2-d resistivity surveying

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In this issue we highlight novel exploration techniques. Paul Bauman gives a tutorial on the uses and potential of resistivity surveying for hydrocarbons. His entertaining and informative article tells us of thehistorical applications of the method, discusses field techniques and shows some modern uses inexploration in the Athabasca oil sands and in gas-charged Quaternary channels. Martyn Unsworthsreview of magnetotelluric methods demonstrates how they can be useful complements to seismic methods.One particular application that should be of interest to many of us is in overthrust belts, where overlyinghigh velocity layers cause problems in seismic imaging. The third paper, by Adam Gersztenkorn et al., isan overview of a new seismic attribute which enables more detailed stratigraphic information to beextracted from seismic data. The more we can get from our seismic data, the better!

Helen Isaac

April 2005 CSEG RECORDER 25

Continued on Page 28

Abstract

2-D resistivity surveying is a technique that has taken advan-tage of solid state relays and modern inversion techniques tomake an evolutionary leap from 1-D resistivity soundings, asurvey method that has changed little after almost 90 years of practice. 2-D resistivity exploration has been used extensivelyfor exploration in the oil sands and, more recently, for veryshallow gas exploration (less than 400 m below groundsurface). While much of present day exploration for new oiland gas resources involves investigating deeper and in moreremote locations, another far less expensive and greatly over-looked play concept exists in reservoirs stranded behindcasing. For most of these potential zones, even minimalgeophysical logging data do not exist. 2-D electrical resis-tivity surveying provides explorationists a second chance todefinitively, and in a cost-effective fashion, explore for "ultra"shallow oil and gas resources in the upper 400 m.

Introduction

For the last seven years we, at Komex, have been routinelyand successfully applying geoelectric techniques in the explo-ration and direct detection of oil sands and, more recently, gascharged fluvial channels nested in the Quaternary buriedvalleys of Alberta. Nevertheless, our successes have been born out of years of personal scepticism based on experience.From 1981 to 1985, as a wireline field engineer and basemanager for Schlumberger in Borneo and Papua NewGuinea, I ran hundreds of logs in the fresh connate waterreservoirs of the Mahakam and Sembakung Deltas, and otherhighly productive oil fields in extreme precipitation equato-rial areas. Even with the latest prototype models of electro-magnetic propagation tools and every induction andresistivity tool available in Schlumbergers arsenal, the ques-tion of whether an interval was oil or fresh water often couldonly be resolved after wireline formation sampling. At thetime, it was clear to me that if one million dollars of geophys-

ical instrumentation could not distinguish oil from water inthe borehole when the formation fluid was millimetres fromthe sensors, direct detection of hydrocarbon from surfacewas impossible.

My scepticism seemed to be confirmed at the University of Waterloo where I worked on the application of geophysicalmethods to the direct detection of dense non-aqueous phaseliquids, or DNAPLs. These non-polar organic fluids arecommon in dry cleaning, wood preservation, asphalt opera-tions, and other previously considered innocuous activities.From an electrical properties perspective, DNAPLs closelyresemble hydrocarbons. In our attempts to directly detectpure phase DNAPLs in groundwater, myself and subsequentresearchers (Brewster et. al., 1994) carried out surveys with awide range of electrical and electromagnetic techniques andinstruments including ground penetrating radar, galvanicre s i s t i v i t y, self potential, complex re s i s t i v i t y, capacitivelycoupled resistivity, and electromagnetic induction systems.While some success was achieved in controlled spills withtime lapse surveys, no technique emerged as being practi-cably useful in real world industrial spill situations. My expe-rience in Alberta of carrying out hundreds of induction andresistivity surveys at contaminated oil and gas sites hasfurther confirmed that while salts, and even trace metals, areviable targets, the hope of reliable and routine direct detec-tion of hydrocarbons from surface is illusory.

Yet, despite intuition to the contrary, there are a number of

types of very economic oil and gas plays for which geoelec-tric techniques for direct detection have been successful, andcontinue to be used. In Alberta, two examples of plays of noteare the Athabasca oil sands, and the gas charged Quaternarychannels of northwestern Alberta. While unusual, these playsare not unique. The characteristics that have made theseplays particularly amenable to geoelectric explorationinclude their shallow occurrence, their significant thicknessrelative to their depth, and their very high resistivity contrastwith the host geology.

