Synchronized digital stacking method and application to induction logging tools

A method of measuring conductivity of a geological formation adjacent to a borehole, comprising the steps of:

• positioning an apparatus including a transmitter and a receiver at a desired position in the borehole;
• generating an oscillating current signal having a fundamental frequency and applying said oscillating current signal to said transmitter, wherein said oscillating current signal induces eddy currents in said formation, the magnitudes of said currents being proportional to said conductivity of said formation;
• receiving an electrical signal induced in said receiver by said eddy currents in said formation;
• generating a clock signal which has a frequency which is an integer multiple of said fundamental frequency of said oscillating current signal, said clock signal synchronized to said oscillating current signal;
• converting said received electrical signal into digital signal samples, wherein said digital signal samples are provided on at least an integer multiple number of cycles of said clock signal, each of said samples representing a different portion of a period of said oscillating current signal;
• summing corresponding digital signal samples obtained at each portion of said period of said oscillating current signal for a plurality of periods of said oscillating current signal, wherein said summed digital signal samples form a stacked signal for each portion of said period of said oscillating signal;
• and
• performing a frequency analysis of said stacked signals to derive an amplitude of an in-phase component and an amplitude of a quadrature component of said stacked signals at said fundamental frequency, wherein said amplitudes are indicative of said conductivity of said formation.

A method for processing signals from a well logging tool, said tool used to determine at least one characteristic of a formation penetrated by a wellbore, said tool comprising at least one transmitter and at least one receiver, said method comprising the steps of:

• activating said at least one transmitter with electrical energy having a periodic waveform, said electrical energy also having a predetermined fundamental frequency and predetermined supplemental frequencies, for a predetermined whole number of cycles of said energy, said cycles corresponding to said predetermined fundamental frequency;
• activating said at least one receiver to generate electrical signals corresponding to a characteristic response of said formation to said energy;
• digitizing said electrical signals into a plurality of digital signal samples corresponding to time segments of a single cycle of said energy;
• repeating said step of digitizing said signals for a number of times equal to said predetermined number of cycles of said energy during subsequent cycles of said energy;
• adding time-correspondent samples from said subsequent cycles of said energy to said time-correspondent samples from previous cycles of said energy, thereby creating stacked signals corresponding to said time segments of said cycle of said energy;
• and
• spectrally analyzing said stacked signals to determine said at least one characteristic response of said formation to said energy.

A logging tool for measuring the conductivity of a subsurface geological formation adjacent to a borehole in association with surface equipment, comprising:

• an oscillating signal generator for providing an oscillating signal having a predetermined frequency and waveform and a clock signal having a frequency which is an integer multiple of the frequency of said oscillating signal, said clock signal synchronized to said oscillating signal,
• at least one transmitting coil coupled to said oscillating signal generator for receiving said oscillating signals and inducing eddy currents in response thereto in the subsurface geological formation proportional to the conductivity of the geological formation,
• at least one receiving coil for detecting said electric fields created by said eddy currents produced in said geological formation by said transmitted signals and generating analog electrical signals in said at least one receiving coil in response thereto,
• an analog-to-digital converter coupled to said at least one receiving coil and said oscillating signal generator for converting said received analog electrical signals proportional to the conductivity of the geological formation into digital signals,
• a digital sampling circuit coupled to said analog-to-digital converter and said oscillating signal generator for converting said digital signals into a predetermined number of digital signal samples occurring during each cycle of said oscillating signal, each of said samples representing a different portion of said oscillating signal cycle,
• a digital stacking circuit coupled to said digital sampling circuit and said oscillating signal generator for summing said digital signal samples obtained at each portion of said oscillating signal cycle for a plurality of oscillating signal cycles and forming a stacked digital signal for each portion of said oscillating signal cycle, and
• a telemetry unit coupled to the surface equipment and said stacking circuit for providing said stacked digital signals for all portions of said oscillating signal cycle to said surface equipment, wherein said surface equipment performs a frequency spectral analysis of said stacked digital signals to determine the conductivity of the geological formation.