METHOD USING ELLIPSES TO INTERPRET SEISMIC REFLECTION DATA

Abstract

The properties of an ellipse can be used to interpret seismic reflection data by using the positions in a vertical section of a shot and a geophone as the foci of an ellipse. With the shot and geophone as the foci, the total time of travel of a reflected seismic wave serves as the constant necessary to define the ellipse. The reflecting surface then is tangent to this ellipse. Therefore, if many ellipses are plotted, the reflecting surfaces may be found by drawing smooth curves that are tangent in common to closely intersecting families of arcs. This basic principle is extended to the interpretation of complex structures that are not perpendicular to the line of traverse and to areas where the seismic velocity changes with depth by the following steps: The shots and geophones are plotted on a graph where the units along both the ordinate and the abscissa are virtual seismic traveltimes. These positions of the shots and geophones are then used as the foci of the ellipses as above. The reflecting surfaces are then drawn tangent to the dark bands of closely intersecting elliptical arcs. From this graph the one‐way time from a shot to a point of reflection, and from the point of reflection to a geophone may be scaled off; this is done by drawing the elliptical radii from the shot and geophone to the point of tangency between the ellipse and reflecting surface. The lengths of these radii are the one‐way times at the time scale of the graph. With the attitude of the wavefront as it returned to the surface at a geophone determined by a spread of three parallel geophone lines, and the one‐way time from the reflection point, one has the necessary and sufficient data to find the point of reflection in space coordinates for the assumed velocity function. Using the ray paths from the shot and geophone to this reflection point, the dip and strike of the reflecting surface at this point are found. This process is then repeated for every shot‐geophone combination for each reflecting surface.