2-D Resistivity Surveying forHydrocarbons-A PrimerPaul BaumanKomex International Ltd., Calgary, Alberta, Canada


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Focus Article Contd


2-D Resistivity

Historically, a number of geoelectric techniques for direct detec-tion of hydrocarbons have been tried in Alberta with varyingdegrees of success. These include 1-D time domain electromag-netic surveys, 1-D resistivity surveys, airborne fixed frequencyand time domain surveys, and magnetotelluric surveys. 2-Dresistivity, also known as electrical resistivity tomography (ERT)

or electrical resistivity imaging, is the most recent entry intoshallow, non-seismic exploration programs. This author believesthis technique to be the most successful of the various electricaltechniques in terms of providing high confidence interpretationsas well as a cost-effective approach given the geology and logis-tics usually encountered in Alberta.

The physics of 2-D resistivity are no different than those of 1-Dresistivity as initially investigated by Conrad Schlumberger inFrance and Frank Wenner in the United States in 1917 and 1918(Burger, 1992). A known direct current (DC) moves in the subsur-face from one electrode to an electrode return at some measureddistance away (Figure 1). Two potential electrodes measure thepotential drop, that is, the force needed for the current to over-

come the resistance of the subsurface earth materials. We knowthe current we put into the subsurface, we measure a voltage,and with Ohms Law, we then calculate a resistance (voltagedivided by current). Given the geometry of our four electrodearray, we can calculate an apparent resistivity of the subsurface.By changing the geometry of the electrodes, we measure anapparent resistivity at a different depth in the earth. The princi-ples of electrical soundings were established in the 1920s; inter-pretation by using two and three layer master curve matchingwas developed in the 1930s (Dahlin, 2000). As an undergraduatein the 1970s, I interpreted the log-log sounding curves of 1-Dresistivity surveys with overlays from books of type curves. As agraduate student in the 1980s I used computer generated curvesand 1-D inversion software to more conveniently produce better

fits (Figure 2). The problems, though, remained. The interpreta-tion of any single 1-D resistivity sounding (or time domainsounding) is inherently non-unique, with infinite unconstrainedmodels providing a solution to the curve fit. And of course, thesolutions by definition assume a 1-D layered model.

Instead of four electrodes, a 2-D resistivity array can beconstructed of any number of electrodes, though in practice thistypically ranges from 42 to 256. The electrodes must be colinearand evenly spaced, though 3-D (Loke and Barker, 1996a; Tsourlosand Ogilvy, 1999; Li and Oldenburg, 2000; Bentley and Gharibi,2004), borehole to borehole, and borehole to surface arrays arealso possible (Ramirez et. al., 1993; LaBrecque et. al., 1996). Whileany one measurement still involves two current and two voltagem e a s u rements, all possible quartets of electrodes can beaddressed by the transmitter/receiver via a patchbox filled withsolid state relays, similar to your local neighbourhood telephonerouting centre (Figure 3). Which electrodes are addressed, andwhen, is determined by software commands laid out by thegeophysicist as governed by the objectives of the explorationprogram (how deep, flat lying or vertical structures, etc.). Allelectrodes are connected to the electronics via one of severaltypes of multicore cable. The survey proceeds in an automatedfashion while the field crew picks up cable from behind and rollsout cable ahead in a fashion very analogous to seismic surveys.With present day transmitters and receivers, up to 8 voltagemeasurements can be made simultaneously, with a single meas-

urement requiring as little as a fraction of a second. In practice,for surveys requiring depths of investigation of 200 to 400 m

below ground surface (bgs), about 1,000 voltages (and apparentresistivities) are measured in an hour. From 2.5 to 6 km of datacan be acquired on a daily basis, depending on the active spreadlengths being used. Stacking, signal discrimination, and otherquality control measures are carried out to ensure the recordingof reliable data. The product of a survey is hundreds or thou-sands of apparent resistivity measurements at various depths

beneath the entire length of the electrode array, or what is knownas a resistivity pseudosection (Figure 4).

