HYDROSTRATIGRAPHIC CHARACTERIZATION OF … In the present study, a preliminary interpreted well log data and VES, provided by Al-Faid Agricultural Development Company, are integrated

ISSN 2321 – 9149 IJAEES (2014) Vol.2, No.1, 39-50

Research Article International Journal of Advancement in Earth and Environmental Sciences

HYDROSTRATIGRAPHIC CHARACTERIZATION

OF GROUNDWATER SYSTEMS IN KHATATBA

AREA USING VERTICAL ELECTRIC SOUNDING

AND WELL LOG DATA, SW NILE DELTA, EGYPT

Abdulaziz M. Abdulaziz

Mining, Petroleum, and Metallurgical Engineering Department, Faculty of Engineering, Cairo

University, Giza, Egypt ( [email protected])

----------------------------------------------------------------------------------------------------------------------------- ---------- ABSTRACT: The study area represents the western extension of the Nile Delta flood plain at the contact of the desert, where

the importance of ground water is rising. This requires a thorough understanding to the developed groundwater systems,

particularly the shallow aquifers. In this study, well log data was integrated to a Vertical Electric Sounding (VES) survey

acquired using Schlumberger configuration at the Khatatba area to characterize the aquifer systems. The results were

interpreted using the published stratigraphic data of the study area, and consequently the extension of the identified aquifers

can be laterally correlated. Two aquifer systems separated by an aquitard or basaltic layer were mapped and characterized in

details. The upper aquifer corresponds to El Khashab formation that produces fresh to brackish water and generally shows

unconfined condition that could locally change to partially semi confined. The second aquifer is commonly confined and

stratigraphically correlates to Qatrani formation with groundwater quality relatively better than the upper aquifer. Detailed

characterization of these aquifers is discussed to improve understanding the groundwater systems of the study area and the

adjacent desert. KEYWORDS: Vertical Electric Sounding, Schlumberger configuration, Khatatba aquifers, aquifer characterization,

groundwater systems.

----------------------------------------------------------------------------------------------------------------------------- ---------- 1. INTRODUCTION: Availability of groundwater resources is a fundamental parameter in initiation and continuity

of desert land development for industrial, urbanization, and agricultural projects [(1), (2)]. Therefore, exploring the

quantity and quality of such valuable resources has the prime importance for these projects. Resistivity sounding method

has proved popular and useful in investigating hydrological, environmental, and engineering problems [(3), (4)]. In a resistivity survey, electric current is injected to the ground through two current electrodes, conventionally called A-B as

shown in Fig:I, and the potential difference is measured between two potential electrodes known to be M-N (Fig:I). The

apparent resistivity of a geologic medium depends basically on the lithologic composition and fluid content (5) and therefore, geoelectric units usually define parastratigraphic units with boundaries discordant with the stratigraphic

boundaries (6).

Fig I: Schlumberger electrode configuration

Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 40

For a geologic medium the apparent resistivity is a function of the resistance value and a geometric factor as

calculated form the following modified Ohm’s equation:

Where ρ is the resistivity in Ω.m, K is a geometrical factor that accounts for the separating distance of both

current and potential electrodes, is the potential difference in mV, and is the electric current in mA.

The value of the geometrical factor can be approximated using parameters presented in Fig:I by the relation;

A variety of VES arrays were developed since the first application by Schlumberger in 1934 (7), but the

Schlumberger array remained the best configuration for deeper soundings. Each VES configuration adopted data

acquisition method and exploit various data processing techniques to convert the resistivity measurements into interpretable 2D and 3D geology information (8). Schlumberger sounding is the most popular sounding method in DC

resistivity survey (9). In this method, the mid- point of the survey remains fixed, while the separating distance between the

electrodes is gradually increased that enables investigating deeper sections and obtaining continuous vertical data (Fig:I).

The interpretation of resistivity data is relatively ambiguous, and sometimes impossible to obtain a unique interpretation (5). But a good control and wise constraints provided by calibrated well logs and modern drilling practices that ensure

minimal changes in rock properties greatly diminish ambiguity and facilitate interpretation.

