Recent advances in VSP tools and VSP analysis techniques can be used to reduce the uncertainities associated with Salt Proximity Surveys. Using these advances, the direction selsmtc energy is traveling at the geophone is measured and used to determine a slngle salt exit point instead of the usual aplanatic curve (the locus of all posslble salt exit points).
The Salt Proximity technique is one of the oldest geophysical techniques known. The Salt Proximity survey is designed to answer an Important questlon critical to successful exploration and production In the vicinity of salt domes, After a well is drilled in the vicinity of a salt dome, whether the well is successful or not, the next question that is asked is how far from the salt mass did the well penetrate. A Salt Proximity can be conducted to try to answer thls questlon. This paper will demonstrate some improvements that have been made to thls old technlque. These improvements were realized by applylng recently developed VSP tools and analysis techniques to Salt Proximity surveys. In particular the current practlce of using three component downhole geophones and arrival direction measurements from hodogram analysis allows some of the uncertalntles of Salt Proximity surveys to be resolved. I will then show how these techniques were used on a real salt dome to find a salt protrusion.
The Salt Proximity survey is based on tihe fact that salt has a significantly higher sonic veloclty than sediments. The salt proximity survey is conducted In the following manner. A set seismic source is placed on the surface above the top of the Salt mass and a geophone ts placed in the well. The first arrival time of the seismic energy is recorded on the geophone. Since the locat ion of the source and the geophone are known, the distance between them is known. In most places the seismic veloct ty of salt is constant and known. If only salt existed between the source and the geophone the first arrival time would be (source to geophone distance)/(salt velocity). Any first arrival time greaterthan this is due to the existence of sediment of lower velocity between the source and the geophone. Furthermore the amount of excess first arrival tlme would be proportional to the amount of sediment bet ween the source and receiver if the energy traveled in a straight line from the source to the receiver. The amount of sediment could be determined if the velocity of the sediment were known or could be guessed. The downhole geophone has undergone a considerable development in the last several years. First it was determlned that the recorded signal could be improved if the geophone tool was clamped to the well bore. Then It was determined that better signals were recorded if the welght of the geophone tool was reduced, This led to the development of tools with high clamptng force to welght ratios.