2-D Resistivity Surveying for Hydrocarbons-A PrimerContinued from Page 25

F i g u re 1. The basic setup for 1-D resistivity soundings, essentially unchanged since1917. (adapted from Dahlin, 1993).

F i g u re 2. A log-log plot of 1-D resitivity sounding data and a corresponding model for a 1992 survey done over salt contaminated soil.

F i g u re 3. A generalised block diagram of an automated 2-D resistivity surveysystem (adapted from Dahlin, 1993).


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Focus Article Contd


The 2-D survey described in the previous paragraph could have been acquired without modern solid state relays and with singlewires dedicated to each electrode, so perhaps a better descriptionof this approach is "automated" 2-D resistivity surveying, for it isthe automation of electrode selection that allows this type of survey to be cost-effective. Regardless of the number of datapoints and speed with which they are acquired, without numer-ical modelling one is left with a pseudosection of apparent resis-tivities plotted at somewhat arbitrary depths. Numericalmodelling techniques developed in the early 1990s (Oldenburgand Li, 1994; Loke and Barker, 1996b) now allow one to invertlarge pseudosections consisting of several thousand field meas-ured apparent resistivities into "true" geoelectric sections in a fewhours. The size of the model blocks, and hence the resolution of the technique, is limited by current spreading and practical limi-tations on how much data can be acquired. While the size of themodel blocks dictates that 2-D resistivity results may neverapproach the vertical resolution of a seismic reflection survey,the ability to invert for true resistivities means that in certaingeologic contexts, the pore fluids of fresh water, salt water, oil,and gas can each be distinguished (Figure 5).

2-D Resistivity Exploration in theAthabasca Oil Sands Deposits

F i g u re 6 is a typical resistivity log through the main units in theAthabasca oil sands. The McMurray Formation bitumen saturatedsand is not homogeneous; nevertheless, resistivities generallyexceed 200 ohm-m. In the overlying marine shales of theClearwater Formation, resistivities are less than 30 ohm-m. In thec a l c a reous marls of the Waterways Formation which underlies theMcMurray sand, resistivities are less than 20 ohm-m.

F i g u re 7 shows a one kilometer geoelectric section across a mine-able ore (oil sand) body. Mineable bitumen bodies appear as targ e t swith resistivities exceeding 300 ohm-m, and even exceeding 500ohm-m. To the west, the McMurray sand is sandwiched betweenconductive Clearwater shale above, and conductive Devonianmarls below. Small resistive targets at or very near surface areQuaternary gravels associated with small creeks or till. The expla-nation of why the indicated wells did not target the highest gradeo re zones is that they were drilled before the geophysical survey.

\ F i g u re 8 shows a second geoelectric section across a high grade bitumen zone in the eastern portion. Resistivities exceed 500 ohm-m. The centre of the section traverses a partially water saturatedesker where resistivities also exceed 500 ohm-m. As in any explo-ration scenario, geoelectric anomalies must be placed in theirp roper geological context using whatever information is at hand -surficial geological maps, aerial photographs, drilling information,

etc. - and subsequently tested by drilling.Looking at a larger scale, Figure 9 shows about 40 km of geoelectricsections assembled into a fence diagram in order to provide a

b road view of the spatial relationship of a number of bitumen satu-rated channels. Figure 10, showing a 1-D resistivity curve extractedf rom a 2-D section and overlaid by an electric log, demonstratesthat although a downhole log has far greater detail than the 2-Dsurface data, the 2-D resistivity survey faithfully identifies thecontacts and true bulk resistivities of the imaged lithologies.


F i g u re 4. The construction of a pseudosection showing how the more broadly spacedquartets of current and voltage electrodes define apparent resistivities at gre a t e rdepths (adapted from Dahlin, 1993).

F i g u re 5. High resolution seismic data is overlain on a modeled 2-D re s i s t i v i t ysection. The seismic data were acquired with a 48 channel seismograph, 40 Hz geophones, a source and receiver spacing of 6 m, and a 0.25 msec sampling interval.Both the seismic and resistivity data sets identify the deepest point of the Rex BuriedValley in eastern Alberta. The seismic data provide superior detail in re s o l v i n g geologic contacts. However, the resistivity data uniquely identify a pro m i s i n gdrilling target as a result of the greater than 30 ohm-m anomaly. The well pro d u c e d70 gallons per minute of potable water. The moderately high resistivities are typicalof intra-till granular aquifers.

