Groundwater-Voyager

8/7/2019 Groundwater-Voyager

1/4Technology Today Summer 20102

W

hile spelunkingand cave divingmake or good,i somewhat

dangerous, recreationalactivities, geologists andhydrologists requentlyrely on instruments carriedby human divers to gener-ate reliable maps o underwater cavernsthrough which groundwater enters, movesabout and then exits rom karst limestoneaquiers. Inormation gathered rom thesedives is collected and analyzed, then gen-eralized to create a reasonable estimate othe size o the cave network, water-bearing capacity, ease o recharge andsensitivity to pollution or contamination.

Aside rom the hazard to humansaety, exploring aquiers and underwatercaverns in this manner is limited in somecases by sheer distance and in others bychannel segments that are too narrow toaccommodate a human diver.

Some limited inormation aboutgroundwater behavior can be gainedindirectly by means o tracers, such asdyes introduced at a recharge eature and

then tracked to the place where theyemerge at a spring or well. However,to map the actual limestone corridorsthrough which groundwater owsrequires a mechanical system that cangather, store and transmit dimensionaldata as it travels with the groundwa-ters ow. This requires a system that issmall, inexpensive, non-polluting, sae,autonomous, sel-powered and able tocommunicate electronically with receiv-ing equipment once it has emerged.

A team o hydrologists and electri-cal engineers at Southwest ResearchInstitute (SwRI) has developed aneutrally buoyant sensor to remotelycharacterize the path, dimension andmorphology o caves and other under-ground conduits and cavities. The

patent-pending systemwas developed underinternal unding, and theunits were constructedusing o-the-shel com-ponents whenever pos-sible to minimize the cos

Neutrally buoy-ant sensors, so-called

because they are designed to neither ridon the surace nor sink to the bottom,gather dimensional and directional datavia an array o ultrasound sensors relativto a compass, or, in this case, a three-axismagnetometer.

The sensor also is equipped with apropulsion system to move it through

the cave and avoid becoming hung on anobstacle or trapped in an eddy. Inorma-tion gathered during travel is collected athe conclusion o the voyage, either byretrieving the sensor and physically transerring the data or by remotely transer-ring the data to a static sensor tethered tthe ground surace close enough to allowremote communication. Spatial scale isdetermined by comparing ultrasoundmeasurements taken o a stationary objec

An SwRI-developed miniature robotsensor creates a map of submergedcaves and channels

Groundwater Voyager

By Ronald T. Green, Ph.D., and Ben Abbott, Ph.D.

Neutrally buoyant sensors are designed to

ride the current within a ooded undergroundchannel or cave and, using miniature

transmitters and receivers, gather information

about the shape and morphology of the chambe

through which they travel

R&D Magazine has recognized SwRIs neutrallybuoyant sensor technology with its R&D 100 Award,

presented annually to the 100 most signicant advancesin technology. In all, SwRI has won 35 o the awards

dating back to 1971.

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by multiple sensors at multipletimes. A sufcient number o

measurements can uniquely determinethe spatial scale and morphology o thecave interior.

Existing systems can perorm someo the unctions o the SwRI-developedsensor using a profling sonar unit or alaser-based range measurement tool,but they are constrained by relativelyhigh cost, excessive size or a need to bedeployed through a borehole. The SwRI-developed remote sensors are uniquein their ability to access small caves andconduit passageways. Their low cost alsomakes them relatively expendable. Manycan be deployed, yet a survey is a successi inormation is retrieved rom only onesensor. Finally, the resolution o mappingis superior to that typically provided bycave divers because o the richness odata collected by multiple ultrasoundsensors.

The science o cave water fow

Besides its importance to water-resource managers, knowledge owater ow through caves and conduits,and the size and shape o the voids, isalso important when karst eatures are

regime (that is, whether it is laminar orturbulent) provide additional meaningulmodel calibration targets to augment theconventional targets o hydraulic headand spring discharge. Methods such astracer tests or mapping by cave diversare o limited applicability, and attemptsto iner conduit locations using geologiceatures such as racture lineaments andsinkholes have not been encouraging.Scientists needed new tools to character-ize conduits to improve the chances o

success using karst aquier ow model-ing tools.

Developing a new sensor

The SwRI teams initial objective wasto develop and demonstrate an inex-pensive sensor designed or placementin conduits up-gradient rom springorifces, with deployment either throughsinkholes or wells that intersect karstconduits. The sensors were instrumentedto record velocity, path traveled and con-duit dimensions as they ow along. Data

would be extracted manually rom thesensors, so they had to be retrieved atthe spring orifces.