In the present study, a preliminary interpreted well log data and VES, provided by Al-Faid Agricultural Development Company, are integrated to the geology of Khatatba and Wadi El Natrun area as observed in field work and

literatures. Such integration helps identifying in details the vertical and horizontal distribution of the subsurface

sedimentary units and delineating the important water-bearing horizons. Accordingly, the hydrostratigraphic framework influencing groundwater occurrence in the Khatatba and the surrounding area is determined.

2. LOCATION AND GEOMORPHOLOGY: Khatatba area constitutes the western boundary of the Nile Delta

encountered 15 Km away from the Rossetta branch of the River Nile. It falls within the northeast territory of Sadat City (Fig:II) and Wadi El Natrun (located 25 Km southwest, Fig:III). Geomorphologically, the study area is covered with hard

siliceous Neogen-sediments which constitute undifferentiated chains of hills arrange along E-W fault as groups of bulges

and hills alternated with small depressions filled with playa or sand (Fig:III). To the south there are small hills scattered throughout large area that was lumped by hydrothermal solutions and volcanic activity. The climate of the study area falls

within arid-semi arid zone with average temperatures (maximum of 28˚C and minimum 14˚C), average humidity 54%,

precipitation depth 41 mm/year, and evaporation 9.5 mm/day. This indicates minimal natural recharge to the shallow aquifer but return irrigation water constitutes a major share to aquifer recharge.

3. HYDROGEOLOGY: The hydrogeological map of Menouf area provides preliminary clues to the dominant

aquifers in Khatatba area and the general groundwater flow system (10). It indicates, based on potentiometric groundwater head distribution, that the Rossetta branch of the Nile recharges the aquifers in Khatatba through hydraulic connection and

the estimated total abstraction of the Quaternary aquifer is about 25*106 m

3/year. Recently, several works studied the

Quaternary aquifer of the study area at El Sadat City and discussed in details the hydrogeology (e.g. [(11), (12)]), Hydrogeochemical characterization [(13), (14)], and subsurface geophysical investigations (15). The study area entails a

relatively shallow aquifer consisting of recent - Pleistocene sand and gravels with intervening discontinuous clay lenses

(14). This stratigraphic architecture results in intermediate productivity due to the effect of the intervening clay and silty-

clay lenses. Well data confirmed the presence of these two aquifers, Quaternary and Miocene, separated by a basaltic or shale

layer usually encountered at 170-200 m depth (Fig:IV). The Quaternary aquifer provides the main groundwater supply for

domestic, irrigation and industrial purposes in the study area. It attains extensive areal distribution to the east of Cairo–Alexandria Desert Road and fuse northward with Nile Delta Quaternary aquifer and southeast with Miocene Moghra

aquifer of Wadi El-Farigh, but towards the southwest merge with the Pliocene aquifer of Wadi El-Natrun depression,

Fig:III and Fig:IV (14). Lithologically, it showed a marked increase in coarse components eastward and is composed mainly of fluviatile sand and gravels occasionally intercalated with clay and usually exhibit variable thickness due to NW-

SE step faults of downthrown to east and north (Fig:III). The clay content increases southward to record 20-25% while the

northern part only shows up to 15% and the major part of the aquifer shows phreatic conditions but locally may change to

Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 41 semi-confined aquifer. Likewise, The saturated zone in the Quaternary aquifer decreases southward direction away of the

Rossetta branch to record approximately 40 m with variable water salinity (TDS: 400-1200 ppm) due to argillaceous components (11). The depth to water below ground-surface ranged between 14 to 25 m in the northern part and 45 to 56 m

at the southern part and east of the Cairo–Alexandria Desert Road. The deeper water table at the southern parts is

primarily attributed to excessive groundwater abstractions at agricultural development zones associating inadequate

aquifer recharge and replenishment (14).

Fig II: Location map of study area, showing locations of the VES profiles and well log, upper right


Fig III: Geologic map to the area around Khatatba modified from Abu Zeid, 1984 (16)

4. METHODS:

4.1 VERTICAL ELECTRIC SOUNDING (VES): To image the dominant subsurface architecture in the study area,

three VES of 600-1000 m length are conducted using Schlumberger configuration. The Schlumberger array provides a

satisfactory resolution in horizontal layers with adequate depth sensitivity but usually adjoins high-signal-to noise ratio (17). The depth of penetration in a resistivity survey is usually approximately one-third of the separating distance between

the current electrodes (18). In the present study, the locations of the VES were accurately surveyed using 30 ground

control points (GCPs), divided equally among the resistivity lines, using a handheld Garmin eTrex GPS unit with an expected horizontal accuracy less than 1 m.

