High-resolution profiling of alluvial aquifer in potential riverbank ...

Journal of Applied Geophysics 66 (2008) 1–14

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Journal of Applied Geophysics

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High-resolution profiling of alluvial aquifer in potential riverbank filtration site by useof combining CMP refraction and reflection seismic methods

Hyoung-Soo Kim a,⁎, Jung-Yul Kim b

a Korea Institute of Water and Environment (KIWE), Korea Water Resources Corporation (KWater), 462-1, Jeonmin-Dong, Yuseong-Gu, Daejeon, 305-730, Republic of Koreab Soam Consultant Co., Ltd., 30, Gajeong-Dong, Yuseong-Gu, Daejeon, 305-350, Republic of Korea

⁎ Corresponding author.E-mail address: [email protected] (H.-S. Kim).

0926-9851/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.jappgeo.2008.08.003

a b s t r a c t
a r t i c l e i n f o
Article history:
Hydrogeological conditions Received 22 August 2007Accepted 5 August 2008
Keywords:Geophysical seismic methodsHigh resolutionCommon mid-point (CMP) refractionP-beamGroundwater tableAlluvial aquiferRiverbank filtration (RBF)Site characterization

, such as aquifer thicknesses, sediments and structure of alluvium, are importantfactors in determining the available intake of water by riverbank filtration (RBF). To understand thehydrogeology of a potential RBF site in Korea, high-resolution seismic surveys were conducted usingrefraction and reflection methods. The depth of the groundwater table in the study area was clearly shown incommon mid-point (CMP) refraction seismic sections and ranged in depth from 2 to 9m below groundsurface. The lower boundary of the alluvial aquifer overlies bed rock and was easily delineated in thereflection sections using the P-beam method. The lower boundary of the alluvium ranges in depth fromabout 30 to 46m below ground surface. The CMP refraction and P-beam methods have many advantages inthe field work and during data analysis because these methods can be applied to data obtained byconventional land seismic reflection surveys. The high-resolution seismic survey, combined with CMPrefraction and P-beam methods, is a powerful tool to obtain subsurface information and evaluate potentialRBF sites.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Analluvial aquifer is a precious natural resource that, because of theinteractions between groundwater and surface water, can be impor-tant for water use and water management. Alluvial aquifers have thecapability of bank storage, which can moderate the effects of floodingand drought. Infiltration of river water to the aquifer cleans the waterthrough physical and microbiological processes. So, the proper use ofan alluvial aquifer can benefit water use and water management.

Riverbank filtration (RBF) is one of the representative alluvialaquifer applications inwater intake and treatment.Many countries useRBF to intake source water for treating contaminated surface water.RBF iswidely used in Europe. InGermany, RBF is used to treat 13% of thedrinking water supply (Hahn, 2006). In Berlin, RBF is used to treat 75%of the City's drinking water supply. RBF does not require chemicaladditives, is simple to operate, and requiresminimalmaintenance. RBFcan reduce concentrations of many pollutants including disinfectionbyproducts (DBP) through a combination of natural processesincluding filtration, biodegradation, and dilution. Other advantagesof RBF include its ability to attenuate contaminant shock loads andreduce water temperature fluctuation (Brandhuber, 2004). Becauseof its advantages, many countries, including Korea, are interested inRBF.

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The detailed profiling and characterization of an alluvial aquifer areimportant factors in selecting suitable well sites and well designswhen evaluating potential RBF sites. Hydrogeological conditions,such as aquifer thicknesses, sediments and ranges of alluvium, areimportant factors in determining the available intake of water by RBF.Because the available intake of water from a well strongly depends onthe hydraulic conductivity of the aquifer and available drawdown inthewell, the alluvial aquifer sediments and thicknesses are critical keyfactors in selecting and developing suitable RBF sites. The objectivesof this study were to assess the depth to the groundwater tableand delineate the alluvial aquifer at a potential RBF site located atJeungsan-Ri, Korea using a high-resolution seismic survey combinedwith common mid-point (CMP) refraction and P-beam seismic pro-cessing methods.

