Wayne Crawford

Seafloor compliance

Seafloor compliance is a measure of how much the seafloor moves under pressure forcing from ocean waves. The softer the ground is, the more the seafloor will move. By measuring this motion as a function of the forcing wavelength, we can determine the subsurface structure as a function of depth. Compliance measurements are most sensitive to the subsurface shear modulus. Low shear modulus bodies such as fluid-bearing sediments, hydrothermal circulation systems or melt bodies create a peak in the compliance that can be analyzed to determine the body's depth, size, and shear modulus.

Compliance measurement Cartoon representation of a compliance measurement (not to scale!), with typical signal wavelengths and amplitudes.

Compliance cartoonWe measure compliance in the period band from 10 seconds to 20 minutes, where the pressure signal comes from ocean surface gravity waves known as infragravity waves. These waves have a very simple dispersion relation, which allows us to calculate their wavelength from the frequency and the water depth. The infragravity waves we measure are from a few hundred meters to tens of kilometers long and a few millimeters tall. Underneath these waves, the seafloor moves a few micrometers vertically. We measure the pressure and displacement simultaneously using sensitive broadband seismometers and differential pressure gauges that we deploy to the seafloor for at least a day and a half at a time. We then throw out bad data caused by earthquakes and fish bumps (or SDEs?) and statistically analyze the remaining data to minimize the noise. During a typical 20-day cruise we can deploy one instrument at 10 different sites. Using several instruments and combining the measurements, we can create a 2D or 3D model of the subsurface shear modulus.

Compliance measurements have been used to study the melt structure beneath mid-ocean ridges and the porosity and physical properties of coastal sediments, and we are preparing an experiment to study sediments beneath basalt flows. Compliance measurements can be used to study any subsurface system where fluids play an important role, such as hydrothermal circulation networks, intraplate volcanoes, and subduction zones. An important future application of compliance measurements is continuous monitoring of time-variant fluid systems (such as the eruptive cycle of magma chambers or the change in a petroleum reservoir during drilling). Since the ocean waves are always "on", continous measurements should give a film of fluid movements that can be correlated to surface events. If the instrument is connected to the surface by a cable, these effects could be measured in near-real time.

References

Explanations of compliance and theoretical studies

  • Yamamoto, T., and T. Torii (1986). Seabed shear modulus profile inversion using surface gravity (water) wave-induced bottom motion, Geophys. J. R. Astr. Soc., 85, 413-431.
  • Trevorrow, M.V., T. Yamamoto, A. Turgut, D. Goodman, and M. Badiey (1989). Very low frequency ocean bottom ambient seismic noise and coupling on the shallow continental shelf, Mar. Geophys. Res., 11, 129-152.
  • Crawford, W.C. (1994). Determination of oceanic crustal shear velocity structure from seafloor compliance measurements. Ph.D. Thesis, University of California, San Diego.
  • Willoughby, E.C., and R.N. Edwards (1997). On the resource evaluation of marine gas-hydrate deposits using seafloor compliance methods, Geophys. J. Int., 131(3), 751-766.
  • Crawford, W.C., S.C. Webb and J.A. Hildebrand (1998). Estimating shear velocities in the oceanic crust from compliance measurements by two-dimensional finite difference modeling. J. Geophys. Res., 103(5), 9895-9916.
    • A 2-D compliance modeling algorithm
  • Latychev, Konstantin (2000). Numerical Modeling of Oceanic Crustal Hydrothermal Systems . Ph.D. Thesis, University of Toronto. - Chapters 6 and 7 discuss compliance, describe a 3-D compliance modeling algorithm (assumes static) Crawford, W. (2000). Seafloor compliance measurements: applications for hydrocarbon exploration. LITHOS, Cambridge, U.K., University of Cambridge, 151-156.
  • Hulme, T., A. Ricolleau, Sara Bazin, W.C. Crawford and S.C. Singh (2003). Shear wave structure from joint analysis of seismic and seafloor compliance data. Geophys. J. Int., 155, 514-520.
    • Shows how compliance can give different velocity results than seismic data because of different sensitivity to anisotropy
  • Latychev , K. and R.N. Edwards (2003). On the compliance method and the assessment of three dimensional sea floor gas hydrate deposits. Geophys. J. Int., 155(3), 923-952.
  • Crawford, W.C. (2004) The sensitivity of seafloor compliance measurements to sub-basalt sediments. Geophys. J. Int., 157, 1130-1145.
    • what it says
  • Hulme, T., W.C. Crawford, and S.C. Singh (2005). The sensitivity of seafloor compliance to two-dimensional low-velocity anomalies. Geophys. J. Int., 163, 547-558.

