Methods and apparatus for determining error in formation parameter determinations
US5661696A · kind A · utility
Assignee
Inventors
Key dates
| Filing date | Aug 11, 1995 |
| Grant date | Aug 26, 1997 |
| Priority date | — |
| Expiry date | Aug 11, 2015 |
Classification
- Technology area (CPC G)Physics
- CPC primaryG01V1/48
- WIPO fieldMeasurement
- WIPO sectorInstruments
Abstract
Methods and apparatus for generating an error bar (variance) log for a formation parameter are disclosed. According to the method, acoustic waveforms are detected at a plurality (M) of detectors which are a function of a source signal which has interacted with the formation and noise, and the detected waveforms are sampled and processed in order to obtain a semblance or a normalized mean square error. Using the semblance or the NMSE which is related to the semblance, a signal to noise ratio 2E/N.sub.0 can be found by recognizing that the semblance function has a non-central beta distribution whose mean is a function of the signal to noise ratio 2E/N.sub.0, the time-bandwidth product N, and M. The signal to noise ratio is then used to find the Cramer-Rao lower bound which is taken as an estimate of the variance. An error bar log is formed by repeating these steps at a plurality of locations along the borehole. The method of the invention can be used in conjunction with both attenuating and non-attenuating waves. The apparatus of the invention relates closely to the method.
Source: USPTO / EPO open patent data. Objective bibliographic and citation counts.