1.1. Brief overview of methods

There are numerous studies on the hydrogeological characteriza-tion of alluvial aquifers using seismic methods (Steeples and Miller,1990; Bradford et al., 1998; Cardimona et al., 1998; Kim et al., 2004;Sumanovac 2006). Most of these studies have used refraction andreflection methods separately but not combined. For optimal resultsfrom refraction surveys, the position and spacing of sources andreceivers must be arranged according to the interpretation methodsand depth of target refractors. In these cases, it is practically impossibleto generate a reflection seismic section from refraction survey data.Also, in the reflection method, refraction signals are regarded as noise

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Fig. 2. Measured radiation pattern due to vertical single force in scaled physicalmodeling.

2 H.-S. Kim, J.-Y. Kim / Journal of Applied Geophysics 66 (2008) 1–14

and the signals are eliminated in the data processing procedure toprevent the distortion or misinterpretation of reflection events.

In this study, we used a roll-along technique which is the conven-tional data acquisitionmethod for land reflection seismic surveys. Thistechnique allows for the processing of both refraction and reflectionsignals. We applied two advanced seismic processing methods to thedata: 1) the CMP refraction seismic method, developed by Germangeophysicists (Gebrande, 1986; Reismers et al., 1991), and 2) the so-called “P-beam” method, developed by Korean geophysicists (Kim,1989; Kim et al., 1994a). The P-beam method enhances the compres-sionwave (P-wave) signal relative to the shear and surfacewave signalsthat are troublesome in refraction and reflection seismic surveys.

1.2. CMP refraction method

The CMP refraction seismic method was introduced by Gebrande(1986), who used numerical modeling in combination with a general-ized reciprocalmethod to data collected from at awaste disposal site inGermany (Orlowsky et al., 1998). Basically, this method uses CMPsorted data and the refraction travel-times aremoved out according totheir ray paths, similar to the move-out in the reflection method. Thismethod could easily be combinedwith the common reflectionmethodin field data acquisition and post-processing of the data.

For many seismic surveys conducted at alluvial aquifer sites, thegroundwater table is the first acoustic boundary at shallow depths

Fig.1. Schematic procedures show an example of CMP refraction stacking (a) Seismic data gatout based on the refracted wave path, (c) Optimum offset trace selection, (d) Stacked seism

(normally within 15m of the ground surface) and produces strongrefraction signals in common roll-along type seismic data acquisition.The refraction signals are eliminated in the reflection method, so as

hered in field (AGC & Noisemuting applied), (b) Seismic data after CMP refractionmove-ic signal.

Fig. 3. Layout for measuring the radiation pattern of scaled physical modeling. A disk-swinger is coupled to PVC plate.

3H.-S. Kim, J.-Y. Kim / Journal of Applied Geophysics 66 (2008) 1–14

not to be confused with reflection events. Use the CMP refractionmethod can provide more detailed information about the ground-water table from the refraction signals.

Fig. 1 shows an example of refraction signals and move-out signalsof CMP sorted seismic data obtained at a test site. The first strongsignals (indicated in the rectangular box in Fig. 1-a) are refractedwaves from the groundwater table at the site. If we apply a move-outcorrection based on the refracted wave path, we can arrange thesignals in a row (Fig. 1-b). Also, we can choose an optimal offset for thesignals and stack for the final section (Fig. 1-c and d). This procedure issimilar to conventional reflection processing, except for the move-outcalculation using the refracted wave path, not the reflected path.

Fig. 4. Radial components (P-wave) recorded from scaled modeling by single verticalsource.

1.3. “P-beam” method

The P-beammethod is a type of source arraymethod that enhancesthe desired wave components. This method was developed by one ofthe authors (Kim, 1989) and details can be found in previous studies(Kim, 1989; Kim et al., 1994a).

In addition to the P-wave component, a vertical single sourceapplied to the surface generates the S-wave and surface wave com-ponents that typically have a considerable energy relative to theP-wave (Mooney, 1976). Thus, the high energy of the S-wave andsurface waves makes it difficult to delineate the refraction and re-flection P-wave signals. However, experience shows that the P-wave

Fig. 5. Transversal components (S-wave) recorded from scaled modeling by singlevertical source.

