Classifications

• • G—PHYSICS
  • G01—MEASURING; TESTING
  • G01V—GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS
  • G01V1/00—Seismology; Seismic or acoustic prospecting or detecting
  • G01V1/28—Processing seismic data, e.g. for interpretation or for event detection
  • G01V1/284—Application of the shear wave component and/or several components of the seismic signal

Definitions

• the present invention relates to a method and algorithms for processing data allowing the use of surface waves.
• the present invention relates to the separation and use of the different propagation modes, as well as a method of '' fast and efficient reversal.
• This method is limited due to its slow execution as well as the imprecision due to an empirical inversion method.
• the SASW (“Spectral-Analysis-of-Surface-aves”) method was developed in the early 806s at the University of Texas (Heisey, J.
• the SASW method consists of three stages: collecting data in the field, evaluating the dispersion curve (phase speed as a function of wavelength) and transforming it into a continuous profile of the speed of shear waves by an inversion process.
• the SASW test is carried out on the ground surface. It consists in recording the Rayleigh waves generated by an impact source, using two space sensors of a distance denoted "D,". The test is repeated for different spacings between the sensors (2 times D ) till4 times D ⁇ s 8 times D., 16 times D., etc.). In general, the sensors are placed at an equal distance on either side of a central point. The source, meanwhile, is located at a distance equal to the spacing between the sensors, relative to the nearest sensor.
• the dispersion curve is determined. first by calculating a cross power spectrum which defines the phase shift, as a function of frequency, between the waves measured by the sensor farthest from the source and those measured by the nearest sensor (equivalent to time necessary for the wave to get from point 1 to point 2).
• the dispersion curve speed of the shear waves as a function of the wavelength
• the medium is represented by a set of N layers and to each are assigned a thickness, a Poisson coefficient, a density and a speed of the shear waves.
• a theoretical dispersion curve corresponding to this medium is then determined and compared with that obtained in the field. If the two curves, theoretical and experimental, coincide, the profile considered corresponds to the solution sought. Otherwise, the speeds assigned to all of the N layers are adjusted until there is agreement between the theoretical dispersion curve and that obtained in the field.
• MASW Multichannel Analysis of Surface Waves
• This method consists in measuring using a number of sensors between 20 and 64 or more, the waves generated using an impact source or using a vibrator (constant frequency) (Park et al, 1999, in Multichannel analysis of surface waves, Geophysics, Vol. 64, N. 3, pp800- 808; Xia et-al, 1999, in Estimation ofnear surface shear wave velocity by inversion of Rayleigh waves, Geophysics, , Vol. 64, N. 3, pp691-700; etc ).
• the so-called CMP (common-mid-point) configuration originally used in the SASW method, which consists of placing the source at a distance equal to the spacing between the first and the last sensor, is used in this method.
• the spacing between the sensors is defined according to the nature of the environment being studied and the depth sought.
• the speed profile determined using the CMP configuration thus represents the medium below the central point of the series of sensors. Park, 1999 indicates that it is best to use a constant source of energy.
• a dispersion curve phase velocity as a function of frequency or wavelength
• Data analysis in this method consists in measuring the degree of coherence between the signals: (1) filters on given frequency bands in the case where the tests are carried out using an impact source or ( 2) measurements directly using a constant energy source (vibrator).
• the dispersion curve (phase velocity as a function of the frequency or wavelength) is determined by evaluating the slope (linear) of each series of signals obtained for the same frequency.
• the MASW method is based on the assumption of an optimal configuration for the generation of the fundamental mode of the dominant Rayleigh waves and a weak energy of the higher modes and the other types of waves (shear and compression).
• the energies of other wave types and higher modes are considered to be noise.
• the determination of the speed profile of shear waves is carried out in the same way as in the SASW method, that is to say by the comparison of the experimental dispersion curve with a curve of theoretical dispersion corresponding to a medium defined by a certain number of layers to which are assigned a thickness, a speed, a density and a Poisson's ratio.
• An object of the present invention is therefore to present a method of modal analysis with the aim of remedying certain limitations of the previous methods, such as those described above, based on Rayleigh waves.
• This new method is known by the acronym S WAP ("Surface Wave Automated Profiling").
• Another object of the present invention is to propose an algorithm allowing the implementation of this method.
• Figure 1 shows in schematic form the steps of the method according to a possible embodiment of the present invention.
• the method according to the present invention does not pose any a priori hypothesis as to the importance of the different modes which contribute to the signals recorded in the field. It consists in determining and separating the different modes of Rayleigh waves in order to use them to better characterize the environment examined.
