The influence of curvilinear form of boundaries on reflected wave field

Abstract

The paper is devoted to the influence of curvilinear reflection boundaries on kinematic and dynamic 1haracteristics of reflected seismic waves. First of all, the paper describes the major reasons of low geological efficiency of CDP seismic survey in the areas of Azerbaijan, characterized by complex surface and deep seismic and geological environmental features and in this aspect, the special attention is paid to curvilinear boundaries. The paper displays statistical data for Shamakhy-Gobustan province, where of 132 anticlinal folds only a few are under production currently. It is considered, how the spatial curvilinear nature of reflection surfaces influence reflection waves propagation. In addition, by use of 3D seismic data the formula is given for breakdown of reflection waves along propagation planes in space. As the example, VSP data from a well in Absheron peninsula are displayed and on the seismic records, it is possible to clearly observe the out of plane waves, which do not propagate in the section plane (in the vertical plane). Velocity analysis is based on evaluation of coherency measure, which allow to assess the correlation level of reference point of seismic traces in the given time window along the hyperbolic trajectory of curvature depending on time and velocity. It is known that in case of curvilinear reflection boundaries the observed travel time graphs are far from hyperbolic by their shape, and due to this, in the process of velocity analysis we derive both optimal stacks, which are the subject to the rule of velocity change versus time, common to this area, and also quite different stacks, which we name as “anomalous”. The great importance is to make clear the nature of these “anomalous” stacks at the initial stage of study as they keep some hidden information about the geology of the area. Analysis display that in addition to folded reflections, the cause of these anomalous stacks may be the curvilinear boundaries, horizontal gradient of the velocity. Curvilinear boundaries influence also the dynamic parameters, wave amplitudes in particular, as in the case of curvilinear boundaries the seismic rays are focusing due to the concaved shape of boundary. It has been noted that if the curvature of a syncline form boundary is not constant, then at the defined reception points the reflections arrive from several parts (points) of syncline and the reflected energy will be recorded as several branches (most frequently as three). As the light is focused when it passes through the lens, the seismic waves are similarly focused by curvilinear boundaries and due to the refraction the seismic rays are bended and we observe intensive reflections on seismic sections.