Zhang Yu jun,Li Changguo
（Center of AeroGeophysics & Remote Sensing（AGRS）of MGMR, 29 Xue Yuan Road,Beijing,100083,P.R.C.）
ABSTRACT This paper describes the gammaspectrometrie well-logging method,apparatus and software, which were originally developed for exploration of potash salt.But it could be used as well for other purposes,such as:the exploration of uranium,the study of geologieal strata.the detection of water-bearing gravel strata or of fractured zones and so on.
Owing to the severe shortage of the potash fertilizer in China during the past more than 30 years great attention has been paid to the study of new methods,including gammaspectrometric well-logging.The gammaspectrometric well-logging method and bore-hole gammaspectrometer were proposed and developed by us in 1963,and several times updated up to the eighties.5 types of the bore-hole gammaspectrometer were developed:single-channel, double-channel and four-channel moddels for both shallow and deep bore-holes.This method has been applied in 6 provinces succesfully.A bore-hole radioactivity-spectra interactive interpretation software system（HRSIS）was de-veloped in 1995,whjch consists of 5 groups of functional programs:the preprocess.the quantitative interpretation.the trend analysis,the graphic display and the integrated interpretation.
Key words:Bore-hole gammaspectrometer,Gammaspeetrometric well-logging method,Gammaspectrometric well-loging software.
METHOD
According to the statistics more than 50%of the total world reserve of sylvinite were discovered in bore-holes.For revelation of sylvine and differentiation of it from the argillaceous thorium-uranium-hearing rocks the gammaspectrometric well-logging method was studied and the bore-hole gammaspectrometer was developed since 1963 and 5 times updated up to the eighties.
It is the natural radioactive isotope K40that lays the physical base for this method.The isotope K40has low isotope abundance 0.0119%and radiation coefficient of gamma photon 11.6%with middling energy 1.46MeV,suitable for detection and spectra analysis.
HARDWARE
The 4 channel bore-hole gammaspectrometer,developed by us,registers the gammaray spectra by a cylindrical NaI（sodium iodide）crystal.The 4 channels are:the channel PK（differential channel for K40 peak of 1.46MeV.The energy threshold is 1.40-1.52MeV）;the channel LI（left integral channel with energy threshold=＞1.30MeV）; the channel RI（right integral channel with energy threshold=＞1.60MeV）and the channel TC（total count channel,its energy threshold is=0.2MeV）.Thefollowing technical points were studied for construction of the bore.hole gammaspectrometer:
*Sensitivity.Because of the low isotope abundance and gamma radiation coefficient it was necessary to raise the sensitivity of the bore-hole gammaspectrometer by increasing the length of the Nal detector up to 100-150mm.Its diameter is limited by the inside diameter of the hole and can not be＞50mm.
Figure 1.The 4 channel bore-hole gammaspectrometer registers the gamma-ray spectra in the hole by a cylindrical NaI（sodium iodide）crystal.The 4 channels are:the channel PK（differential channel for K40 peak of 1.46MeV.The energy threshold is 1.40-1.52MeV）；the channel LI（left integral channel with energy threshold=＞1.30MeV）；the channel RI（right integral channel with energy threshold=＞1.60MeV）and the channel TC（total count channel, its energy threshold is=＞0.2MeV）.
*Impulse transfer.In order to keep the resolution the impulses（from photomuitiplier after preamplifier）must be transferred from the down hole unit through a cable,longer than 3600m,without serious de-formation in shape and in amplitude.A voltage-current type transfer was accepted.The resolution for Cs137after 3600m cable transfer kept the same level of resolution（10%）as without the long cable.
* Stability.The vertical temperature gradient of the earth is about 1 degree Centigrade for each 33 m,so at the depth 3500m the environmental temperature in bore-hole can be as high as 100 degrees Centigrade.For the reliable stabilization of the spectrometer the following 3 measures were adopted:the selective use of high temperature NaI crystal and photomultiptier；the compensation of the negative temperature coefficient of the photomultiplier by the positive temperature coefficient of the high voltage source for the photomultiplier；and the use of an autostabilization circuit for isotope Cs137 to keep the spectral peak position.
