CT理论与应用研究 CT Theor y andApplicationsVol.9, Supplement Sept., 2000Analysis on applied effectiveness of near-surface aquifers explorationwith NMR technique in North Western arid areaYangGuixin WuYiGuoJianqiangandZhuQingjun(No.135, QiYi middler o ad, Baoding, Hebeipr. 071051)ABSTRACT: The surfaceNMR technique prospectinggroundwater is a new applied field, which is an only technique for a direct detection of groundwater in the world at present.NMR instrument allows invasive measurements of aNMR signal f r omsubsurfacewater-saturated lay- ers.Direct measurementof thesignal f r omwatermoleculesguaranteesahigh reliability of water detection. Interpretation of experimental date reveals location of aquifers, their depth and water content. Mean size of water-saturated rock pores can also be estimated. Sinethe NMR instrument was introduced into our institute in 1999, It had been used to test the dif f erent near-surface aq- uifers explorations inthe loess landof Shanxi Pr.andthe southofNingxia Pr., and a cer t ain field results were obtained. Based on analysis of these results, this ar t icle discusses on the existing problem of the prospecting reliability in strong noises background ， and the pr o specting results comparison using a transmit t ing antenna of square loop and figure 8 sh即ed loop at the same testing site. Key words:NMR, Groundwater, Near-surface, Shanxi, Ningxia1IntroductionSurface Nuclear Magnetic Resonance (NMR) is a new method for exploration of ground- water. It is main advantage of the surface NMR method, compared with other geophysical methods for water prospecting is that the surface measurement of the NMR signal from water molecules in the subsurface ensures that it only responds tothe subsurface water. Its principle of the method is based on excitation of hydrogen protons by artif i cial magnetic fields oscillating with the local Larmor frequency and on recording the magnetic fields emit t ed by relaxation of pr e cessingprotonsusingthetransmit t er loop. TheNumis equipment developed by FrenchIRIScompany is an advanced groundwater ex- ploratinginstrument withNMR technique at present. It uses a squar e loopof75mside length or a coil shaped like f i gure8 consisting of both square loopsof 37.5mside length laid on the ground as the transmit t ing/r e ceiving antenna. Af t er the signalof magnetic field measured ar e processed, the inversion r e sults which indicates estimates of the water contentand the depth of each aq- uifer in the studied zone are shown in the diagrams, the studiedzone is approximately depended ona IDdeepcylinder of about 2D indiameter(D:thetransmit t ingantenna diameter).2Exploration testing resultsInordertofind anewadaptabletechnique andmethod for groundwater exploration innorth- western acidareas, WehaveusedtheNumisequipment fromabroadto do agreat dealof work in the area, and got a cer t ain results. The explorating information of dif f erent types of near-surface aquiferare given r e spectively as follows. 2.1Near-surface porous water The Haiyuan 10 testing set lies in the Haiyuan area of Ningxia Pr. The quar t emar y sandy52CTTheor y and ApplicationsV 01 .9 gravel strata change greatly inthickness, which is a main aquifer, underlying the tertiar y layer of clay stone, clayey sand-stone and concreted gravel with poor permeability and poor water- bearing condition. Thetesting site liesonthe upper section of an old river bed, a known drilling hole at near, the water level 16m with a relative big production of water, 0-30m depth of the upper aquifer is mainly consistingofcoarse sand filled gravel,30- 50mdepth is mainlyfinesand and silty sa n d, below50m istheTertiar y clayeysandstone. The noises sourcepoint of the testing site is 300m far f r omthe long wave radio station a n d elect r ic power lines which increase st r ong coher e nt noise. The high quality data is obtained by using the eight shaped loop which is ver y ef f ective in suppressing st r ong coherentnoise.8 figure loop measuring resultsof Haiyuan 10 are shown in figure Ia, square loop measuring results of the same site are shown in figure Ib. Alt- hough the noise is ver y st r ong of 100-3000nv, the two displaying results are fit well with the known information with regard to reasonably high signal-to-noise ratio a n d relatively good aringconditionof st r ata.。 ， 深度 (米 ):5加巧加知印加6 0含水t(% )1 : i oi s之 o257o 0 52 0二11.Fig laHaiyuan10 between31.9-42.5mdeep st r atacontains water content 8% corresponding to the structural fracture aquifer. The measuring r e sult is fit with theSupplementYangGuixin etal.:Analysis onapplied ef fectiveness ofnear-surface aquifers exploration53 knowngeological data。 ，深度 m)水f (%) s o ar-一- -一 oz1se, a1 zi s1 巴1 日 2020日1 .1 .，水 :c叭 ，已，.2-! !; 几.月 !Fig.2aQingiian2 “8“ f i gure loopmeasuring resultsFig.2bQingjian2 square loop measuring results 图Z a清涧2 “ 8”字型线圈勘查结果图2b清涧2 正方形线圈勘查结果3 Analysis on applied effectiveness and present problem1. The field test results have demonstrated th