simulation, using the same input and boundary conditions as our original model simulation1 impedance contrast at Lusi is sufficient to focus seismic energy into the mud layer.

The dashed line marks the top of the mud layer. The results from this simulation show that the inferred Additional information.

Supplementary information is available in the online version of the paper., we adopted a published velocity profile2 described as check-shot data, which we used as an input constraint for our  in Supplementary Fig. 2 are from a different lithology than that at Lusi, the physics is lithology-independent,  therefore remain unchanged. We appreciate this opportunity to correct the record.

Nature Geosciences

SUPPLEMENTARY INFORMATION

DOI: 10.1038/NGEO2239

Lusi mud eruption triggered by geometric focusing of seismic waves

M. Lupi, E. H. Saenger, F. Fuchs and S. A. Miller

Supplementary Figure 1. Measured Vp and Vs profiles (a) from the BJP-1 borehole.

The Vp record extends to about 300 m depth, while the Vs record does not begin until the casing shoe at about 1100 m depth. Notice that Vs is about 380 m/s in the mud layer.

A zoom-in of a portion of the record (b) shows increasing Vp with depth above the mud layer, between about 700 m and 875 m depth (blue line), indicative of a normally compacting horizon.

A reduction in Vp at depths from 875 m to  1150 m indicates significantly reduced effective stress from the over-pressured and under-consolidated mud layer.

Supplementary Figure 2. Experimental data showing large reductions in Vp/Vs ratios with increasing effective stress.

We used the measured Vp of 2000 m/s at the top of the mud layer to estimate a Vs of about  750 m/s at this boundary.

The recorded S-wave velocity of 380 m/s in the mud layer (Vp/Vs=4.5, supplementary fig 4) indicates low effective stress representative of an under-consolidated and over-pressured horizon, typically referred to as low-velocity zones. Modified from [Lee, 2010].

Supplementary Figure 3. The complete montage of the well log recorded for the BJP1 borehole.

In our original study, we interpreted the top of the mud layer to exist at about 1,100 m depth based on a published velocity profile.

However,  well log data recorded at borehole BJP1 show that the mud layer begins at ~900 m and we have adjusted our analyses  and interpretations accordingly.

Supplementary Figure 4. Measured Vp/Vs ratios showing persistently elevated ratios of  about 4.5 within the mud layer indicative of a low effective stress (high pore pressure) environment.

References:

Lee, M.W., (2006c), A simple method of predicting S-wave velocity: Geophysics, v. 71, p. F161–F164.

Lee , M. W. (2010), Predicting S-Wave Velocities for Unconsolidated Sediments at Low Effective Pressure, USGS Scientific Investigations report 2010-5138.

Walton, K., (1987), The effective elastic moduli of a random packing of spheres: Journal of the Mechanics and Physics of Solids, v. 35, p. 213–226.

Zimmer, M.A., (2003), Seismic velocities in unconsolidated sands―Measurements of pressure, sorting, and compaction effects: Palo Alto, Calif., Stanford, Ph. D thesis 204 p.