Any photo-peak of an isotope is added on the compton continuum of photo-peaks with higher energy.It is the theoretical base of the stripping method and the principal component analysisfor K,that the background is proportional to the RI（Fig.1）.
SOFTWARE
A bore-hole radioactivity-spectra interpretation software system（HRSIS）was developed in 1995, which consists of 5 groups offunctional programs:the preprocess（A: PRP）,the quantitative interpretation（B:KTU）,the trend analysis（C:TRD）,the graphic display（D:GRP）and the integrated interpretation（E:ITR）.The sixth group is for the HELPs（F:HLP）.The HRSIS system was written on PCs with C language.Fig.2 shows the functions of this HRSIS system.
Figure 2.User interface, listing the functions of the bore-hole radioactivity-spectra interpretation software system（HRSIS）.A:PRP the preprocess,B:KTU the quantitative interpretation,C:TRD the trend analysis, D:GRP the graphic sidplay and E:ITR the integrated interpretation.Tht last group F:HLP is for the HELPs.
Figure 3.The example of well-logging images for the borehole ZK4 of Lajin area in YunNan province.
* On the left there are the 3 concentration curves.
*In the middle thcre are the 4 original gammaspectrometric logs:RI,LI, PK&TC.The 2 enriched ischelite layers at depths 274m and 325 m are revealed by low RI,high LI,high PK and high TC.
* On the right there are 2 trend analysis curves for RI&LI.
Figure 4.The correlation profile by tend anaysis curves for RI and LI,where the periods of lake sedimentation are seen clearly.
For quantitative interpretation both double-window and three-window methods were used.Fig.3 illustrates the well-logging image,as one example of the processing results by HRSIS system.On this image（Fig.3）2 KCl layers at depths 274m and 325m are revealed by high LI,high PK,high TC and low Rl.Fig.4 displays the correlation profile by tend analysis curves for RI and LI,where the periods of lake sedimentation are seen clearly.Fig.5 is the scattergram of Th and K concentrations,determined by functions of group B:KTU.
Figure 5,The scattergram of Th and K concentrations,determined hy func-tions of group B:KTU.
APPLICATIONS
Gammaspectrometric well-logging method has been applied in 6 provinces（YunNan,HuBei.SiChuan.ShanDong,XinJing and QingHai）successfully.The sylvinite-bearing layers with thickness=＞0.5m and KCl concentration=＞2%can be detected bv bore-hole gammaspectrometer reliably in MengYeJing and other salt deposits.The policy of“Exploration for both Oil and Sylvite Concurrently”was made by Ministry of Petroleum of PRC.Following this policy a aphthitalite layer with thickness 0.8m and KCl concentration as high as 16.8%was found in JiangHan oil field using this gammaspectrometric well-logging.In province SiChuan the ischelite layers with thickness of=＞0.2m were detected.
In 1991 in YunNan province in a new drilling area,when all of the drilling core of the first borehole was lost,only thanks to the gammaspectrometric well logging a big amount of money was saved by avoidance of redrilling.
This method can be used as well for other purposes,such as:the exploration of uranium,the study of geological strata,the detection of the water-bearing aquifers or of the fractured zones and so on.
ACKNOWLEDGEMENTS
We thank Mi Shuang-Jie,Li Show-Tian,Zhang Ming-Lun,Chen Yun-Long,Li Shu-Yi,Chen Xian-Yao and others,who have taken part in the development of the bore-hole gammaspectrometer and the study of the method.
REFERENCES
1.Zhang Yu-Jun,Wang Nai-Dong and Zhang Zhi-Min.A software package for processing of data of airborne gamma-ray multichannel spectrometer,Computing Techniques for Geophysical and Geochemical Exploration.11,11-21（1989）.
2.Zhang Yu-Jun.Digital image processing of airborne radiometric and magnetic data from central ChaiDaMu Basin,An Overview of Exploration Geophysics in China.American Society of Exploration Geophysics,Tulsa.517-535（1989）.
原載《Proceedings of ISEEG,97:International Symposium on Engineering and Environmental Geophysics》，1997,10.