F i g u re 6. Typical natural gamma and resistivity logs through a mineable oil sandb o d y. High resistivities, large layer thicknesses, and strong resisitivity contrastswith the overlying and underlying lithologies combine to make bitumen saturated McMurray an excellent target for 2-D resistivity surveys.


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Focus Article Contd


2-D Resistivity Exploration for Gas ChargedQuaternary Channels

2-D resistivity surveying has been used routinely and success-fully in the exploration for unusually shallow gas reservoirs innorthwestern, central, and eastern Alberta. Typically, as with theSousa and Rainbow fields near High Level, the reservoirs are atless than 100 m depth, and consist of gas charged Quaternarychannels (Canadian Discovery Digest, 2001). The channels areincised and stacked within broader buried valleys (Pawlowicz etal., 2004) that are often recognized in seismic reflection data orairborne geophysical surveys. These gas-charged reservoirs areparticularly easy to identify in northwestern Alberta, where bedrock is ubiquitously of marine origin - there are no coals orextensive freshwater bedrock aquifers that could also potentially be imaged as highly resistive anomalies. To date, approximately15 bcf of gas have been produced from these shallow targets westof High Level.

Figure 11 shows two gas charged zones each displaying resistiv-ities exceeding 100 ohm-m. The reservoir to the west is straddled by three gas producing wells with total depths ranging from 62

to 93 m bgs. The Shaftesbury shale defining the bottom and sidesof the buried valley can be seen subcropping in the east. The elec-trically conductive overburden trap, though thin, is well definedabove the reservoir. Depths to the tops of the reservoirs, as wellas the lateral boundaries of the reservoirs, are generally veryprecisely and accurately defined. The bottoms of the reservoirsare often overestimated due to the strong resistivity contrastwith the shales below, and subsequent "contamination" of thedeeper and larger numerical model blocks at depth.

While unusual, these gas charged Quaternary channels in theSousa and Rainbow fields are not unique in Alberta, nor uniqueto Alberta. Similar channels have been identified in eastern andcentral Alberta, northeastern British Columbia and the south-

west of the Northwest Territories, as well as in Quebec,Denmark, Russia, and Patagonia. Economic shallow Quaternarygas reservoirs of biogenic origin are presently being produced inChina (Lin et al., 2004). Direct detection of gas in shallow bedrock reservoirs such as the Belly River has also met withsome success. In July 2003, the Alberta Energy and UtilitiesBoard successfully capped "Old Salty," an orphan well 80 m fromthe Peace River that had been flowing 340,000 cubic feet of natural gas a day for 87 years (Calgary Herald, 2004). Even aslate as 1982, relief efforts were not able to safely drill below 99 m bgs because of the pressures. Clearly, significant volumes of gasin the top 400 m remain to be discovered in Alberta, and it isunlikely that seismic reflection will be a cost-effective explo-ration tool at these depths.

The Near Future

Geophysics textbooks of the 1980s put severe limitations on theapplications of resistivity techniques. Te l f o rd et al. (1986), forinstance, noted that "the chief drawback is its large sensitivity tominor variations in conductivity near surface," hence, re s i s t i v i t ysurveys were usually avoided in dry, frozen, or other soil typesexhibiting extreme electrical contrasts. Te l f o rdet al. also noted asecond severe limitation in "the practical difficulty involved indragging several electrodes and long wires over rough woodedterrain." Pro g ress in hard w a re, software, an understanding of the

physics of the measurement, and in logistical aids has removed theabove noted obstacles in less than a decade (Figures 12, 13, and 14).

Notwithstanding, very significant developments in the method-ology of 2-D resistivity surveying are presently occurring. Theh a rd w a re development of improved and expanded multichanneltransmitters and receivers will directly effect the speed of acquisi-tion, and hence, the cost-effectiveness of both 2-D and 3-D

surveying. Pulled resistivity arrays, already in use in Europe forover a decade (Sorensen, 1994), may be particularly appropriate forthe wide open prairies of Western Canada. 2-D arrays from surface,used by Komex for monitoring air sparging at contaminated indus-


F i g u re 7. In this 1 km section of 2-D resistivity data, bitumen saturated McMurraySand is clearly visible as those large anomalies with resistivities exceeding 300 ohm-mThe small, near surface resistive features are Quaternary gravels in the drift, thoughthe large near surface resistive body at the right hand edge of the section is mineable oreThe fine grained low energy deposits overlying the ore, and the calcareous marls at thebase of the section are strong conductors (less than 50 ohm-m).

