Passive Surface Wave Survey And Its Application In Urban Area

Passive seismic measurement has been intensively studied for the purpose to make up the shortage of the active measurement.In highly populated urban areas,the passive surface wave methods particularly grasp its chance to blossom because traditional geophysics surveys can not be properly applied.Compared with the active surface wave methods,the passive surface wave methods possess the advantages ?1 to complement active survey methods by exploring greater depth due to an extending the source spectrum to lower frequencies;?2 to save costs associated with active sources in field operations;?3 to monitor the long term mechanical evolution of structures and grounds in civil engineering scale.Despite the clear trend in implementingthe passive surface wave methods to characterize underground structure on multiple scale,challenges still exist due to thenon-uniform noise source distribution.?1 Handling with the azimuthal effects caused by directional noise sources.The azimuthal effects will inevitably bring biases into dispersion measurements due to the fact that true randomness of ambient noise cannot be achieved in reality.?2 Improving the vertical resolution of the passive surface wave measurements.The passive sources is capable to generate low-frequency surface waves with deeper penetration,but cannot ensure the near-surface resolution.If the passive surface wave methods can provide phase velocities up to several tens of Hz,the active surface wave method might not need to be performed and the amount of field work could be dramatically reduced.?3 Classifying passive surface waves.In practice,the duration(or the record-length)of the observed seismic noise data is always limited,or expected to be shorten as soon as possible,to save costs in field work.Temporal averaging would not be sufficient to fulfill requirements for the stationary phase assumption.It is crucial to apply classification or data selection on passive surface waves to minimize the contributions from non-stationary sources.In this thesis,I advance the methodology of the passive surface wave survey and at the same time study the applications of passive surface wave methods in urban area using traffic-induced seismic noise.To handle with azimuthal effects caused by directional noise sources,multichannel analysis of passive surface(MAPS)waves based on long noise sequence cross-correlations is proposed.I apply seismic interferometry to produce common virtual source gather from1 hour multichannel passive surface waves.Common virtual source gathers are then used to do dispersion analysis with an active surface wave scheme based on phase-shift measurement.This thesis indicates the superiority of MAPS over the roadside passive multichannel analysis of surface waves(MASW)on the validity of azimuth detection,feasibility of combining the active MASW and MAPS,and accuracy in determining dispersion energy trends,especially at a relative low-frequency range(1 ? 10Hz)in urbanareas.In order to provide phase velocities up to several tens of Hz to image very shallow structure,the active surface wave methods are usually required to increase thefrequency range beyond the power of the passive surface wave survey.Considering that they can provide high-frequency phase velocity information up to several tens of Hz,the active surface wave survey would be omitted and the amount of field work could be dramatically reduced.I propose a strategy for high frequency(> 10 Hz)passive surface wave survey by imposing active sources during continuous passive surface wave measurement,which is therefore called “mixed-source surface wave measurement”.A series of shortduration(< 10 mins)comparison experiments manifest the flexibility and applicability of the proposed method in high-frequency phase velocity analysis.I suggest that it will be constructive for seismic investigation to perform mixed-source surface wave survey,rather than exclusively performing either active surface wave measurement or passive surface wave measurement.However,the high-frequency dispersion energy image in the passive surface wave survey would usually be polluted by a type of “crossed” artifacts.It is common in the bidirectional noise distribution case with a linear receiver array deployed along roads or railways.I review several frequently used passive surface wave methods and derive the underlying physics for the existence of the “crossed” artifacts.I prove that the“crossed” artifacts would cross the true surface wave energy at fixed points in the f-v domain and propose a FK-based data selection technique to attenuate the artifacts in order to re-trieve the high-frequency information.Numerical tests further demonstrate the existence of the “crossed” artifacts and indicate that the well-known wave field separation method,the traditional FK filter,does not work for the selection of directional noise data.Real-world applications manifest the feasibility of the proposed FK-based technique to improve passive surface wave methods by a priori data selection.Due to logistical constraints,passive seismic acquisition in urban areas is mostly limited to short recording periods.Non-stationary sources contributions would not be canceled out because of insufficient temporal averaging,then the dispersion measurements would be smeared,especially on the low-frequency band.I formulate a criterion in the taup domain for selective stacking of dispersion measurements from passive surface waves and apply it to high-frequency(> 1 Hz)traffic noise.The criterion is based on the automated detection of input data with a high signal to noise ratio in a desired velocity range.Modelling tests demonstrate how the criterion captures the contributions from the non-stationary sources.Real-world application shows that the proposed data selection approach significantly improves the dispersion measurements by extending the lower frequency band(< 5 Hz)and attenuating the distortion at the higher frequency band(6~13Hz).