Fig. 6.Hodograph analysis of the P-wave and S-wavewith radiation pattern about singlevertical source.


component can be emphasized by the proper arrangement of sourcepositions. The results from two-dimensional physical modeling canbe used to understand this phenomenon in better detail (Kim andBehrens, 1986; Dresen and Rueter, 1994).

Fig. 7. Measurements on physical modeling show the energy distribution pattern of P-wavelocated equidistantly (14 mm). The left diagrams show the sources and the right are measureand (c) 7 vertical sources.

A vertical single force applied to the surface generates a P-wavewith considerable shear wave as illustrated in Fig. 2. The layout formeasuring this radiation pattern is shown on Fig. 3. The correspondingradial components (P-wave) and transversal components (S-wave) areillustrated in Figs. 4 and 5 respectively. Fig. 6 shows the correspondinghodography with the radiation pattern.

Fig. 7 shows the change of radiation, as the amount of sources(vertical single forced types) being located equidistantly is increased,resulting in a very sharply elongated shape with almost P-waveenergy, just like a search beam (called “P-beam”). The radial com-ponents corresponding to seven vertical sources (in case of Fig. 7-c)are depicted in Fig. 8 and the hodography in Fig. 9.

As shown in Figs. 7 and 8, almost exclusive P-wave energy radia-tions, with a minimum of S-wave energy could be attained within anarrow region of radiation angles if the successive vertical singlesources are used concurrently or stacked serially. This P-waveenhancing source array method is called, as a matter of convenience,the “P-beam” source. The influence of the surface wave is minimal andthe P-wave reflection and refraction signals predominate.

Not only the gathers of P-beam sources could be obtained by realsource arrays in the field, but those also could be obtained easilyby systematic rearrangement and stacking of successive singlesources. Fig. 10 illustrates the rearrangement for the CMP gathers of

and S-wave components when successive vertical sources are stacked. The sources ared energy patterns of P and S components for (a) 3 vertical sources, (b) 5 vertical sources,

Fig. 9. Hodograph analysis of P-wave and S-wave with radiation pattern about 7successive vertical sources.

Fig. 8. Radial components (P-wave) recorded from scaled modeling by 7 successivevertical sources (in case of Fig. 7-c) show clear directed P-wave radiation. Thosechannels located around vertical direction show comparatively strong signals.


the P-beam source compared with the conventional CMP sortedgathers from the single-source shot gathers in roll-along measure-ment. For comparisonpurposes, Fig.11 shows examples of shot gathersof a single source and those of a P-beam source using rearrangementand stacking methods from the same data set.

2. Seismic field surveys in potential riverbank filtration sites

2.1. General description of field sites

Seismicfield datawere acquired at the Jeungsan-Ri site located alongthe Nakdong River in southern Korea to assess the feasibility of RBF.This site consisted of alluvial deposits overlying bedrock. The seismicsurveys were part of a water supply project for the supply of municipaland industrialwater to nearbycities. Fig.12 shows the location of the sitewith seismic survey lines. The results fromthe feasibilityassessmenthadbeen favorable and planning for construction of RBF facilities andreviewing of relating regulations were undergoing.

The Jeungsan-Ri site is located at one of the largest alluvial depositregions in Korea associated with the Nakdong River. There are alreadyseveral RBF sites within 20km downstream and upstream of theJeungsan-Ri site. The nearest RBF site is Buk-Myeon, which is in theopposite side of the study area across the river, about 2km down-stream. The capacity of facilities at the Buk-Myeon site is about10,000m3/day and the facilities have supplied treated water since2002. Another RBF site is the Daesan-Myeon which is about 9kmdownstream from study area. This site is one of the largest RBF site inKorea and the capacity of facilities at the Daesan-Myeon is about120,000m3/day in these days. Moreover, there are plans for the nearfuture to expand the facilities to 180,000m3/day.

