Response Characteristics Of Electromagnetic Wave Field And Its Application In Detecting The Bottom Boundary Of Riprap Of Seawall

China is one of the most frequent flooding countries in the world,and the seawall project plays an important role in the defense of coastal areas against typhoon storm surge disasters.In the engineering measures,the method of throwing stones to form the riprap layer is often used to enhance the stability and anti-erosion ability of the seawall.In practice,to evaluate the measurement and quality of seawall engineering,it is usually necessary to determine the depth of the bottom boundary of the riprap layer.In recent years,ground penetrating radar(GPR)technology has been gradually applied in the field of riprap layer detection due to its high detection efficiency and non-destructive.Aiming at the problem of inaccurate detection of the bottom boundary of riprap of seawall by GPR,based on the analysis of the current situation,the following research work has been carried out and relevant insights have been gained:(1)Based on the geoelectric information of a seawall,the detection models of the bottom boundary of riprap of seawall were constructed.By numerical simulation,the wave field propagation and response characteristics of electromagnetic wave at different excitation frequencies,different bottom morphology of riprap of seawall and different stratigraphic structures of seawall were studied.The results show that:(1)The electromagnetic wave with excitation frequency of 50 MHz is the most suitable for the requirements of field test.(2)The electromagnetic wave is only reflected and transmitted at the regular bottom boundary of riprap layer.The reflected wave is continuous in the same phase axis and time,which reflects the bottom boundary of the riprap layer intuitively;The electromagnetic wave is reflected,transmitted and diffused at the irregular bottom boundary of the riprap layer.The in-phase axis of the reflected wave is obviously misaligned.The ability to trace the bottom boundary of the riprap layer is weaker than that of the regular bottom boundary,but the accuracy of the determination of the morphology of the boundary is still acceptable.(3)When the covering layer thickness is less than 2.5 m,the covering layer only has a great influence on the picking of the initial arrival of the electromagnetic wave,but does not affect the determination of the bottom boundary of the riprap layer;When the thickness of covering layer and riprap layer changes,the reflection time of electromagnetic wave at the stratigraphic interface changes.The time difference of reflected wave at the upper and lower interface is linear with the thickness of covering layer and riprap layer;When the depth of the bottom boundary of the riprap layer is 10-13 m,the energy attenuation of electromagnetic wave is small,and the signal amplitude changes little.The electromagnetic wave responds well to the bottom boundary of the riprap layer,and the depth of the bottom boundary of the riprap layer can be identified reliably.(2)Considering the importance of electromagnetic wave velocity to determine the depth of the bottom boundary of riprap,a comprehensive "GPR + drilling " method was proposed.It uses information such as the lithology and depth of the strata exposed by the borehole to calibrate the velocity of the electromagnetic wave.Then the depth profile of electromagnetic wave is obtained by the time-depth conversion and the bottom boundary of riprap is tracked.The results of field test with comprehensive detection method show that: this method is more accurate to calibrate the propagation velocity of electromagnetic wave inside the seawall,and reliable to determine the depth of the bottom boundary of the riprap layer.It provides a strong technical support for the quality evaluation of engineering of riprap of seawall,so as to ensure the safety and normal operation of the seawall.Figure [28] table [12] reference [109]...