THE AIM OF THIS PROJECT IS TO DEVELOP AND TEST COMBINED TOMOGRAPHIC INVERSION METHODS FOR TRANSMISSION AND REFLECTION OFFSET VSP DATA AND TO DEVELOP METHODS BASED ON SEISMIC MODELLING TO USE THE VSP RESULTS FOR A TARGET ORIENTED PROCESSING OF REFLECTIONS.
The aim of the research programme was the development of a tomography method to process, invert and evaluate travel time and amplitude data, to explore targets such as hydrocarbon reservoirs by means of reflection seismics. This method can be used for any source receiver geometries such as vertical seismic profiling (VSP), horizontal seismic profiling (HSP), cross hole geometries and their combinations. For the inversion, not only the first time (amplitude) onsets but also the later arrivals and amplitudes from refracted and reflected waves are used. Additionally, the derived model from the tomographic inversion for a target orientated processing based on beam focussing techniques is used in selected parts of the model.
In the forward modelling phase, the Gaussian beam method was selected and its reliability and accuracy tested. In the inversion phase, iterative reconstruction techniques were studied. The forward modelling was then fitted to the iterative inversion process. Results from a synthetic reverse VSP model were compared with the standard tomographic inversion.
A tomography method involving the adaptation of the Gaussian beam method to the simultaneous iterative reconstruction technique (SIRT) algorithm, was developed to process, invert and evaluate traveltime and amplitude data. This method can be used for any source receiver geometries like vertical seismic profiling (VSP), horizontal seismic profiling (HSP), crosshole geometries and their combinations. For the inversion not only the first time (amplitude) onsets but also the later arrivals and amplitudes from refracted and reflected waves are used. Considering late arrivals increases rely coverage, which leads to an improvement of the uniqueness and resolution of the velocity distribution. The derived model from the tomographic inversion was used for target oriented processing based on beam focusing techniques in selected parts of the model.
The conventional single point source is not appropriate for a target oriented exploration. A linear source array can be used to generate or simulate directed P-waves which illuminate specialunderground targets. The radiation angle can be determined by an empirical equation. This equation is based on the depth of the main impedance contrast in the prospecting area. In case of a simple model with a thin gas bearing layer beneath a high impedance contrast the signal to noise ratio of the reflection signal could be improved. This method can be applied when special targets are of great interest during tomographic measurements. The application of directed vertically polarized shear waves was investigated using a 2-dimensional analogue model with a thin gas bearing layer as the simulated target of the exploration.
If the goal of an exploration is a maximum resolution of seismic parameters in an area between 2 boreholes or around 1 borehole both methods, the tomographic inversion and the target oriented processing, are recommended in common. Tomography leads to a very good image of the underground structure which can be used to determ ine incidence angles for a target oriented processing with directed P-waves and S-waves. The tomography data can be used again for this second step and the results confirm the tomographic inversion results.
A COMBINED TOMOGRAPHIC INVERSION METHOD FOR TRANSMISSION- AND REFLECTION OFFSET VSP DATA WILL BE DEVELOPED. TRAVELTIMES AND AMPLITUDES OF FIRST AND LATER ARRIVALS CAN BE USED FROM TRANSMISSION DATA FOR A TOMOGRAHIC INVERSION RESULTING IN A TWO DIMENSIONAL VELOCITY AND ABSORPTION DISTRIBUTION IN THE OFFSET VSP PLANE. IN MANY LATERALLY INHOMOGENIOUS AREAS RAY TRACING, SEISMIC MODELLING AND THE USE OF CONSTRAINTS ARE NECESSARY TO IMPROVE THE INVERSION RESULTS. IN ADDITION, USING LATER ARRIVALS PICKED FROM THE VSP SEISMOGRAMS OR THE ENTIRE SEISMIC TRACES (VSP-CDP CONVERSION), THE EXPLORED AREA CAN BE EXTENDED.
THE RESULTS OF A SOPHISTICATED INTERPRETATION OF OFFSET VSP SURVEYS, AS DESCRIBED ABOVE, GIVE VERY SUITABLE PARAMETERS AND CONSTRAINTS FOR A TARGET ORIENTED PROCESSING OF DATA FROM REFLECTION LINES PASSING THE DRILL SITE. IN MANY FIELD CASES THE TOTAL SECTION IN LATERAL AND VERTICAL EXTENSION IS NOT OF THE SAME INTEREST AS SPECIAL SMALL SIZED AREAS. A SPECIAL PROCESSING AND INTERPRETATION (STRATIGRAPHIC, LITHOLOGIC) IS NEEDED FOR A LIMITED PART (TARGET) WITHIN THE SECTION. KNOWLEDGE OF MACRO-MODEL (VELOCITY AND ABSORPTION) FROM THE VSP PROCESSING AND A COMBINATION OF RAY TRACING WITH ARRAY TECHNIQUES ON THE SHOT AND RECEIVER SIDE CAN FOCUS THE PROCESSING AND INTERPRETATION ON THE PREDEFINED TARGET. RELATED INVESTIGATIONS WILL BE PERFORMED BY ANALOGUE AND NUMERICAL SEISMIC MODEL TECHNIQUES.