F i g u re 9. A 3-D perspective perspective of about 40 km of geoelectric data across mineable oil sands prospect. Many visualization techniques used in seismic inter p retation can be applied to resistivity data.

F i g u re 8. A 2-D resistivity section imaging both bitumen saturated McMurray anda partially water saturated esker. Where air or partially fresh water saturated gran -ular material is juxtaposed with bitumen saturated sand, electrical methods areuseful, but cannot, on their own, define the precise contact.


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Focus Article Contd

trial sites, could be expanded for temporal monitoring of steamchamber development in SAGD operations. The use of perma-nently installed arrays (Dahlin and Johansson, 1995) and telemetrywill make 4-D monitoring inexpensive and essentially continuous.M o re powerful computers and accompanying algorithms willfacilitate faster inversions and joint inversions with other geoelec-trical and non-geoelectrical data. Increased capabilities of inversion

s o f t w a re to incorporate known constraints will improve accuracy,resolution, and depth of investigation. The survey costs of re s i s-tivity surveying, already approximately 25% of the per kilometrecost of high resolution shallow seismic reflection, will continue to

be proportionately less as the above developments increase thespeed and quality of acquisition and pro c e s s i n g .

Conclusion

After being presented with possibilities of alternative geophys-ical exploration methods for certain hydrocarbon plays, I haveoccasionally heard oil and gas explorationists resignedly refer toseismic reflection as the "white bread" of geophysics. Yet, 80years after seismic reflection was first used in oil exploration(Telford, 1986), it still holds a well deserved overwhelmingdominance as an exploration tool due to its great depth of inves-tigation while preserving useful vertical and lateral resolution.Nevertheless, seismic reflection has some fundamental failingsincluding a very low probability for direct detection, and thereality of measuring a parameter at surface (acoustic impedance)sensitive to physical properties (density and velocity) that aredifferent from those measured in the borehole to identify theresource (resistivity and porosity). Still, for most hydrocarbonexploration programs, electrical methods should not even beconsidered. Insufficient resolution and/or depth of investigationare the usual limitations. The niche applications, however, arenot insignificant. In Alberta, shallow oil sand resources arespread across an area the size of Ireland. The three componentsof a Quaternary gas play - a shallow primary gas reservoir, anincised channel breaching this reservoir, and a low permeabilitytrap - exist over most of Alberta and much of Western Canada.And while more efficient exploitation of gas reserves stranded


F i g u re 11. Three producing gas wells drilled into resistive targets within a buried paleovalley near High Level, in Northwestern Alberta. Resistivities of the gas zones,as imaged from surface, exceed 100 ohm-m and peak near 200 ohm-m.

F i g u re 12. 2-D resistivity surveying on the Athabasca River for a then (winter,2001) undeveloped oil sands lease.

F i g u re 13. 2-D resistivity program in Wadi Hadhramaut in Yemen imagingaquifers and salt water intrusion to depths of 400 m. Similar surveys have beensuccessfully carried out by Komex on the largest sand dunes of the Pacific Coast.

F i g u re 10. 1-D resistivity data extracted from a 2-D geoelectric section, and super -imposed with an electric log through a well at the same location. One can visualize2-D resistivity as dragging a coarse resolution downhole electric log through a slicea c ross the exploration are a .


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behind surface casing will not be the answer to a looming energycrisis, such zones are cost-effective targets for exploration and

delineation by surface electrical methods. Other fields similar toSousa and Rainbow are surely waiting to be discovered.

ReferencesBentley, L. and Gharibi, M., 2004, Two- and three-dimensional electrical resistivityimaging at a heterogeneous remediation site, Geophysics 69(3), 674-680.Bly, D., October 2004, Peace River well blowout capped after 87 long years, CalgaryHerald.

