Investigation of underground cavities in a two‐layer model using the refraction seismic method

Abstract

ABSTRACTThis paper presents a new approach to the investigation of underground cavities. Our technique is based on the refraction seismic method. We have studied a two‐dimensional, two‐layer geological model. In our model, the lower seismic velocity layer is situated above the higher seismic velocity layer, with a circular cavity positioned within the upper layer. We have investigated the influence exerted by the cavity on the first arrivals of seismic waves. The obtained traveltimes are solutions of the eikonal equation and are presented using the time‐distance graph. All refracted waves encountering the cavity have to circumvent it, as it represents an impediment to the propagation of seismic energy. This circumvention causes delays in the first arrivals of the seismic waves at the surface as compared to traveltimes with no cavity present. These delays create a characteristic shape of the time‐distance graph, characterized by the peak point in which the plot line has a discontinuous change. Using this graph and analysing the delays of the first arrivals, we have derived expressions for determining both the position and size of the circular cavity. The practical application of the derived relations has been tested on a model test site built in a natural rock setting. This simple method indicates the presence of the cavity. The accuracy of the calculated cavity parameters: the horizontal position , the depth and cavity radius depends on the geophone spacing. For geophone spacing equal to or less than the cavity radius, the accuracy of the method is shown to be acceptable. The maximal estimated error is equal to a half geophone spacing. Finally, we have demonstrated that this method is also applicable in the detection of non‐circular cavities.