Prototypes were tested under variouslaboratory and feld settings to demon-strate and assess their capabilities. Field

located near damsor beneath roadsand buildings. In2004, SwRI scien-tists and engineersbegan an initiativeto develop toolsor enhanced char-acterization andrepresentation oow through karstaquiers. A new

MODFLOW com-puter code variant,MODFLOW-DCMV2.0, was created aspart o this project.MODFLOW-DCMmodels groundwa-

ter ow through conduits within porousmedia. Three karst aquiers have beenmodeled using MODFLOW-DCM: theBarton Springs segment o the EdwardsAquier in South Central Texas, the SantaFe River Sink/Rise system o the FloridanAquier in North Central Florida, and

the Blue Spring system o the FloridanAquier in Volusia County in NortheastFlorida.

Efcient and eective applicationo MODFLOW-DCM to karst aquiershinges on identiying conduit locationand morphologycharacterization.In particular,reducing uncer-tainty in conduitlocation andproperties suchas geometry and

size will improvethe prospect thatthe MODFLOW-DCM model willsuccessully simu-late karst aquierow regimes.Measurements ogroundwater owvelocity and ow

Dr. Ben Abbott, left, is an Institute engineer in the Communications

and Embedded Systems Department of the Automation and Data

Systems Division. He has extensive expertise in wireless sensor

network technologies and has participated in development of several

network-centric data acquisition, recording and telemetry systems.

Dr. Ronald T. Green, right, is a hydrogeologist with additional

expertise in geology and geophysics. He is an Institute scientist in the

Department of Earth, Material and Planetary Sciences within SwRIs

Geosciences and Engineering Division.

Different sizes of

prototype sensors

were evaluated,

and a number of

modications,such as ns and

propellers, were

added to improve

performance or

address deciencies

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testing was perormed at the SpringCreek Cave and Honey Creek Cave nearSan Antonio. Several sizes and versionso sensors were employed to addressvarious technological challenges encoun-tered during development and deploy-ment. Sensor sizes varied rom4 centimeters in diameter (gol ball size)to 8 cm (sotball size) and 22 cm (smallsoccer ball size). For proo-o-concept,there was no eort to miniaturize thesensor components.

Development eorts ocused ontwo unctionalities: an instrumentationpackage capable o measuring key attri-

butes o a conduit in a karst aquier; andthe ability to negotiate through the owregime o a ully saturated conduit undernatural conditions. The frst unctionalitywas straightorward. Meeting the secondunctionality, however, presented morechallenges than initially anticipated. Theprototype design evolved during theproject as the SwRI team addressed unan-ticipated challenges, such as how to keepthe sensor in the main ow channel o asemi-saturated conduit.

Instrumentation unctionality

The prototype sensors were assem-bled with commercially available compo-nents. The need to characterize conduitgeometry, ow path and ow rate led to adesign that included ultrasound sensors,dual-axis magnetometers and acceler-ometers. The magnetometers enable thesensor to gauge its pointing directionrelative to magnetic North, and the accel-erometer enables the determination o

motion dynamics as the sensor travelsthrough a conduit. The most importantaspect o the design, however, was relat-ed to the use o ultrasonic transducersto characterize conduit geometry and,ultimately, velocity.

The SwRI team decided to use sixpairs o ultrasonic transducers, equallypositioned around the sensor enclosure,to transmit sonar pings outward tothe conduit walls. Each pair consistedo a transmitter that sent out ultrasoundpulses normal to the sensor node, anda receiver that recorded the reectedultrasound pulse. Accurate distances tothe surrounding conduit eatures wouldbe determined using the time o arrivalo the reected pulses. This sonar rang-

ing would not only accurately character-ize the conduit geometry but also detectwall eatures that can be used or veloc-ity calculations. Post-processing o datarom all components enables calculationo real-time velocity along the conduitpath as well as the shape and size o theconduit.

An initial, non-submersible versiono a sensor prototype was evaluated in abuilding hallway to demonstrate that the

application was unctional in an open-airenvironment. Ater successul hallwaytests, a submersible prototype was con-structed using o-the-shel electronic

components and a printed circuit boardor the main circuitry. Six pairs o water-proo ultrasonic sensors were connectedto the board via coaxial cabling. For easeo access, the SwRI team mounted theassembly inside a 22-cm, clear-plasticball. For later deployments, a motorizedpropeller and control circuitry were ft-ted to the ball or navigating conduitterrain.

Flow dynamic unctionality

Developing the sensors ow unc-

tionality was an iterative process in whichnew eatures were added to succeed-ing generations o prototypes deployedinto Spring Creek Cave. First-generationspherical prototypes tended to oat outo the main ow channel and stall at thecave walls due to a ow vector patterncalled Poiseuilles ow. This pattern, nor-mally observed in pipelines, exhibits thegreatest ow stream in the center and noow at the walls. This initial batch o

Signals transmitted from a sensor drifting

through the ooded cave are transmitted in three

directions, and the return signals are processed

and stored for analysis following recovery of the

sensor after it emerges from the cave (right).

