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Sonic logs commonly were not included as part of the logging program on older wells. In certain areas, wells are drilled with air. The absence of drilling fluid in the borehole eliminates the possibility of recording a sonic log. In other areas, sonic logs are either of poor quality or, for one reason or another, omitted from the logging program. A method has been developed to predict interval transit times using nuclear well logs. It involves combining neutron, density, and gamma-ray log measurements into a log of predicted interval transit times referred to as a synthetic sonic log.
The method involves a combination of commonly used and accepted well-log interpretation techniques. It effectively accounts for lithology, shale, porosity and hydrocarbon effects. The method requires only 3 parameters, which may be selected based on the well-log data. The synthetic sonic log agrees well with actual sonic-log measurements over a wide variety of geographic areas and borehole depths. The method is effective for formations commonly encountered in geophysical well logging including salt, shale, sandstone, and carbonate.
The synthetic sonic log is an excellent substitute for use in generating synthetic seismograms and establishing time-depth relationships.
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