Detecting Changes In An Environmental Condition Along A Wellbore

A system that detects a change in an environmental condition along a wellbore, the system comprising:

• an optical waveguide positioned in the wellbore;
• a light source that introduces an optical signal into the waveguide;
• and
• a receiver that receives a backscattered signal from the optical waveguide and produces a trace which represents an intensity of the backscattered signal along the optical waveguide, wherein the intensity represents the environmental condition along the wellbore,
• wherein the light source introduces light at a first frequency into the optical waveguide during a first time period and the receiver produces a first trace in response to reception of the backscattered signal from the waveguide,
• wherein the light source introduces light at a second frequency into the optical waveguide during a second time period and the receiver produces a second trace in response to reception of the backscattered signal from the optical waveguide,
• wherein the second frequency is different from the first frequency,
• wherein the first trace is substantially equal to the second trace, and
• a change in the environmental condition along the wellbore is calculated based on the difference between the first and second frequencies.

A method for detecting a change in an environmental condition along a wellbore, the method comprising:

• positioning an optical waveguide along the wellbore;
• introducing each one of first optical signals from a light source into the optical waveguide at one of multiple first frequencies during a first time period;
• receiving first backscattered signals from the optical waveguide in response to the introduction of the first optical signals and producing a set of first traces;
• making a set of baseline traces equal to the set of the first traces;
• initiating a change in the environmental condition;
• introducing each one of second optical signals from the light source into the optical waveguide at one of multiple second frequencies after at least a portion of the environmental condition has occurred;
• receiving second backscattered signals from the optical waveguide in response to the introduction of the second optical signals and producing a set of second traces;
• comparing the baseline traces to the second traces;
• determining that at least one of the baseline traces correlates to at least one of the second traces;
• determining an incremental change in the environmental condition based on differences between frequencies that are associated with the at least one of the baseline traces and the at least one of the second traces;
• adjusting the multiple second frequencies;
• repeating 1) the introducing the second optical signals, 2) the receiving the second backscattered signals, 3) the comparing the baseline traces to the second traces, 4) the determining the correlation, 5) the determining the incremental change and 6) the adjusting the multiple second frequencies until the environmental condition is stable;
• and
• combining the incremental environmental condition changes to determine a total environmental change in the wellbore.