Experiments using compliance

  • Trevorrow, M.V., T. Yamamoto, M. Badiey, A. Turgut, and C. Conner (1988). Experimental verification of sea-bed shear modulus profile inversions using surface gravity (water) wave-induced sea-bed motion, Geophysical Journal, 93, 419-436.
  • Yamamoto, T., M.V. Trevorrow, M. Badiey, and A. Turgut (1989). Determination of the seabed porosity and shear modulus profiles using a gravity wave inversion, Geophys. J. Intl., 98(1), 173-182.
  • Trevorrow, M.V., and T. Yamamoto (1991). Summary of marine sedimentary shear modulus and acoustic speed profile results using a gravity wave inversion technique, J. Acoust. Soc. Am., 90(1), 441-456.
  • Crawford, W.C., S.C. Webb and J.A. Hildebrand (1991). Seafloor compliance observed by long-period pressure and displacement measurements. J. Geophys. Res., 96, 16151-16160.
  • Nye, T., and T. Yamamoto (1994). Concurrent measurements of the directional spectra of microseismic energy and surface gravity waves, J. Geophys. Res., 99(C7), 14321-14338.
  • Crawford, W.C., S.C. Webb and J.A. Hildebrand (1999). Constraints on melt in the lower crust and Moho at the East Pacific Rise, 9¡48'N, using seafloor compliance measurements. J. Geophys. Res., 104(2), 2923-2939.
  • Willoughby, E.C., and R.N. Edwards (2000). Shear velocities in Cascadia from seafloor compliance measurements, Geophys. Res. Lett., 27(7), 1021-1024, 2000.
  • Crawford, W.C. and S.C. Webb (2002). Variations in the distribution of magma in the lower crust and at the Moho beneath the East Pacific Rise at 9°-10°N. Earth Plan. Sci. Lett., 203(1), 117-130.
    • Same site, more data

Things related to measuring compliance

  • Webb, S.C. and W.C. Crawford (1999). Long period seafloor seismology and deformation under ocean waves. Bull. Seis. Soc. Am., 89(6), 1535-1542.
    • How compliance affects long period seafloor seismology
  • Crawford, W.C. and S.C. Webb (2000). Removing tilt noise from low frequency (<0.1 Hz) seafloor vertical seismic data. Bull. Seis. Soc. Am., 90(4), 952-963.
    • A potential noise source for compliance measurements, and how to remove it

Related papers

  • Beaumont, C., and A. Lambert, (1972). Crustal Structure from surface load tilts, using a finite element model, Geophys. J. R. Astr. Soc., 29 (2), 203-226.
    • Using tides to do "compliance" well before we or Yamamoto did
  • Sorrels, G.G., and T.T. Goforth (1973). Low frequency earth motion generated by slowly propagating, partially organized pressure fields, Bull. Seismol. Soc. Am., 63, 1583-1601, 1973.
    • The first formulation of the compliance of a half-space, but many notational errors
  • Webb, S.C., X. Zhang and W.C. Crawford (1991). Infragravity waves in the deep ocean. J. Geophys. Res. 96(C2), 2723-2736.