Therewere 12 boreholes drilled and onewater-well installed at theJeungsan-Ri site to collect hydrogeological information, such as thick-

ness of alluvium, material of sediment and depth of the groundwatertable in the area. These data show that the thickness of alluvium at theJeungsan-Ri site ranges from about 35 to 50m with increasingthickness generally toward the river. These data also show that thegroundwater table in the study area ranges in depth from about 2 to9m below the ground surface. The alluvial sediments in the study areaare composed mainly of sandy gravel, coarse sand, and sandy silt. It isbelieved that the sandy gravel is the most productive water bearingzone in this area.

2.2. Data acquisition and processing

Seismic field data were acquired from the Jengsan-Ri to asses thehydrogeological conditions. A map showing the seismic survey lines isprovided in Fig. 12. Seismic measurements were taken along fivesurvey lines for a total length 1325m (Table 1). The common shotgather datawere acquired bymeans of the roll-along techniques usingan end-on shot array. The acquisition system consisted of a weight-drop type “Bumser” (Kim et al, 1994b) seismic source that is com-parable to vertical single source, and a SUMMIT (developed by DMTGimH, Germany) telemetry data acquisition system with 24-bitdynamic range. Representative acquisition parameters and surveyline information are presented in Table 1.

The seismic data processing software packages “FOCUS,” (Para-digm) and CWP/SU (Seismic Unix developed and distributed by theCenter for Wave Phenomena of Colorado School of Mines), were usedfor data processing.

As described above, we used P-beam source gather data byrearranging and stacking the single-shot gather data to enhance the P-wave component. Also, in this study, the P-beam source gather datawere treated as the shot gather data in normal seismic processingprocedures.

To process the refraction data, we selected the first arrivals ofcorresponding direct and refracted waves. The arrivals were displayedin the form of a travel time curve with 10 successive shot gathers inthe interval of every 60 shots. From the travel time curves, we couldestimate the change of depth to refractor and the change of velocityhorizontally. Fig. 13 shows an example of travel time curves andestimation of velocities from those curves from the survey lineSUR1404. The depth and velocity information is critical for calculatingthe NMO of refracted waves in the CMP refraction method. Afterrefraction move-out, we selected some traces in an optimal windowand stacked the traces. Fig. 14 shows the CMP refraction stack sectionof survey line SUR1402. The CMP refraction stack sections in the studycould be migrated and finally converted into depth sections by use ofvelocity information in the sections, as in the reflection stack section.

In processing of reflection data, the conventional procedures (f–kfiltering, gain control, muting of refractions, CMP sorting, velocity

Fig. 10. Array chart gives the reconstruction of P-beam shot gathers from the conventional roll-along source and receiver arrays of single-shot gathers.


analysis, static and NMO corrections and stacking) were applied toP-beam source gather data. Fig. 15 shows an example of a CMP gatherundergoing prestack processing before NMO correction and stacking.Fig. 16 shows the final reflection stack sections from the survey lineSUR1405 using single and P-beam sources. The section clearly showsthe groundwater table and the alluvium/bedrock contact. Thegroundwater tables from reflection stack sections of the study areacould be compared with the results from refraction stack sections.To obtain the depth information of the alluvial sediments, the finalstack sections were migrated and converted into depth sections usingcommon reflection seismic procedures.

3. Results and discussion

The CMP refraction seismic sections (Fig. 17), which wereconverted into depth, suggest that the groundwater table is clearlydelineated by the CMP refraction method. The depth of the ground-water table from the borehole datamatchedwith the seismic sections.Furthermore, the slight undulation of the groundwater table in thealluvium could be checked in the sections and we could find thehydraulic gradient, which is themost important factor in groundwaterflow evaluation by numerical modeling of the aquifer and in evalu-ating RBF feasibility.

There are several advantages in using the CMP refraction methodfor mapping the alluvial groundwater table. There is no need forseparate surveys to get the refraction signals because we can usethe refraction signals acquired in a conventional reflection survey.Also, the CMP refraction sections are easily obtained by conventionalreflection processing procedures if we evaluate the normal move-outaccording to the refracted ray path instead of the reflected ray path.Another important advantage of the CMP refraction method is that itcan be used to obtain high-resolution and continuous informationabout the groundwater table because this method uses all CMP sortedrefraction signals in the survey line.

