Decomposing full-waveform sonic data into propagating waves for characterizing a wellbore and its immediate surroundings
A method for estimating parameters of a propagating wave field, such as the direction of propagation in 3D space of an acoustic wave from borehole-acoustic data. The estimation technique uses phase delays between recordings made by the individual receivers related to a plane wave travelling across the receiver array. The estimated plane waves include the wave field generated directly by the acoustic source, as well as refracted and reflected components of these fields. The technique can be used with overlapping wave fields. This will provide significant improvements in the quality of formation properties estimated from full-waveform data that are obtained either from wireline or from while-drilling sonic data. The inventive method enables characterization of abnormalities outside multiple casing strings where acoustic signals are transmitted from a source located inside said multiple casing string.
1. A method for characterizing properties outside a multiple casing string, comprising transmitting acoustic signals from a source located inside said multiple casing string; recording received signals on an array of acoustic receivers mounted on a longitudinal cylindrical tool inside said multiple casing string, and processing received signals by performing the following steps:
a. separating received acoustic signals into components of propagating plane waves by measuring the phase delay of received signals;
b. estimating propagation parameters for each component of propagating plane waves for differentiating different propagating wave modes;
c. sorting propagating wave modes into borehole modes and body wave modes;
d. processing and characterizing body wave modes for characterizing properties outside of the multiple casing string; and
e. estimating direction of abnormalities based on ray parameters of the plane wave components.
2. The method according to claim 1 , where the estimation of propagation parameters for each component of propagating plane waves is based on signal power and semblance measured.
3. The method according to claim 1 , where characterized properties includes air pockets and quality of cement outside the casing string.