Monitoring a microseismic event

• US 6,856,575 B2
• Filed: 10/21/2003
• Issued: 02/15/2005
• Est. Priority Date: 10/21/2002
• Status: Expired due to Fees

• Current Assignee: Vetco Gray Controls Inc (Baker Hughes Company)
• Original Assignee: ABB Offshore Systems Limited (General Electric Company)
• Inventors: Jones, Robert Hughes
• Primary Examiner(s): Lobo, Ian J.
• Application Number:
  US10/690,374
• Publication Number:
  US 20040125695A1
• Time in Patent Office: 483 Days
• Field of Search: 367/38, 367/43, 702/17
• US Class Current: 367/38
• CPC Class Codes: G01V 1/01 Measuring or predicting earthquakes
• CPC Class Codes: G01V 1/362 Effecting static or dynamic corrections; Stacking
• CPC Class Codes: G01V 2210/32 Noise reduction

Abstract

A method of monitoring a microseismic event includes detecting the event to produce a first signal dependent on the event. The first signal includes noise at a frequency of, for example 50 Hz. A first sample of the first signal is taken. Then a second sample of the first signal is taken, the second sample occurring n/f seconds after the first sample, where n is an integer (e.g. 1). Subtracting the first and second samples from each other produces a farther signal dependent on the event in which the noise has been at least partly compensated for.

Claims

1. A method of monitoring a microseismic event, comprising:

detecting said event to produce a first signal dependent on said event, the first signal including noise at a frequency of f Hz;

taking a first sample of said first signal;

taking a second sample of said first signal, the second sample occurring n/f seconds after the first sample, where n is an integer; and

subtracting the first and second samples from each other to produce a further signal dependent on said event in which said noise has been at least partly compensated for.

2. A method according to claim 1, wherein n=1.

3. A method according to claim 1, wherein f=50.

4. A method according to claim 1, wherein the microseismic event is one occurring in a fluid producing well.

5. A method according to claim 1, wherein a source of the noise signal is a cathodic protection direct-current (DC) derived from an alternating current main supply by rectification.

6. A method of monitoring a microseismic event, the method comprising the steps of:

detecting a microseismic event to produce a first signal dependent on said event, the first signal including a noise signal produced by cathodic protection currents having a frequency of f Hz;

taking a first sample of said first signal;

taking a second sample of said first signal, the second sample taken n/f seconds after the first sample; and

determining a difference between the first and second samples to produce a third signal dependent on said microseismic event in which said noise signal has been at least partly removed.

7. A method according to claim 6, wherein a source of the noise signal is a cathodic protection direct-current (DC) derived from an alternating current main supply by rectification.

8. A method according to claim 7, wherein f is approximately 50 Hz.

9. A method according to claim 8, where n is an integer, and wherein n=1.

10. A method of monitoring a microseismic event in a fluid producing well, comprising the steps of:

rectifying an AC main supply to produce DC current, the DC current containing a noise signal having a base frequency f Hz equivalent to the frequency of the AC main supply and harmonics thereof;

detecting a microseismic event to produce a first signal dependent on said event, the first signal including the noise signal;

taking a first sample of said first signal;

taking a second sample of said first signal, the second sample taken n/f seconds after the first sample;

determining a difference between the first and second samples to produce a third signal dependent on said microseismic event in which said noise signal has been at least partly removed; and

applying the third signal to an existing trigger detection algorithm to detect the microseismic event.