As described by Barseem et al., 2013 (19) a series of constant separation measurements with successive increase

in electrode separation was applied to identify datum levels. The measurement procedure usually starts using unit spacing (usually 2, 3, 5, or 10 m) in the first traverse that increases through increasing the electrode separation by adding a unit

spacing to acquire a measurement of the subsequent traverse. Thus for 5 m initial electrode spacing, the following

traverses are obtained successively using electrode spacings of 10, 15, 20…and so on. The present VES experiments

enabled approximately 200 to 325 m depth of penetration. For each measurement, the potential difference (ΔV) between potential electrodes is measured and recorded by a highly sensitive Voltmeter while the electric current between the

current electrodes using micro-Ammeter. The power supply utilized a group of dry cells connected in series to secure

normally 400 to 600 volt as a final potential difference that sometimes approaches 1000 volt in long profiles. Therefore, insulated cables of armored copper are highly recommended to be utilized during field work and data acquisition. To

minimize the soil effect, a saline solution is dispensed on electrode to maintain the contact resistance below or within 2

kΩ.


Fig IV: Hydrogeological cross-section of the near shallow aquifers between Rossetta branch and Wadi El Natrun (11)

4.2 WELL LOGGING DATA: Well log data record including SP (MV), Gamma Ray log (CPS), and Resistivity log

(Ohm-m) for a nearby well (Fig:II) penetrating the stratigraphic section of the study area are used in the interpretation of resistivity data and subsurface characterization. Resistivity log measurements involve two configuration; Normal with

16N and 64N measurements and lateral configurations. The log measurements extend down to approximately 260 m and

all data record are conventionally corrected for hole diameter and bed thickness. The resulting records are further processed for visual and numerical interpretations that help identifying the potential water zones and the suitable well

specifications.

4.3 DATA PROCESSING AND RESULTS: VES data analysis was completed using a hybrid technique in which a

preliminary manual interpretation was accomplished and the final results were tested using RESIX-IP program (supported

by Advance Geosciences Incorporation, AGI) designated to Schlumberger curves interpretation. As a start, all data set are

investigated for consistency using log-log plot of the apparent resistivity (ρ) versus AB/2 (Fig:V). The main advantage of this step is data smoothing that not only emphasizes near-surface resistivity variations but also attenuate resistivity

variations at greater depths, which also involves filtering out the spurious data points that unfit to the overall profile of the

VES. This step is crucial to the final result as interpretation depends fundamentally on the minute variations in resistivity values encountered at shallow depths. Thus, a great caution is increasingly required in this step to prevent replacing real

field reading by false data points (20). The resulting curve of each VES is compared to the standard curves (21), where the

obtained experimental data curve is subdivided into a group of small curves. Each of these small curves represents a

Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 44 geoelectric unit of known thickness (m) and resistivity (Ω.m) that actually designates a geologic stratum of physical

properties differs from those located above and below.

Fig V: Interpreted field curve data for VES profiles shown in Fig:II

The resulting data (7 layers of different thickness and resistivity) are subsequently fed in the RESIX-IP program

to verify validity and carry out fine tuning to the final output. Typically, the resulting geoelectric section doesn’t precisely correspond to the stratigraphic column of the studied section. Therefore, a correlation between the known stratigraphic

column of the study area (Fig:VI) and the geoelectric section using visual interpretation of a nearby well log data

including Gamma ray, SP, and Resistivity logs (Fig:VII) is crucial for geoelectric data interpretation. The final output

(Fig:IIX) is a composite hydrostratigraphic section that not only provides complete / accurate stratigraphic units but also indicates the potentiometric water level and approximate water salinity (Fig:IIX). In addition, the lateral extension of the

hydrostratigraphic units can be authentically verified and hence, the average thickness/dominant slopes and geologic

structures (if present) can be, straightforward, recognized. Visual interpretation of Gamma ray and resistivity well logs (Fig:VII) helps identify several horizons of

contingent value that corresponds to homogeneous lithology and probably water content. Several petrophysical

parameters such as lithology, shale content, and water salinity can be approximated using the available well logs data.

