Method for initializing receiver channels in a cement bond logging tool

1. A method for processing transducer signals in a well logging tool, comprising the steps of:

operating a multi-element piezoelectric receiving transducer in the logging tool, configured to provide a plurality of m receiver channels, wherein each element of the receiving transducer is operating in an anti-resonant mode at a frequency below the resonant frequency thereof;

initializing the input of each of the m receiver channels to a quiescent reference value corresponding to absence of a transmitted excitation signal;

calibrating each of the plurality of m receiver channels of the receiving transducer, while operating the receiving transducer in the anti-resonant mode, to obtain and store a set of digitized gain factors Fc for each receiver channel in memory in the tool; and

normalizing the output signals from the m receiver channels while operating the receiving transducer of the logging tool in an anti-resonant mode during a logging operation down hole to produce output signals from each of the plurality of m receiver channels.

2. The method of claim 1 , wherein the step of operating a multi-element transducer includes the step of:

providing a receiving transducer formed of a plurality of piezoelectric rings each reduced in axial length such that an excitation frequency applied during logging occurs more than one octave below a fundamental resonance mode of each ring of the receiving transducer.

3. The method of claim 2 , wherein the step of providing a receiving transducer includes the step of:

stacking the plurality of piezoelectric rings along a longitudinal axis wherein each adjacent pair of rings includes an acoustic isolation layer imposed there between.

4. The method of claim 1 , wherein the step of initializing the input of each of the m receiver channels comprises the steps of:

setting a jumper in the logging tool to enter an initialization mode;

disabling application of firing signals to a transmitter transducer in the logging tool;

running a predetermined number Q of run cycles in the logging tool to determine an average output signal value K to be stored in memory, for each of the m receiver channels;

determining whether the average value is within an acceptable range; and

repeating the foregoing steps until the averaged reference value is stored for all m receiver channels.

5. The method of claim 4 , wherein, the step of disabling comprises the step of:

ensuring that any input signal present at a receiver input is fully attenuated.

6. The method of claim 4 , wherein the step of running comprises the steps of:

storing read and digitized quiescent reference values P for each of the m receiver channels;

deleting the Q/4 lowest values and the Q/4 highest values of P for each of the m receiver channels; and

averaging the remaining values of P, storing the average value K in a mapped memory location for each of the m receiver channels.

7. The method of claim 4 , wherein the step of determining comprises the step of:

setting an acceptable range at +/−16 bits.

8. The method of claim 1 , wherein the step of calibrating comprises the steps of:

executing a calibration process to produce digital gain factors based on an average value N of a plurality of representative samples of a received signal; and

obtaining the digital gain factor Fc for use with each m receiver channel in the receiving transducer in the logging tool.

9. The method of claim 8 , wherein the steps of obtaining comprises the steps of:

selecting a standard value for a received input signal, Vin, std.;

calculating a digital gain factor Fc=(Vin,std.)÷N; and

storing values for Fc for all m channels in a memory in the logging tool for use during logging as calibration factors.

10. The method of claim 1 , wherein the step of normalizing comprises the steps of:

transmitting an excitation signal from a transducer in the logging tool;

digitizing and storing electrical logging signals corresponding to the received acoustic signals for each m channel; and

normalizing the digitized logging signals by revising their amplitudes according to the stored digital gain factors Fc applied to the digitized logging signals as calibration factors.