Publications

The goal of this paper is to present a very simple approach for the correction of a ",pushdown", effect generated in the shadow of faults. This procedure has been implemented and tested in TOTALINDONESIE. It is an uncomplicated and a more costeffective method than is generally proposed by contractors (Tomography or PSDM), and its accuracy has been successfully tested by several deviated wells in the Mahakam Delta, East Kalimantan. It is applicable in both exploration and delineation projects.These ",pushdown", effects can be observed on seismic time images beneath dipping extensional faults, and are due to seismic rays that first travel through the hanging wall block of the fault before imaging the footwall block. In the hanging wall block, sediments are usually less compacted and the velocities slower than in the footwall compartment. Consequently, the traveltimes are longer than expected, the result is a ",pushdown", image observed beneath the fault plane.The reality of wave propagation around a fault plane is probably very complex (diffractions at the edges of reflectors, refractions along the fault plane), however the main parameters that control this fault shadow effect can easily be assessed through an iterative and interpretative method.An initial 2D-depth model is built using dip seismic lines and a regional velocity function, which is then manually edited to remove the ",pushdown",. The interpreter must make an assumption concerning the amplitude of the fault shadow. Some zero-offset ray tracing is then carried out using image rays to compute the traveltimes, which correspond to the time migration associated with a constant interval velocity for each layer in the model. Finally, computed traveltimes are compared to the traveltimes observed in the original seismic data. The geometry of the model is then updated in order to minimize the difference between computed and observed traveltimes.Zero-offset seismic image-ray tracing is a simple trial and error approach, which can be very useful in predicting wave propagation artifacts such as fault shadow effects.