THEMATIC ISSUE
Types and genesis of induced polarization anomalies in the water-rich structures of the Taian karst area
Yi-xiang Chen •Fu-ping Gan •Yu-ling Chen •
Cong-hui Wu •Pan Ke •Ji-yi Wei •Chen-jie Lu
Received:3May 2014/Accepted:12January 2015ÓSpringer-Verlag Berlin Heidelberg 2015
Abstract From December 2010to April 2011,agricul-tural production in North China and Huang-Huai basin experienced difficulties becau of vere drought.Bad on samples from the development of deep karst ground-water resources,a t of laws has been established to reflect the types and genesis of induced polarization (IP)anoma-lies in a range of water-rich karst structures in Taian area.The curves of IP anomalies in the water-rich structures can be classified into ramps,slow downs,low-resistance plat-forms and low-resistance V patterns.IP anomalies obrved in apparent resistivities contours can be classified as low-resistance steps,low-resistance gradients and low-resistance U patterns.In water-rich areas hosted mainly in caves,IP anomalies include low-apparent
resistivity,low-visible polarization,low half decay time,low attenuation and low multi-parameter synchroneity.The water-rich areas mainly consist of fault structures,where the IP anomalies have a low-apparent resistivity and low-vision polarization,and exhibit relatively long half decay time,maximum attenuation value exceptions,apparent resistiv-ities and multi-parameter ‘‘rever pha’’anomalies.
Keywords Karst ÁInduced polarization sounding ÁAnomaly ÁTypes and genesis
Introduction
From December 2010to April 2011,Northern China and the Huang-Huai area experienced a once-in-a-century drought.There is little surface water in the Taian karst area,and it is difficult to supply sufficient water that is suitable for human and animal consumption while also providing for agricultural production.Conquently,during the vere drought,wells were drilled in the Taian area as an effective way to produce groundwater resources from deep karst aquifers.To characterize the aquifers,geo-physical methods including resistivity (Chen et al.2012;Go
´mez-Ortiz et al.2012;Jia et al.1996;Li et al.1988;Liu et al.2006;Loke et al.1995;Kondo 1997),induced polarization (Fu et al.1993;Li 1994;Li et al.1990;Su 1998;Yang 2012;Ye et al.2006;Zhou et al.2002),c
harging (Leucci et al.2004),audio magnetotelluric sounding (Chen et al.2012;Meng et al.2014),and shallow ismic exploration (De-Giorgi et al.2014))were employed.Among the methods apparent resistivity and induced polarization were the most common (Meng et al.2014).The configuration and depth of the karst terrain within the working area is very complicated.The con-ductivity of water-rich underground karst structures is disperd,which results in complicated anomaly patterns for the apparent resistivity and induced polarization sounding curves.Many induced polarization datats were collected by the Institute of Karst Geology in spring 2011to characterize the underground water in the Taian karst area.The data provide evidence for the genesis of vari-ous anomaly types in the area,which depend to a greater or
Y.Chen (&)ÁF.Gan ÁY.Chen ÁC.Wu ÁJ.Wei ÁC.Lu Karst Dynamics Laboratory,MLR and GZAR,Institute
of Karst Geology,Chine Academy of Geological Sciences,50Qixing Rd,Guilin,Guangxi 541004,China e-mail:chenyixiang@
C.Wu
School of Nuclear Engineering and Geophysical Exploration,East China Institute of Technology,Nanc
hang,Jiangxi 330013,China
P.Ke
Guangxi General Team of Geological Exploration,Center of China Building Material Industry,Guilin,Guangxi 541002,China
Environ Earth Sci
DOI 10.1007/s12665-015-4065-8
lesr extent on the different types of karst.The paper discuss the classification of induced polarization anom-alies bad on the abnormal shapes and strength of the apparent resistivity and IP sounding curves.
