OBTAINING AN IMAGE OF AN UNDERGROUND FORMATION

摘要

1. A method of obtaining an image of an underground formation around a borehole extending through the underground formation, which method comprises the steps of (a) selecting a number of positions for an omnidirectional source and a three-component receiver in the borehole, selecting a number of underground positions in the formation, and attributing a value of zero to the underground positions; (b) arranging an omnidirectional source and a three-component receiver in a first position in the borehole; (c) activating the omnidirectional source to generate seismic energy and recording with the three-component receiver data in the form of the components of the reflected seismic energy; (d) determining from the components of the reflected seismic energy the directions from which the reflected seismic energy arrives at the three-component receiver as a function of two-way travel time; (e) selecting a first underground position; (f) calculating the arrival direction of a ray extending from the omnidirectional source to the underground position and back to the three-component receiver and the two-way travel time of seismic energy passing along the ray; (g) accepting the data if the calculated arrival direction is substantially equal to an arrival direction as obtained in step (d) pertaining to reflected seismic energy having the same two-way travel time, and adding the accepted data to the value attributed to the underground position; (h) selecting a next underground position and repeating steps (f) and (g) until the last underground position; and (i) arranging the omnidirectional source and the three-component receiver in a next position in the borehole, and repeating the steps (b) through (h) until the last position along the borehole to obtain the image of the underground formation comprising a set of data mapped on underground positions. 2. The method according to claim 1, wherein step (g) comprises determining the difference between the calculated arrival direction and the arrival direction as obtained in step (c) pertaining to reflected seismic energy having the same two-way travel time, multiplying the data with a weight factor which is a predetermined function of this difference, and mapping the weighted data on the underground position. 3. The method according to claim 1 or 2, wherein the data used in step (g) is the sum of the components of the reflected seismic energy recorded in step (c). 4. The method according to any one of the claims 1-3, wherein the omnidirectional source and the three-component receiver are coincident. 5. The method according to any one of the claims 1-4, wherein the three-component receiver further includes a pressure sensor.