Application Of Higher-mode Surface Wave In Shallow Imaging

Using higher-mode dispersion curves extracted from microtremor recordings to inverse S wave velocity structure is a new geophysical method in urban exploration.The urban microtremor exploration mainly utilizes low frequency microtremor data caused by natural phenomena and high frequency microtremor data caused by human activities.It has the advantages of environment friendly,convenient data acquisition,cost-effective process and few limitations of survey region.So that,it is widely applied in many fields,such as engineering prospecting,environmental monitoring and resource exploration.The microtremor recordings include body wave and surface wave,and the surface wave is energy-dominant,thus we can extract surface dispersion curves from microtremor recordings.In general,the fundamental dispersion curve is the major component of the overall energy.But when there is a velocity reversal or strongly velocity change in underground structures,the higher-mode dispersion curves are stronger than the fundamental dispersion curve,or have a mode-kissing phenomenon.The higher-mode dispersion curves have more information of underground structure,and enhance the constraint on solutions of inversion problems,so it can effectively reduce the non-uniqueness and uncertainty of solutions.This paper will study the urban engineering exploration using higher-mode surface wave dispersion characteristics.In this paper,we firstly introduced the basic theory of the frequency-Bessel transform method(abbreviation as F-J method).Then,it was applied to actual urban mi-crotremor recordings from Shanghai,and we inversed S wave velocity using fundamental and higher-mode dispersion curves by the particle swarm optimization.Finally,we obtained the profile of S wave velocity with the depth range of 0?70 m and detected the low velocity zone caused by the ancient river.The results were evaluated and interpreted based on the borehole information.Through this survey,we summarized a scheme of acquisition,processing and evaluation of urban exploration,and it can provide a reference for urban engineering.Through theory analysis and realistic data verification,after comparing with the traditional urban exploration method,we draw the following conclusions:(1)the F-J method has strong anti-interference ability and low cost,and the data acquisition is efficient and convenient;(2)the F-J method can effectively extract the higher-mode dispersion curves and improve the accuracy of inversion.Therefore,the F-J method has wide application prospect in urban exploration.