Principles of the induced polarization method for water exploration
The u of induced polarization(IP)methods for geological prospectingfirst began in the early20th century.Since the 1950’s,scholars from around the world have studied the IP phenomenon in water bearing rocks and investigated the possibility of IP methods in groundwater arches.Valuable result
s have been obtained and published.During the late 1960’s,IP methods were employed to arch for ground-water in some areas of drought and water shortage in China’s provinces of Shanxi,Shanxi(another province),and inner Mongolia.Some IP parameters such as half decay time, attenuation,excitation ratio,and deviation related to the occurrence of groundwater were successively put forward. Accordingly,some geophysical methods were propod. This also led to the development and manufacture of certain geophysical prospecting instruments.Gradually,China has rin to the cutting edge of the IP method of rearch,par-ticularly in the detection of groundwater resources.
The IP method,with a standard four-electrode DC resistivity spread can be illustrated as the voltage(D V2) between the potential electrodes(M,N),which will grad-ually decay rather slowly to zero when the current(D V1)in electrodes(A,B)is abruptly interrupted(Fig.1a).
The induced polarization(IP)method in general, including time-domain and frequency-domain IP,is one of the geophysical prospecting methods employed in both mineral prospecting and in solving problems in hydroge-ology and engineering geology.It makes u of contrasts in resistivity and IP of rocks and minerals by measuring and studying changes in the IPfield induced by an artificial current.There are differences in the IP effect and fre-quency respon for different types of rocks,min
erals,and rocks with varying moisture contents.Thus,the decay velocity of the condaryfield is also not the same in time-domain IP.So,the occurrence of karst water can be deduced using parameters such as apparent resistivity, apparent IP,half decay time,and attenuation,which can be calculated from decay curves.Thefield arrays for IP include profiling ,composite profiling,Sch-lumberger profiling,central gradient array,and dipole–dipole profiling)and sounding ,Schlumber-ger sounding,pole–dipole sounding and dipole–dipole sounding).The most common methods ud in karst detection consist of composite profiling,Schlumberger sounding(e Fig.1b)and pole–dipole sounding.
First,in a preliminary survey,composite profiling is employed mainly to define the position of karst fractures in a plane and then a Schlumberger or pole–dipole sounding will be ud to investigate the spatial distribution of karst water bearing structures by resistivity variation with depth, while afixed center is t in an expanding spread[the current electrodes(A,B),respectively move same distance outward from the array center].
Karst features,such asfissures,dissolved pores,caves, and karst fault fractures,are good storage spaces and migration channels for karst groundwater,exhibiting the characteristics of lower resistivity and lower IP,which are very helpful to provide reliable geophysical conditions for IP in a karst ground
water arch.
Table1lists some characteristic resistivities and IP values for various minerals,rocks,and karst water bearing structures and shows the obvious IP effect contrasts in the survey area.The geophysical information in the IP sounding curves,in combination with hydrogeological data,can be ud to detect the karst water bearing struc-tures underground and can also be employed to include only anomalies generated by the resistivity methods. Cas
The geophysical study area in Taian,Shandong Province, is located in the Luxi anticline,in the western tectonic units of the North China platform.It is underlain by a bament of Archean metamorphic rocks,compod of the Taishan Group,which produces a strong bending fracture.The fracture zone consists of a group of NW–WNW striking high-angle normal faults located in the south-side of the raid block and another group striking NW or NE is dis-tributed in the north(Pan et al.2014).The developmental direction of the fractures is consistent with the orientation of the major tectonic line,giving priority to the NW and NE-oriented groups(Fig.2a).Transformed by Yanshan and Himalayan movements,the landscape has developed into a rift basin(Fig.2b).