The resulting three-dimensional graphic (inset)

represents the size and morphology of the

cave segments interior walls as determined by

processing the sensors stored data.

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sensors traveled no more than 30 metersin two days ater being deployed.

The SwRI team attached fns to thesensors to avoid this stalling tendencyand added bottom weights to maintaina constant attitude and prevent rotation.These proved eective in reducing thetendency to rotate out o the ow feld.

Meanwhile, the importance o neu-tral buoyancy became apparent as sen-sors that oated on the surace tendedto snag on stalactites and other caveeatures and those that sank to the caveoor departed rom the ow channeland stopped moving. In response, theSwRI team attached the propeller mecha-nism. Set at an angle near the bottom oeach sensor shell, the propeller was pro-grammed to engage at preset intervalsto frst provide orward rotation to drivethe sensor downward, then reverse rota-tion to drive the sensor upward to avoidbecoming embedded in mud or silt onthe cave oor.

Three o the propeller-equippedsensors were deployed at various loca-tions near the cave mouth to observewhether the new unctionality wouldenable them to navigate past restrictionsin the ow regime. They perormed wellenough to collect ultrasound, magne-tometer and accelerometer data or asegment o the cave.

Data analysis

Data rom these prototypes werecollected to ascertain the ability to

remotely characterize a wet cave. Sonarand magnetometer data proved moreuseul than accelerometer data. The frststep in reducing sonar data was to calcu-late the sensor velocity along the conduitow path. This consisted o cross-corre-lating the ront and rear sonar signals onboth sides o the sensor to determine therelative sample delays between detectedeatures. A correlation was made or eachsample over time windows. Centering

the delay window around each sample,the distance window was calculatedusing the ront sonar reading or the frstsample o the window and the rear sonarreading or the last sample o the window.

Dividing the distance by the delay,the velocity at each sample was deter-mined. The frst and last windows o thesample were smoothed to the averagevelocity o the nearest known sample.For each sample, the ront and rear sonarreadings were used to calculate the nor-mal distances to each side o the conduit.The top and bottom samples providedthe distances to the ceiling and oor othe conduit, respectively. Preliminary dis-tances were converted to fnal distancesusing calibrated values derived as multi-plication actors to correlate water traveltimes to distance. Assuming a smoothtransition arch around theconduit, additional distanceswere interpolated around theconduit geometry.

Future applications

Future generations o sen-sors might be equipped withadditional instrumentation tocollect environmental data,such as temperature, gas com-position and water chemistry.They also could be miniatur-ized to a diameter as small as2 cm to 5 cm. Meanwhile, thepropulsion system might bereplaced with a more sophisti-

cated buoyancy system that would acti-vate only as needed.

Neutrally buoyant sensors could beused in applications other than caves,such as pipes that are limited in diameteror whose interior size has been reduceddue to sediment deposition or corrosion;sanitary sewers in older cities whereaccurate maps and records are not avail-able and whose condition precludes saehuman access; or geotechnical settings,such as mines or conduits that are notsae or manned entry. v

Questions about this story?Contact Green at (210) 522-5305or [email protected], or Abbott at(210) 522-2802 or [email protected].

ReerencesAlexander, E.C., Jr. and J.F. Quinlan. 1992. Practical Tracing o

Groundwater, with Emphasis on Karst Terrains. Geological Society

of America, Boulder, Colorado. 2 Vol. pp. 195 & 133.

Ford, D.C. and P.W. Williams. 1993. Karst Geomorphology and

Hydrology. Chapman and Hall. New York, NY. 601 p.

Painter, S.L., A. Sun, and R.T. Green. 2006. Enhanced Charac

terization and Representation of Flow through Karst Aquifers. Final

Report. AWWA Research Foundation. Project 2987.Painter, S.L., A. Sun, and R.T. Green. 2007. Enhanced Char-

acterization and Representation of Flow Through Karst Aquifers

Phase II. Final Project Report to the Edwards Aquifer Authority and

the Southwest Florida Water Management District. P. 101.

Quinlan, J.F., and R.O. Ewers. 1989. Subsurface Drainage

in the Mammoth Cave Area, in White, W.B., and White, E.L., eds.,

Karst Hydrology: Concepts from the Mammoth Cave Area: New

York, Van Nostrand Reinhold, pp. 65103.

Smart., P.L. and I.M.S. Laidlaw. 1977. An Evaluation of Some

Fluorescent Dyes for Water Tracing. Water Resources Research. Vo

13. pp. 15-33.

Using internal funding,

SwRI staff members

developed a wireless-

sensor-based system to

map and characterize

water-lled cave passages

using neutrally buoyant

wireless sensors. Drifting

through the passagesand using internal

propulsion systems to

navigate around obstacles

the sensors autonomously

map the pathway, ow

velocity and dimensions

of these important

groundwater conduits to

improve management of

karst aquifers.

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Groundwater-Voyager