# US 2013/0155812 A1 · Using Higher Order Harmonics to Determine Acoustic Impedance of Material Behind Borehole Casings > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 2013/0155812 A1 - Document ID: US20130155812A1 - Status: Published application - Kind: A1 - Application number: 13/330,676 - Filed: 2011-12-20 - Published: 2013-06-20 - Earliest claimed: 2011-12-20 - Assignees: Schlumberger Technology Corporation - Inventors: Benoit Froelich; Andrew J. Hayman; Kenneth K. Liang - Web page: https://patentgrep.com/patent/US20130155812A1 ## CPC - G01V 1/306: {for determining physical properties of the subsurface, e.g. impedance, porosity or attenuation profiles} - E21B 47/005: Monitoring or checking of cementation quality or level - G01V 1/50: Analysing data - G01V: GEOPHYSICS; GRAVITATIONAL MEASUREMENTS; DETECTING MASSES OR OBJECTS; TAGS (means for indicating the location of accidentally buried, e.g. snow-buried, persons A63B29/02) - E21B: EARTH OR ROCK DRILLING; OBTAINING OIL, GAS, WATER, SOLUBLE OR MELTABLE MATERIALS OR A SLURRY OF MINERALS FROM WELLS ## Abstract An acoustic logging tool emits a wideband acoustic pulse toward an inside surface of a borehole casing and an acoustic response is received. The acoustic response is analyzed at one or more higher order harmonics of the thickness mode resonance of the casing to determine the acoustic impedance of the material behind the casing. ## Claims ### Claim 1 A method of determining an acoustic impedance of a material behind a casing, the method comprising: emitting a wideband acoustic pulse toward an inside surface of the casing; receiving an acoustic response corresponding to the emitted acoustic pulse; identifying at least one second or higher order harmonic of a thickness mode resonance frequency associated with the casing; and analyzing the acoustic response at the at least one second or higher order harmonic to determine the acoustic impedance of the material. ### Claim 2 (depends on 1) The method of claim 1, wherein identifying the at least one second or higher order harmonic includes identifying a portion of the acoustic response based on a thickness of the casing. ### Claim 3 (depends on 1) The method of claim 1, further comprising: identifying the thickness mode resonance frequency associated with the casing; and analyzing the acoustic response at the thickness mode resonance frequency and at the at least one second or higher order harmonic to determine the acoustic impedance of the material. ### Claim 4 (depends on 3) The method of claim 3, wherein the at least one second or higher order harmonic includes a plurality of second, third and/or higher order harmonics. ### Claim 5 (depends on 4) The method of claim 4, wherein analyzing the acoustic response at the thickness mode resonance frequency and at the at least one second or higher order harmonic comprises analyzing the acoustic response at the thickness mode resonance frequency and at each of the plurality of second, third and/or higher order harmonics to generate a set of acoustic-impedance values. ### Claim 6 (depends on 5) The method of claim 5, further comprising performing a comparison of the set of acoustic-impedance values and determining the acoustic impedance based on the comparison. ### Claim 7 (depends on 6) The method of claim 6, further comprising determining the acoustic impedance based on a combination of members of the set of acoustic-impedance values. ### Claim 8 (depends on 1) The method of claim 1, wherein analyzing the acoustic response comprises processing the acoustic response using an inversion algorithm, wherein the inversion algorithm analyzes the acoustic response at the at least one second or higher order harmonic. ### Claim 9 (depends on 1) The method of claim 1, further comprising determining that the casing comprises carbon steel, wherein analyzing the acoustic response further comprises determining a corrected acoustic impedance associated with carbon steel. ### Claim 10 (depends on 1) The method of claim 1, further comprising determining a wave velocity associated with the casing, the wave velocity being one of a compression wave velocity and a shear wave velocity, wherein analyzing the acoustic response further comprises determining a corrected acoustic impedance associated with the wave velocity. ### Claim 11 (depends on 1) The method of claim 1, further comprising determining a density associated with the casing, wherein analyzing the acoustic response further comprises determining a corrected acoustic impedance associated with the density. ### Claim 12 (depends on 1) The method of claim 1, further comprising: determining a first ratio, wherein the first ratio is a ratio of the second harmonic frequency with the thickness mode resonance frequency; determining a second ratio based on the first ratio, wherein the second ratio is a ratio of compression-wave velocity to shear-wave velocity associated with the casing; and determining a corrected acoustic impedance by applying a correction based on the second ratio. ### Claim 13 A method of determining a an acoustic impedance of a material behind a casing, the method comprising: emitting a wideband acoustic pulse toward an inside surface of the casing; receiving an acoustic response corresponding to the emitted acoustic pulse; identifying at least one second or higher order harmonic of a thickness mode resonance frequency associated with the casing by analyzing the acoustic response; determining an acoustic impedance of the material by analyzing the acoustic response at the at least one second or higher order harmonic; and determining a corrected acoustic impedance of the material by applying a correction to the acoustic impedance of the material. ### Claim 14 (depends on 13) The method of claim 13, wherein determining the acoustic impedance of the material comprises analyzing the acoustic response at the thickness mode resonance frequency and at each of a plurality of higher order harmonics to generate a set of acoustic-impedance values and determining the acoustic impedance based on a combination of members of the set of acoustic-impedance values. ### Claim 15 (depends on 13) The method of claim 13, wherein identifying the at least second or higher order harmonic includes identifying a portion of the acoustic response based on a thickness of the casing. ### Claim 16 A system for determining an acoustic impedance of a material behind a casing, the system comprising: an ultrasonic-imaging (USI) tool having an acoustic transducer that emits a wideband acoustic pulse toward the casing and receives an acoustic response thereto; and a computing device including a processor and a computer-readable medium having computer-executable instructions embodied thereon that, when executed by the processor, causes the computing device to determine an acoustic impedance of the material, the computer-executable instructions comprising a multi-harmonic inversion component that identifies at least one second or higher order harmonic of a thickness mode resonance frequency associated with the casing by analyzing the acoustic response, wherein the multi-harmonic inversion component analyzes the acoustic response at the at least one second or higher order harmonic to determine the acoustic impedance of the material. ### Claim 17 (depends on 16) The system of claim 16, wherein the computer-executable instructions further comprise a correction component that determines a corrected acoustic impedance of the material by applying a correction to the determined acoustic impedance of the material. ### Claim 18 (depends on 16) The system of claim 16, wherein identifying the second or higher order harmonic frequency includes identifying a portion of the acoustic response based on a thickness of the casing. ### Claim 19 (depends on 16) The system of claim 16, wherein the at least one second or higher order harmonic includes a plurality of second, third and/or higher order harmonics. ### Claim 20 (depends on 19) The system of claim 19, wherein the inversion component analyzes the acoustic response at the thickness mode resonance frequency and at each of the plurality of second, third and/or higher order harmonics to generate a set of acoustic-impedance values and wherein the multi-harmonic inversion component determines the acoustic impedance based on a combination of members of the set of acoustic-impedance values.