The most expod unit is the Taishan Group of Archean metamorphic rocks.Granitic gneiss is the m
ain lithology, with regional metamorphism and widespread migmatiza-tion occurring.Drilling into metamorphic bedrockfissure water results in small amounts of water:5–15m3/h is usual.Quaternary overburden is impacted by new struc-tures resulting from ongoing uplift;in the nearby valleys, the units are generally less than2-m thick and groundwater exploitation is of little value.In addition,in areas expod to Tertiary shale units,water resistance is poor.Karst units provide the most important groundwater aquifers.How-ever,carbonate rock outcrops are not extensive.They were mainly expod in the early Paleozoic Cambrian and
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Ordovician,mainly in the eastern part of the study area near the towns of Culai,in the corner of a valley,Daw-enkou in the south of Taian and Daolang in the west of Taian.The carbonate aquifers in this area are mainly the thin limestone of the early Paleozoic Cambrian ([)Mantou Fm ([1m ),the thick Oolitic limestone of the Zhang Xia Fm ([2z ),the thick limestone and dolomitic limestone of the Sanshanzi Fm ([3s ).The thick pure limestone sandwiched with leopard limestone and argillaceous dolomitic lime-stone is of middle Ordovician (O 2)age.The karst water system in the study area varies widely owing to different lithological combinations,structural characteristics,and scale developments.However,it is still subject to the dual control of the regional tectonic frame-work and th
e karst groundwater discharge plane on the whole.Becau the Taishan group metamorphic rocks (art)are expod as intrusive rock outcrops,the NW–SE,NE–SW,and E–W striking fault structures cut the forma-tions in the region in a staggered pattern,undermining the integrity of the surface layers of karst water.
Faults
Fig.1a Charging and discharging curves of the
induced polarization (IP)effect,b Schematic diagram of the induced polarization sounding.1clay,2limestone,3
stratigraphic boundary,4karst cave,5groundwater table,6current electrodes A,B,7potential electrodes M,N,8primary electric power line,9condary electric power line
Table 1Electrical and IP properties of rocks and minerals in the karst area of Taian Rocks and minerals
Resistivity q s (X m)Chargeability g s (%)Half decay time S 0.5(S )Attenuation D Water bearing fractured limestones 100–250 1.5–2.30.9–2.80.3–0.5Competent limestones 2,500–10,0000.5–31\0.2–0.3Carbon limestones
5–9,0005–201\0.2–0.3Disminated polymetal mines
100–9,000
18–35
1\
0.2–0.6
Environ Earth Sci
between each block form main guide channels and storage space,as such,fault structures and buried karst have become major targets in the arch to find water for periods of drought.The groundwater quality of tested zones meets the Standards for Drinking Water Quality of the People’s Republic of China (GB5749-2006).
The plane position of water-rich karst can generally be surveyed using apparent resistivity composite profiling and the audio frequency magnetoelectrotelluric method.Bad on general investigation results,using an apparent electrical sounding method to make a detailed investigation of the spatial distribution of water-rich karst fissures,and
employing the IP sounding method to identify and distin-guish the properties of the apparent resistivity anomaly can significantly improve the well drilling rate.Application of IP sounding to a wate
r-rich zone between different lithologies
The following example comes from the village of Erhu,located in Jiaoyu,Taian,Shandong Province.Water con-rvancy projects are required there to prevent drinking water problems.The survey area involves hilly topography,plains,and a flat terrain covered with the layer
of
Fig.2a Regional geology and structural system diagram in Taian study area,b An integrated hydrogeologic Section.1Quaternary,2Tertiary,3Cretaceous,4Jurassic,5Permian,6Caboniferous,7Ordovician,8Cambrian,9Taishan Group,10diorite,11fault,12anticline,13syncline,14geological boundary,15fault number,16ction plane A-B,17Pore fissure water in Jurassic clastic rocks,18fractured karst water in Ordovician carbonate rocks,19pore fissure water in Tertiary clastic rocks,20karst fissure water in Ordovician carbonate rocks intercalated with clastic rocks,21under-ground water level (m)/ascending spring,22no./elevation,23hole depth (m),24mineralization of water (g/L),25water yield (m 3/day),26Drawdown (m)
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Quaternary diments about 2-m thick.The Mantou For-mation fuchsia shale,underlying a Cambrian argillaceous limestone and laminated argillaceous limestone exposure near the survey area,has a layer of limestone about 7-m thick.The Zhushadong Formation siliceous limestone,located to the north of the village is underlain by Archean granite/gneiss.The stratum generally strikes northward at 18°–25°,with its dip increasing northward,suggesting a monoclinic structure (e Fig.3).