The stack and depth sections of reflection seismic survey (SUR 1401)are presented in Fig. 18 with the CMP velocity models. There aredistinctive reflection seismic events and velocity changes in thesesections. These sections clearly show the upper and lower boundaries ofalluvium aquifer and some sedimentary structures within the alluvium.The reflection events from theboundary betweenalluviumandbed rockcould be further enhanced by the P-beammethod. Also, the ground-rolleffect could be diminished easily by the P-beam method.

Fig. 19 illustrates the enhancement of the P-wave in the P-beamdata. Fig. 19 shows the acquired P-beam and single-shot gathers withthe frequency–wave number (f–k) spectra. This diminished surfacewave allows for a more detailed interpretation of reflection. Moreover,

Fig. 11. Examples of comparison between shot gathers derived from single-source and the P-beam source. It is recognized that P-wave reflections (indicated by arrowmarks) derivedfrom the P-beam source are more enhanced than those from the single source.


the applied P-beam method used the rearrangement and stackingprocesses from the single-source shot gathers; it is not necessary tomake additional comparisons with conventional seismic reflectionsurveys. This practical aspect is another advantage of this method.

Fig. 20 shows the final interpretations of survey line SUR1404 andSUR1405 with geological logging of boreholes at the study site. Thegroundwater table was delineated on the basis of the CMP refractionsection and the sedimentary structure in the alluvium and theboundary between the alluvium and bedrock were determined on thebasis of the reflection depth section. Fig. 20 presents aquifer structureand groundwater depth information. Interpretative drawings of theother survey lines alsowere reviewed. The results include estimates ofthe lateral hydraulic gradient of the groundwater table and thedetailed thickness and structure of the alluvial aquifer to furtherevaluate the feasibility of RBF at this site.

4. Conclusions

In the determination and characterization of a suitable RBF site,it is important to know the depth of the groundwater table and

the distribution and structure of alluvial sediments. Because theavailable intake of water from a well strongly depends on hydraulicconductivity of aquifer and available drawdown in the well, thesediments and aquifer thickness are critical key factors in selectingand developing suitable RBF sites. High-resolution seismic methodscan play an important role in evaluating these hydrogeologicalparameters.

To obtain hydrogeological information about potential RBF sites,we used a high-resolution seismic method that includes CMPrefraction and P-beam methods. The results from these methodsclearly showed the depth of the groundwater table and the structureof the alluvial aquifer at the study sites.

The CMP refraction method has many advantages in mapping thegroundwater table. A slight undulation of the groundwater tablewas detected easily with this method. CMP refraction, moreover, iseffective in the field work because the refraction signals can beobtained from conventional reflection surveys. The P-beam methodalso has many advantages in delineating the structure of alluvialsediments in the near-surface. We could map the lower boundary ofan alluvial aquifer more clearly and find more reflection events which

Fig. 12. Map shows the seismic field survey area, Jeungsan-Ri in the Nakdong River watershed and the detailed survey lines. The arrow on the seismic survey lines indicates thedirection of the seismic section.


might be undetectable without the P-beam method because of strongground-roll signals.

This study strongly suggests that a high-resolution seismic survey,combining CMP refraction and P-beammethods, is a powerful tool forobtaining hydrogeological information and evaluating potential RBFsites.