Brewster, M. and Annan, A., 1994, Ground-penetrating radar monitoring of a controlledDNAPL release: 200 MHz radar, Geophysics9(8), 1211-1221.Burger, R., 1992, Exploration geophysics of the shallow subsurface, New Jersey, PrenticeHall, 489 pp.

Canadian Discovery Digest, 2001, Sousa Northwestern Alberta Twp111, Rge 2 W6Quaternary Gas, January/February , pp. 25-39.

Dahlin, T., 1993, On the automation of 2D resistivity surveying for engineering and envi -ronmental applications, Ph.D. thesis, Department of Engineering Geology, LundUniversity, 187 pp.

Dahlin, T., 2001, The development of DC resistivity imaging techniques, Computers &Geosciences, 27, 1019-1029.Dahlin, T. and Johansson, S., 1995, Resistivity variations in an earth embankment damin Sweden, Proceedings of the 1st Meeting Environmental and EngineeringGeophysics, Torino, Italy, 308-311.LaBrecque, D., Ramirez, A., Daily, W., Binley, M. and Schima, S., 1996, ERT moni -toring of environmental remediation process, Measurement Science & Technology, 7, 375-383.Li, Y. and Oldenburg, D., 2000, 3D inversion of induced polarisation data, Geophysics 65,1931-1945.Lin, C., Gu, L., Li, G., Zhao, Y. and Jiang, W., 2004, Geology and formation mechanismof late Quaternary shallow biogenic gas reservoirs in the Hangzhou Bay area, easternChina, AAPG, 88 (5), 613-625.Loke, M. and Barker, R., 1996a, Practical techniques for 3D resistivity surveys and datainversion techniques, Geophysical Prospecting 44 (3), 499-524.

Loke, M. and Barker, R., 1996b, Rapid least squares inversion of apparent resistivity pseudosections by a quasi-Newton method, Geophysical Prospecting, 44, 131-152.Oldenburg, D. and Li, Y., 1994, Inversion of induced polarization data, Geophysics59,1327-1341.Pawlowicz, J., Hickin, A., Nicoll, T., Fenton, M., Paulen, R., Plouffe, A. and Smith, I,2004, Shallow gas in drift: northwestern Alberta, Alberta Energy and Utilities Board,EUB/AGS Information Series 130.Ramirez, A., Daily, W., LaBreque, D., Owen, E., and Chesnut, D., 1993, Monitoringan underground steam injection process using electrical resistance tomography, WaterResources Research, Vol. 29, No. 1, 73-87.Sorensen, K., 1994, Pulled array continuous electrical profiling, Proceedings of theSymposium on the Application of Geophysics to Engineering and EnvironmentalProblems, Boston, Massachusetts, 2, 977-983.Telford, W., Geldart, L., Sheriff, R. and Keys, D., 1986, Applied geophysics, New York,Cambridge University Press, 860 pp.

Tourlos, P. and Ogilvy, R., 1999, An algorithm for the 3-D inversion of tomographic resistivity and induced polarisation data: preliminary re s u l t s, Journal of the BalkanGeophysical Society, 2(2), 30-45. R

Focus Article Contd2-D Resistivity Surveying for Hydrocarbons-A PrimerContinued from Page 32

F i g u re 14. Geoelectrical exploration for shallow gas near High Level, Alberta.Telford et. al. (1986) are still occasionally correct in that logistical difficulties mayi n t e r f e re with the pro g ress of a survey.

Paul Bauman is a Professional Engineer with over 20 years of geophysical exploration

experience in the environmental, water resource, mining, oil and gas, and archaeologysectors. Paul has a B.Sc.E. in geological engineering from Princeton University, a minor inNear Eastern Studies also from Princeton, and an M.Sc. from the University of Waterloo inhydrogeology. He worked for Schlumberger International from 1981 to 1986 in Borneo andPapua New Guinea. In 1990, Paul created and has since managed the near surfacegeophysics group at Komex International Ltd. This group numbers about 12 geophysicistsand has worked on every continent in the world. Recently, Paul was asked by UNICEF tofind fresh water for the earthquake/psumami victims, and as a result he had to takeoff,cancelling his June 2005 luncheon talk.

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