A small brook that runs east of the survey area supplies the groundwater.Bad on drilling data,the depth of the groundwater is less than 20m.The main groundwater aquifers are a purple shale layer with laminated limestone and karst limestone including karst fissure water and muddy water,of which the groundwater storage is located in the limestone,with an aquifer thickness of about 5m and an abundance of water.
From the apparent resistivity (q s )profile of survey line no.2in the composite profiling method,resistance is en to vary from high to low at point 605/2,resulting from the granite coming into contact with a medium thick limestone and argillaceous limestone of the contacted anomalous zone.
As can be en from the apparent resistivity (q s )ction contour map of survey line no.2that ud a three-pole
induced polarization sounding (e Fig.4a),the same type of anomaly that coincides with the apparent resistivity (q s )anomaly in survey line no.2in the composite profiling method,appears at point 600/2.In fact,a contacted low resistance step-gradient type with a low-resistivity clod loop anomaly appears in the 660/2–680/2gment (AB/2=150–250m),of which the low-resistivity anomaly
reveals the tip of the contacted karst zone or the water-rich area of water saturated rock ,the Zhusha-dong Formation that contains a medium-thick gray lime-stone,argillaceous limestone and granite.The type of anomaly obrved in the single electrical sounding q s curve at measuring point 690/2(Fig.5a)is a ramp (or more complex)type of a multi-layer sounding curve,reflecting vertical developmental heterogeneity in the karst.
Bad on the apparent polarization rate g s of the three-pole IP sounding method,the half-time S 0.5,the degree of decay D of sounding curves at 690/2(Fig.5a),the polar-ization rate g s \3%,the minimum of decay half-time S 0.5and degree of decay D,respectively,turn up in AB/2=150–250m.The typical multi-parameter curve is synchronized with the anomalous low in apparent resis-tivity q s ,an apparent polarization rate g s ,a half-time S 0.5,and the degree of decay D.Comprehensive analysis of the apparent resistivity q s ,which depends on the polarization g s ,half-time S 0.5and degree of decay of the D sounding curve anomaly,suggests that full water caves and other karst limestone units that are clo to different lithological contacted zones can cau a low-resistivity anomaly with measuring point 690/2at the center.
During the Air DTH hammer drilling conducted at measuring point 680/2,with a hole depth of 155m,the drilling revealed a honeycomb limestone and cave area,with a shaft-forming well inflow lar
ger than 720m 3/day,that was verified by an IP anomaly sounding.Application of IP sounding to layered karst aquifer prospecting
The next example comes from the town of Dawenkou,located in the southernmost part of Daiyue (a town belonging to Taian).The topography of the surveyed area contains plains and flat terrain,with an overlying Quater-nary soil layer thickness of about 2–8m.Near the drilling location,the next layer down consists of sand and gravel with a Quaternary soil thickness of about 15m.This is underlain by Middle Ordovician carbonate rocks.Accord-ing to the district’s exploration of mineral resources survey and evaluation plan (e Fig.6),a speculative east–west striking fault is possibly buried near the measurement area.In the vicinity of the surveyed area along the Wen River to the south,the underlying Middle Ordovician pure-quality carbonate rocks are thickly distributed
and
Fig.3Karst hydrogeological map at Jiaoyu.1bedrock fissure water,2karst-fractured groundwater,3cavern water,4surface water,5survey area and exploration well,6wells,7village,8fault
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