• the method consists in generating surface waves using an impact source, and in detecting it using sensors, placed at one or at different distance intervals, defined as a function of the depth over which the test is carried out as well as the nature of the terrain being studied ( Figure 2).
• the sensors will be all the more distant the greater the depth to be studied, for example.
• the distance between sensors is adjusted as a function of the attenuation characterizing the medium to be studied.
• the number of sensors used is a compromise between the cost incurred and the desired detection sensitivity. A greater number of sensors allows greater accuracy of the measurements. Nevertheless, analyzes show that the use of a number of 16 sensors is a good compromise between the cost of the equipment and the precision sought. It is however conceivable to use a greater number of signals in the analysis (24 or 32) by using a greater number of sensors or by carrying out two successive SWAP tests while keeping the same source of energy as shown in FIG. 2. The realization of two successive SWAP tests allows the determination of a greater number of speed profiles shear waves by processing different combinations of 16 successive signals (Figure 2). The determination of a large number of velocity profiles then makes it possible to present the results in the form of a tomography of velocity of the shear waves in two dimensions (FIG. 3).
• the This method analyzes the signals collected in the frequency-wave number plane (also called time-space plane), so as to determine an energy spectrum of the signals in these two domains. This procedure requires triggering the recording of signals from the moment of impact.
• the processing of the signals is done adaptively using a filtering procedure which makes it possible to adjust the resolution at the analysis frequency and which can be assimilated to a wavelet analysis.
• the entire analysis process including, separation, identification and selection of different modes for reversal end is automated.
• the SWAP method also incorporates a weighting system which allows the energies of the different signals to be modulated so as to give more importance to a precise point in space (located within the distance covered by the sensors).
• This system not only allows a reduction in the disturbance of the dispersion points which can be produced by significant variations in the environment investigated, but also allows the production, using different weighting systems, of at least three dispersion curves which represent different places. within the distance covered by the 16 sensors.
• the inversion that is to say the determination of the speed profile of the shear waves, is done from the dispersion curves of at least two modes of the Rayleigh waves.
• the inversion technique proposed in the present invention is based on the comparison of experimental and theoretical dispersion curves, in terms of difference as well as in terms of shape, and allows faster reversal and better exploitation of the curves. of experimental dispersion.
• the SWAP method proposed in the present invention differs from methods such as SASW and MASW in particular, by the fact that no assumption is made at the outset as to the dominance of the fundamental mode of the Rayleigh waves.
• the present method tackles the problem as a whole by first identifying the different modes of Rayleigh waves, and by reconstituting, in a second time, the medium which corresponds to all of these modes.
• the SWAP method not only allows an unambiguous identification of the different components of the terrain (since it may happen that a higher mode dominates over a certain range of frequencies), but also the evaluation of a Poisson coefficient profile in addition to a speed profile of shear waves N s .
• the different stages of the method according to the present invention are carried out by means of original algorithms.
• the inversion process is based on the comparison of the calculated and experimental dispersion curves, not only in terms of phase speed difference, but also in terms of the shape of the dispersion curve.
• This reversal process according to the present invention is automated using an algorithm designated by the acronym I ⁇ NSS.
• I ⁇ NSS an algorithm designated by the acronym I ⁇ NSS.
• the use of form criteria in addition to the difference allows a very fast convergence of the process.
• the processing of signals recorded in the field for the determination of the different modes of the Rayleigh wave is automated using an algorithm compatible with DSTVSS.
• the method also innovates by using the group speed which corresponds to the propagation of the wave train or energy and which is used for cleaning, identification and verification (consistency) of the different types of waves.
• the SWAP method of the present invention makes it possible to establish a standard by the arrangement of a defined number of 16 sensors b of the spacings established according to the study envisaged. It goes without saying that the present invention has been described purely by way of indication and that it can accommodate several other arrangements and variants without however exceeding the scope of the present invention as defined by the claims which follow.

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• Engineering & Computer Science (AREA)
• Remote Sensing (AREA)
• Physics & Mathematics (AREA)
• Life Sciences & Earth Sciences (AREA)
• Geology (AREA)
• Environmental & Geological Engineering (AREA)
• Acoustics & Sound (AREA)
• General Life Sciences & Earth Sciences (AREA)
• General Physics & Mathematics (AREA)
• Geophysics (AREA)
• Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
• Geophysics And Detection Of Objects (AREA)
• Investigating Or Analyzing Materials By The Use Of Ultrasonic Waves (AREA)

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