Shale content (Fig:VII) is estimated using shale index (Ish) that can be approximated as;

Where γlog is gamma ray response at the zone of interest, γclean is a representative gamma ray response in the clean formation, and γSh is the representative gamma ray response shale.

Using Larenov equation for Tertiary rocks (22), shale volume (VSh) shown in Fig:VII is calculated as;

The formation water resistivity (Rw) is approximated using several techniques, but the SP methods are widely applied. In the present work, Rw was calculated using SP log as described in Bassiouni (23). Groundwater salinity in

clean water bearing formation (Sln), measured in ppm of NaCl equivalent, can be approximated graphically using the

corresponding Rw (Ohm.m) and temperature (26 °C) by Gen-9 chart of Schlumberger (23) and checked for consistency

using the following empirical formula modified from (24)


Fig VI: Composite stratigraphic column for the West Nile Delta area (25)

Using mud filtrate resistivity (Rmf) of 1.65 Ohm.m and the borehole temperature 90 ˚F, the synthetic groundwater

salinity log for groundwater is presented in Fig:VII. The complete well log data analysis and interpretation is presented in

Tables:I, II, and III

5. DISCUSSION: The three VES (VES-1, VES-2, and VES-3) showed a relatively similar field curves that

indicated the presence of 7 distinct geoelectric unites of comparable geoelectric characteristics. All these layers have been

documented in the three VES except the seventh layer (Fig:IIX) that was not detected in VES-3 located down-dip and therefore the 7th. geoelectric layer was located deeper to fall within the detection field. Generally, the final composite

hydrostratigraphic section relatively correlates with the corresponding resistivity logs of the nearby well (Fig:IIX). Based

on the well logs analysis and the subsurface geoelectric model the subsurface of the study area can be characterized into three hydrostratigraphic horizons (Table:IV). Each horizon is alternatively subdivided into geoelectric units of unique

resistivity characteristics based on lithologic composition and water content (Table:IV).


Fig VII: Resistivity, Gamma ray, and SP Well log data for the well shown in Fig:II and preliminary interpretation of shale

content and expected TDS of the groundwater.

The first horizon occupies the vadose zone of the near surface unconfined aquifer and includes two geoelectric

units (Fig:IIX and Table:IV). The first unit shows a resistivity of 68 to 76 Ω.m (Fig:IIX) that extends 6 to 9 m below the land surface and consists of Pleistocene friable sand and gravel of fluvial origin and low gamma ray counts (Table:II).

This unit usually contains the excess water of return irrigation of agricultural activities and therefore the water content

depends significantly on agricultural season and practices. At dry barren land of the study area this zone is fused with

Zone Two and in most cases it is hard to distinguish them into two independent zones. The second geoelectric unit showed high resistivity values, 180 to 250 Ω.m, that encountered down to 27-38 m below surface and correspond to the

transition zone. This zone is obviously recognized on electric logs (Fig:VII) with notable separation between the response

of shallow and deep tools induced by the invasion of drilling fluids. Typically, this unit is made of clean medium-grained sand as presented on Gamma ray log (Fig:VII and Table:II) but may admix with clay, calcareous silt and conglomerate to

reach 20 to 39 m thick.

Depth Interval (m) Resistivity Analysis


Table I: Well log analysis and interpretation for resistivity logs presented in Fig:VII

Table II: Full log analysis and interpretation Gamma Ray log shown in Fig:VII

Table III: The expected Salinity as calculated in clean sand formation using SP log presented in Fig:VII

The second hydrostratigraphic horizon represents a phreatic zone in the hydrogeological system of the study area

