Borehole measurements using a fast and high energy resolution gamma ray detector assembly

Abstract

A gamma ray detector assembly for a borehole logging system that requires the measure of gamma radiation with optimized gamma ray energy resolution and with fast emission times required to obtain meaningful measurements in high radiation fields. The detector assembly comprises a lanthanum bromide (LaBr3) scintillation crystal and a digital spectrometer that cooperates with the crystal to maximize pulse processing throughput by digital filtering and digital pile-up inspection of the pulses. The detector assembly is capable of digital pulse measurement and digital pile-up inspection with dead-time less than 600 nanoseconds per event. Pulse height can be accurately measured (corrected for pile-up effects) for 2 pulses separated by as little as 150 nanoseconds. Although the invention is applicable to virtually any borehole logging methodology that uses the measure of gamma radiation in harsh borehole conditions, the invention is particularly applicable to carbon / oxygen logging.

Description

[0001]This application is a reissue application of U.S. Pat. No. 7,999,220, which was derived from U.S. patent application Ser. No. 12 / 129,812, filed on May 30, 2008, which is herein incorporated by reference in its entirety for all purposes.BACKGROUND[0002]Borehole well logging systems that emit bursts of high energy (of the order of 14 million electron volts (MeV)) neutrons are routinely used in geophysical exploration, recovery and monitoring operations. These systems are typically used in cased boreholes, although some uncased or “open hole” applications are known in the art. As examples, pulsed neutron logging systems are used to measure formation density in cased boreholes, to determine formation lithology, to detect gas within formation pore space, and to identify and to optionally measure the flow of water behind casing.[0003]The earliest commercial pulsed neutron logging system was to delineate saline formation liquid from non-saline liquid, which was assumed to be liquid h...

AI Technical Summary

Benefits of technology

The invention is a gamma ray detector assembly for a borehole logging system that requires the measure of gamma radiation with optimized gamma ray energy resolution and fast emission times in high radiation fields. The detector assembly comprises a lanthanum bromide (LaBr3) scintillation crystal that exhibits outstanding peak resolution and emission time compared to other types of scintillation crystals. The lack of oxygen in the crystal matrix offers an incremental boost in C / O sensitivity per unit volume, compared to fast, higher density detectors such as LYSO, GSO, LUAP that contain oxygen. In intense gamma radiation fields, the speed of detector pulse processing and pulse throughput are important, and a digital spectrometer has been designed to maximize processing throughput and maintain high resolution requires fast measurement and pile-up inspection of the pulses. The detector assembly comprising the LaBr3 detector crystal and the digital filtering and digital pile-up inspection spectrometer is limited by filtering of electronics noise and digital sampling rates, but is capable of accurate measurements and high-temperature operations. The performance is facilitated by the combination of the LaBr3 detector crystal and the digital filtering and digital pile-up inspection spectrometer.

Problems solved by technology

In practice, the detector assembly comprising the digital spectrometer and the LaBr3 crystal is limited by filtering of electronics noise and digital sampling rates.

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