Acknowledgement

This work was financially supported by a grant (code 3-4-3) fromthe Sustainable Water Resource Research Center of 21st CenturyFrontier Research Program and Kwater (Korea Water ResourcesCorporation) Research Program. This financial assistance is gratefullyacknowledged. The authors wish to thank the staff of KIWE (Korea

Table 1Field parameters for each seismic survey line at the Jeungsan-Ri site

Survey line Length Near-offset No. ofchannels

Stationinterval

Shotinterval

Samplingrate

SUR1401 251 mSUR1402 171 mSUR1403 301 m 2 m 50 1 m 1 m 0.25 msSUR1404 251 mSUR1405 351 m

Institute of Water and Environment) and Soam Consultant Co., Ltd. fortheir helpful field work and preparation of figures. The first authorwishes to express his appreciation to the staff of the Department ofGeology, Portland State University, who supported this research workand extended an invitation to him to be an adjunct professor at theuniversity during preparation of this paper. The first author wishesalso to thank Eric Collins and Jill Carroll of GSI Water Solutions, Inc.for their reading and correcting of the manuscript. Special thanksare to anonymous reviewers for their critical revisions and helpfulsuggestions.

References

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Brandhuber, P.J., 2004. Trends in drinking water treatment. Waterscapes 15 (4), 4–5HDR.

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Dresen, L., Tuetter, H., 1994. Seismic coal exploration, Part B: in-seam seismics.Handbook of Geophysical exploration section 1. seismic exploration, vol. 16B.

Gebrande, H., 1986. CMP-Refraktionsseismik. In: Dresen, L., Fertig, J., Ruter, H., Budach,W. (Eds.), Seismik auf neuen Wegen, 6. Mintrip-Seminar, Unikontakt, RuhrUniversitat Bochum, pp. 191–206 (in German).

Hahn, J.H., 2006. Detecting groundwater-surface water interaction in RBF systems usingbiological methods. International Symposium on Artificial Recharge of Ground-water 2006, Kwater (Korea Water Resources Corporation), pp. 57–63.

Kim, J.-Y., 1989. Directivity of P-radiation caused by a seismic source array. Korea Inst.Miner. Min. Eng. 26, 28–33 (in Korean).

Fig. 13. Examples of first arrival time picking and refractor velocity estimation from shot gathers of survey line SUR1404 in Jeungsan-Ri site, comparing single-shot gathered data andP-beam shot gathered data. The P-beam shot gathered data show more clear refraction events from the groundwater table than those of the single shot.


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Kim, J.-Y., Kim, Y.-S., Hyun, H.-J., Kim, K.-S., 1994a. Applicability of P beam source to theshallow reflection seismics. Korea Inst. Miner. Energy Resour. Eng. 31, 407–412(in Korean).

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Kim, H.-S., Kim, J.-Y., Kim, Y.-S., 2004. The introduction of successive CMP refractionstaking method and its application to investigate groundwater table in alluvialaquifer. Proceedings for 2004 Spring Meeting of Korea Society of Soil andGroundwater Environment (in Korean).

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Orlowsky, D., Ruter, H., Dresen, L., 1998. Combination of common-midpoint-refractionseismics with the generalized reciprocal method. J. Appl. Geophys. 39, 221–235.

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Fig. 14. An example of CMP refraction stacked section for survey line SUR1402. This section shows the groundwater table of the area clearly and precisely.

Fig. 15. Examples of seismic reflection data processing. This is the conventional processing procedure for reflection seismic data before NMO and stacking in this study.


Fig. 16. Comparison of reflection stacked sections of survey line SUR1405 by use of (a) single source and (b) P-beam source. The section of P-beam shows drastically clear reflectionevents comparing that of single source.

Fig. 17. The depth sections derived from CMP refraction stack sections of survey line (a) SUR1401 and (b) SUR1402. These sections show the strong and clear signal refracted from thegroundwater table.


Fig. 18. The reflection stacked section (a) and depth section (b) of SUR1401 with CMP velocity model. The signals of groundwater table in depth section show almost similar resultwith that of the depth section (see Fig. 17-a) derived from CMP refraction method.


Fig. 19. Single (a) and P-beam (b) shot gathers seismic data after f–k filtering with the f–k spectra. Arrows indicate signals (ground-roll in (a), reflection in (b)) and their correspondingspectra.


Fig. 20. Interpretation drawings of survey line SUR1404 (a) and SUR1405 (b) with geological logging of borehole at the site. The interpretation line of the groundwater table is basedon the CMP refraction section. The structure in the alluvium and the boundary between the alluvium and the bedrock are based on the reflection depth section.


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