(Fig:VI). It involves three geoelectric units on the resistivity log (Fig:VII) presented as a relatively low resistivity (less than 10 Ω.m) unit between two moderate resistivity (65 Ω.m >Resistivity< 25Ω.m) units that stratigraphically correspond

to El Khashab formation (Fig:VI and Fig:IIX). The upper unit in the phreatic zone is made of yellowish to grey clean

sandstone that showed a resistivity of 40 to 65 Ω.m and may reach 30 m thick (Fig:VI). The middle unit is approximately 40 m thick and made of intercalations of sand and clay/gypseous clays. Due to the presence of clay and/or gypsum the

reported resistivity is relatively low (8.1 to 9.2 Ω.m) and several water wells taping the aquifer may produce brackish

water when the well drainage area falls within the proximity of this zone. The lower unit in the phreatic zone extends to 90 m thick and normally shows slightly low resistivity (30-42 Ω.m) compared to the upper zone due to the effect of thin

clay intercalation.

Well log data of the second hydrostratigraphic horizon showed detailed lithologic composition of various water

contents (Fig:VII). Gamma ray log indicated four distinct units range in radioactivity from low to high gamma ray count that was interpreted as intermediate sand to argillaceous sand (Table:II). Such lithology has notably influenced the

response of the resistivity log as the high clay content units correlates well to the low resistivity units while the clean units

showed moderate resistivity records (Table:I). In addition, clay content and the change in water salinity with depth may explain the response of resistivity log as the percolated water from excess irrigation water usually increase the water

salinity at the upper parts of the aquifer (Table:III). Also the evaporitic components in the upper zone further deteriorate

From To Resistivity level / Value (Ohm.m) Interpretation and justification

0 12 No Reading Absence of clay

12 42 High / 281.6 Dry sand

42 90 Medium / 55.6 Brackish water sand

90 100 Low / 34.7 Sand clay intercalations

100 132 Medium / 67.2 Brackish water sand

132 137 Low / 87.6 Less brackish water/Argillaceous sand

137 192 Medium / 115.5 Fresh water sand

192 221 Low / 55.3 Argillaceous sand

221 246 Medium / 101.1 Fresh water sand

246 267 Low / 12.2 Argillaceous sand

Depth interval (m) Gamma ray log analysis

From To Average Gamma Ray Level / count (cps) Interpretation

0 9 low / (7.5) Alluvial sand

9 90 Low / (6.1) Medium grained sand

90 100 Intermediate to high / (30.2) Sand – clay intercalations


132 137 Relatively high / (35.6) Argillaceous sand


192 221 Relatively high / (35) Argillaceous sand

221 246 Intermediate / (28) Fine to medium grained sand

246 267 High / (55.6) clay

Depth (m) Expected Salinity (ppm)

From To

138 144 400 to 500

162 192 250 to 350

222 246 300 to 450

Abdulaziz, A.M./ International Journal of Advancement in Earth and Environmental Sciences, Vol.2, No. 1 48 the water quality where encountered (Fig:VII). This is well shown in Table:I and correlates well with the gamma ray

response (Fig:VII).

Fig IIX: Composite geoelectric cross section as interpreted from VES profiles shown in Fig:II

The Phreatic horizon is entirely alluvial and stratigraphically known as Gebel Khashab formation (27) that corresponds to Raml formation (26) and extends over a wide area in the Northern Western Desert. It is exposed southward

to the surface at Gebel El Khashab (NE Fayium), Gebel Khasm El Qaud northward, Qattara Depression in the west, and

the Nile Delta in the east. The lower boundary of Kashab formation is erosional that may overly scours in the top of

Widan El Faras basalt or unconformably overly the Qatrani formation (Fig:VI) that is, in this case, hard to precisely delineate their contact at many localities due to the similar lithology (25). At the neighboring Wadi Natrun area and its

environ (Fig:III), marine fossils might be found in some Lower Miocene beds (16) which eliminates these beds from El

Khashab formation to the slightly younger fluviomarine Moghra formation of the Delta subfacies (25). At Wadi El Farigh (Fig:III), this unit is exposed as low ridges parallel to the trace of Wadi El Farigh, where the silicified wood and vertebrate

fossils ascertain the Early Miocene age (27).

