Method for real-time extraction of ocean bottom properties

• US 5,475,651 A
• Filed: 10/18/1994
• Issued: 12/12/1995
• Est. Priority Date: 10/18/1994
• Status: Expired due to Fees

• Current Assignee: the united states of america as represented by the secretary of the navy
• Original Assignee: the united states of america as represented by the secretary of the navy
• Inventors: Sundvik, Michael T., Grande, David W., Bishop, Judith L.
• Primary Examiner(s): Pihulic, Daniel T.
• Application Number:
  US08/330,142
• Time in Patent Office: 420 Days
• Field of Search: 367/88, 367/131, 367/135, 367/117, 367/124, 364/423, 364/923.4
• US Class Current: 367/88
• CPC Class Codes: G01S 15/89 for mapping or imaging
• CPC Class Codes: G01S 7/539 using analysis of echo signal for target characterisation; Target signature; Target cross-section
• CPC Class Codes: G01V 1/38 specially adapted for water-covered areas G01V1/28 takes precedence

Abstract

A method for characterizing ocean bottom properties in real-time by explong the multipath structure of reverberation to extract values for bottom loss and the bottom scattering coefficient. The bottom parameters are extracted from a measured reverberation energy envelope generated from the reverberation returns of one or more pings from a sonar system. The measured reverberation envelope is compared to a reference reverberation model to identify a period of time in which the measured reverberation energy envelope exhibits properties that allow the extraction of bottom parameters. The bottom parameters are then extracted by direct comparison of the measured reverberation to the reference reverberation model within the time period or by iteratively changing the value of bottom loss or bottom scatter coefficient and recalculating the reference reverberation for the identified time period until the reference reverberation matches the representative reverberation. The accuracy of the reference reverberation model is maintained by continuously monitoring the current sonar state configuration and the current environmental parameters including bottom depth, sound speed profile, and wind speed and rebuilding the model should either the sonar configuration or the environmental parameters change in a manner which would cause a change in the model.

Claims

1. A method for extraction of ocean bottom parameters in real-time comprising the steps of:

periodically determining environmental parameters;

periodically determining a sonar system state configuration;

periodically obtaining a reverberation energy envelope;

monitoring the environmental parameters and the current sonar system state configuration for changes;

creating an eigenray computation model in response to changes in the environmental parameters or changes in the sonar system state configuration;

generating a reference reverberation model using the eigenray computation model; and

characterizing ocean bottom parameters from the reverberation energy envelope by using the eigenray computation model and the reference reverberation model. PG,44

2. The method of claim 1, wherein the characterizing ocean bottom parameters step further comprises the steps of:

identifying periods of time in which the reverberation energy envelope exhibits characteristic properties; and

extracting bottom parameters by processing portions of the reverberation energy envelope and the reference reverberation model falling within the identified periods of time.

3. The method of claim 2, wherein the identifying periods of time step further comprises the steps of:

identifying bottom scattering time periods; and

identifying bottom loss time periods.

4. The method of claim 3, wherein the extracting bottom parameters step further comprises the steps of:

calculating a bottom scattering coefficient by comparing the reverberation energy envelope with the reference reverberation model for each bottom scattering time period; and

calculating a bottom loss value by calculating a predicted reverberation value, using the results from the calculating a bottom scattering coefficient step, for each bottom loss time period and comparing the reverberation energy envelope with the predicted reverberation value for each bottom scattering time period.

5. The method of claim 2, wherein the identifying periods of time step further comprises the steps of:

separating the reverberation energy envelope into a portion representing noise and a portion representing reverberation;

calculating a reduced reverberation envelope by subtracting the portion of the reverberation envelope representing reverberation; and

identifying a decay time period wherein the reduced reverberation envelope exhibits a linear decay rate.

6. The method of claim 5, wherein the extracting bottom parameters step further comprises the steps of:

calculating a bottom scattering strength coefficient by obtaining the intercept of a straight line fit to the reduced reverberation within the decay time period; and

calculating a bottom loss value by successively changing the value of bottom loss and recalculating the reference reverberation model within the decay time period until the slope of the recalculated reference reverberation closely matches the slope of the straight line fit to the reduced reverberation within the decay time period.

7. The method of claim 2, wherein the monitoring the environmental parameters and the current sonar system state configuration for changes step further comprises the steps of:

calculating a baseline reverberation average;

calculating a current reverberation average; and

comparing the baseline reverberation average with the current reverberation average to identify variations between the baseline reverberation average and the current reverberation average.

8. The method of claim 7, wherein the identifying periods of time step further comprises the steps of:

identifying bottom scattering time periods; and

identifying bottom loss time periods.

9. The method of claim 8, wherein the extracting bottom parameters step further comprises the steps of:

calculating a bottom scattering coefficient by comparing the reverberation energy envelope with the reference reverberation model for each bottom scattering time period; and

calculating a bottom loss value by calculating a predicted reverberation value, using the results from the calculating a bottom scattering coefficient step, for each bottom loss time period and comparing the reverberation energy envelope with the predicted reverberation value for each bottom scattering time period.

10. The method of claim 7, wherein the identifying periods of time step further comprises the steps of:

separating the reverberation energy envelope into a portion representing noise and a portion representing reverberation;

calculating a reduced reverberation envelope by subtracting the portion of the reverberation envelope representing reverberation; and

identifying a decay time period wherein the reduced reverberation envelope exhibits a linear decay rate.

11. The method of claim 10, wherein the extracting bottom parameters step further comprises the steps of:

calculating a bottom scattering strength coefficient by obtaining the intercept of a straight line fit to the reduced reverberation within the decay time period; and

calculating a bottom loss value by successively changing the value of bottom loss and recalculating the reference reverberation model within the decay time period until the slope of the recalculated reference reverberation closely matches the slope of the straight line fit to the reduced reverberation within the decay time period.