Objectif It is well known that vertical seismic profiles (VSP) are useful tools for predicting lithology ahead of the drill bit. The goal of the research is to verify whether the seismic waves, both P ad S, generated by the bit during drilling, could be utilized to generate a reciprocal VSP by properly disposed receivers on the surface and in the subsurface.During the reseacrh :- a theoretical study and analysis of the data collected around two production wells was performed, to discriminate the signal produced by the rotating bit from amongst the large number of wave fields produced by the drilling machinery;- new data processing techniques were developed for the noise determination and reduction;- a graphic workstation was used for processing the seismic data in the field, for demonstrating the feasibility of a prototype system capable of producing a reciprocal VSP directly in the field;- the signal produced by the bit while fracturing the rock was identified and separated from other spurious unwanted noise;- using such a signal, a reciprocal VSP was produced in which reflections below the drilling level were both present and in agreement with events evident in the surface seismic sections;- the drill-bit signal first arrivals were detected along the seismic line, demonstrating the possibility of using tomographic techniquesfor evaluating the velocity field around the well.The first problem to solve is the identification of both the position and signature of the source.It is not true that the terrain is energized only at the position of the bit; the drill touches the sides of the well and disturbances are generated. Moreover the bit generates S waves as well as P waves during its rotation and hammering.The undesired waves should be removed using their different frequency, wavelength polarization using frequency filters, geophone groups and 3D geophones.It is necessary to stress that some of these waves might be useful, subsequently, for lithological purposes. In fact, S waves might very well enrich our data base, provided that they are properly separated.A graded experiment that removes, one by one, the uncertaines, would certainly be useful since no modeling is currently able to simulate all the various, at present unpredictable, effects on the bit.Once the signature of the source is found, a crosscorrelation of that signature with the signalsrecorded by the geophones will allow the determination of :- differential propagation times between bit, geophones on the surface and buried, and therefore seismic velocities- bit location, using seismological techniques.Finally once the location of the bit is determined, it will be necessary to solve the problem of the prediction of the lithological characteristics in the layers beyond the actual penetration.If the upgoing wave only is recorded, filtering out all other non relevant waves, the autocorrelation of that signal, for positive times only, will provide a signal that will contain, after the direct arrival, all the other reflections at their proper times. Programme(s) ENG-HYDROCARB 2C - Programme (EEC) of support for technological development in the hydrocarbons sector, 1986-1989 Thème(s) 1.4 - GEOPHYSICS AND PROSPECTING Appel à propositions Data not available Régime de financement DEM - Demonstration contracts Coordinateur ISTITUTO NAZIONALE DI OCEANOGRAFIA E DI GEOFISICA SPERIMENTALE Contribution de l’UE Aucune donnée Adresse Borgo Grotta Gigante 42/C SGONICO-TRIESTE Italie Voir sur la carte Coût total Aucune donnée