The lower hydrostratigraphic horizon represents an aquifer system with low resistivity aquitard overlying a moderate resistivity water bearing unit. The aquitard consists of 30-35 m clay layer of 6.0 Ω.m resistivity, while the

aquifer unit is distinctly represented as moderate resistivity unit, 52-65 Ω.m (Table:IV). This aquitard could be

underlained or totally replaced by a basaltic layer (Widan El Faras formation, Fig:VI) up to 25 m thick particularly

eastward but such a unit is not encountered either by the VES or the well log data at the investigated site. The aquifer unit is composed of sandstone of Qatrani formation (27) and is usually encountered at 225 m depth below ground surface in

the study area (Fig:VI). It is easily distinguished from the underlying green/gray shale beds of Kasr El Saghah formation

by the brightly variegated sandstone and gravely sandstone that rarely change to mudstone. This horizon usually lacks the presence of fossils except for the upper part and most likely was deposited in estuarine to fluviomarine environment [(28),

(29)].


Table IV: correlation between geoelectric model of Fig:IIX and well log data presented in Fig:VII

Generally, a slight vertical and lateral change in facies is reported in shifting the environment from slope deposits to valley fill deposits, but perceptible vertical and lateral changes are only observed on changing depositional environment

from continental slope deposits into coastal flood plains surrounded by swamps. This is probably where the gamma ray

log recorded medium to relatively high gamma sand with fresh water content as indicated by the expected salinity log that

typically correlates to the flood plain environment (Table:II and Fig:VII). Such interpretation is augmented by the correlating resistivity log and VES data (Table:I and Fig:IIX). In the study area it is normally reported as basaltic

fragments mixed with fine grained sand and silt. Field work indicated that the lower aquifer is not greatly exploited

compared to the Phreatic zone due to the high cost of well drilling. In addition, aquifer replenishment takes place through Nile water infiltration from the nearby Al Bohairy Canal and Rossetta branch. Therefore, the lower aquifer still acquires

good quality groundwater of TDS less than 450 ppm.

6. CONCLUSION: Integration of well log data and VES conducted in the study area indicated the existence of two

aquifer systems: phreatic and confined. The phreatic aquifer has resistivity between 30 and 65 Ohm.m based on clay

content that markedly dominates the middle part of the aquifer and consequently decreases measured resistivity below 10

Ohm.m. The confined aquifer is encountered 250 m below ground surface and is made of clean sand bearing fresh water as depicted by low gamma ray and resistivity record of 52-65 Ohm.m. Generally, the two aquifers showed good water

quality with the lower aquifer representing a potential groundwater resource of better quality compared to the upper

aquifer. Such information elucidates the importance of integration between VES and well log data in evaluating groundwater resources.

ACKNOWLEDGEMENT: The author acknowledges very much the technical and logistical support provided by Al-Faid Agricultural Development Company, Sadat city, Egypt. The fruitful discussion at the early stages of this work with

Dr. Abdalla Faid, The National Authority of Remote Sensing and Space Sciences (NARSS), is greatly appreciated.

Comments and critical reviews by Professor Dr. Fouad Khalaf (Professor Petroleum Engineering) and Professor Dr.

Abdel-Zaher Abouzeid (Professor of Mining Engineering) at Faculty of Engineering, Cairo University were very helpful and are greatly appreciated.

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Hydrostratigraphic Zonation Geoelectric

layer Resistivity (Ohm.m)

Log response Water salinity Remarks Hydrogeological

condition Lithology

Vadose zone

Fluvial Sand & Gravel

Layer 1 68-76 Low Gamma Highly

fluctuating soil effect

Medium grained sand

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Low Gamma 200-400

Infiltration water

Phreatic zone

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250-500 Groundwater

level

Shale-sand intercalations

Layer 4 8.1-9.2 Oscillating

medium to low 250-500

Argillaceous medium sand

Layer 5 30-42 Low resistivity Med. Gamma

250-500

Lower Aquifer system

Clay layer Layer 6 6.0-6.5 Low resistivity High Gamma

High Value Shale effect

Medium sand

Layer 7 52-65 Low resistivity Low Gamma

300-450


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Fayum Depression, Egypt, U. S. Geol. Surv. Profess. Paper 1452, 60 pp.

HYDROSTRATIGRAPHIC CHARACTERIZATION OF … In the present study, a preliminary interpreted well log data and VES, provided by Al-Faid Agricultural Development Company, are integrated

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