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Active mud volcanism observed with Landsat 7 ETM+

Matthew Patrick �, Kenneson Dean, Jonathan DehnAlaska Volcano Observatory, Geophysical Institute, University of Alaska Fairbanks, 903 Koyukuk Dr., P.O. Box 757320,

Fairbanks, AK 99775-7320, USA

Received 12 December 2002; accepted 17 October 2003

Abstract

Mud volcanoes are relatively small spatter cones that erupt water-laden mud and gases, and occur throughout theworld. For many mud volcanoes, the eruption of warm mud (10^40‡C) can be detected with high-resolution thermalsatellite imagery. We demonstrate the utility of Landsat 7 Enhanced Thematic Mapper Plus (ETM+) imagery forthermal monitoring of active mud volcanism. We constrain the temperature and area of active mud discharge andestimate surface heat flux for two isolated mud volcanoes in the Copper River Basin, Alaska using Band 6 (10.4^12.5Wm). The heat flux results span a wide range due to uncertainties in the environmental conditions at the time of imageacquisition, but can be constrained to be less than 0.24 MW for each of the two mud volcanoes considering previouslypublished field measurements. With this higher-resolution Band 6 on the ETM+ sensor, as well as the high-resolutionthermal bands on the ASTER sensor, reliable monitoring of mud volcanism on this scale is possible for the first time.: 2003 Elsevier B.V. All rights reserved.

Keywords: mud volcano; satellite monitoring; Klawasi; Alaska; Landsat; ASTER

1. Introduction

The bubbling mud pits of Yellowstone and Ice-land are vivid testimony to the mobility of soiland water surrounding shallow plutons. In somelocales this activity manifests itself in a phenom-enon called mud volcanism, which is character-ized by spatter cones, often meters to tens of me-ters high, erupting water-laden mud and gases.Southeastern Alaska is home to a number of ac-tive mud volcanoes, including three on the lowerwest £ank of Mt. Drum, a Pleistocene stratovol-

cano. For the Drum mud volcanoes, and otherslike them, the erupted material is tens of degreesCelsius above background temperature and theactive mud discharge areas are tens of metersacross. These qualities permit measurement ofthe mud discharge regions by high-resolutionthermal sensors such as the Enhanced ThematicMapper Plus (ETM+) on Landsat 7 and the Ad-vanced Spaceborne Thermal Emission and Re£ec-tion Radiometer (ASTER) sensor on the Terraspacecraft. In this study, the Drum mud volca-noes are analyzed using a Landsat 7 ETM+ imagein order to demonstrate the e⁄cacy of such datafor monitoring this unique type of activity. Weestimate temperature and area of the activemud, as well as constrain heat £ux, using a two-component model of the pixel-integrated radi-

0377-0273 / 03 / $ ^ see front matter : 2003 Elsevier B.V. All rights reserved.doi:10.1016/S0377-0273(03)00383-4

* Corresponding author. Present address: HIGP/SOEST,University of Hawaii Manoa, 1680 East-West Road, Honolu-lu, HI 96822, USA.

E-mail address: [email protected] (M. Patrick).

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Journal of Volcanology and Geothermal Research 131 (2004) 307^320

R

Available online at www.sciencedirect.com

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ance. The accuracy of our satellite-based tech-nique is evaluated by comparing our results tothe range of values measured in the ¢eld overthe last 40 years.

Many active mud volcanoes on Earth are situ-ated in benign and restricted settings such as na-tional parks, but a small number are located peril-ously close to population centers and couldpotentially pose some risk to their inhabitants.This fact was demonstrated on February 22,1997 in south-central Trinidad, when PiparoMud Volcano violently erupted more than25 000 m3 of mud into the nearby village, withmud fountaining up to 100 m. Although no hu-man deaths or injuries occurred, over 100 peoplewere displaced with 50 of them losing theirhomes, dozens of farm animals were killed and15 acres of crop land were damaged (CaribbeanDisaster Emergency Response Agency, 1997). An-other violent mud volcano eruption occurred justtwo miles north of Lake City, California in 1951in what may be the largest mud eruption eventever recorded. More than 150 000 m3 of mudwere erupted in £ows averaging 4 m in thickness

(White, 1955). Mud bombs were erupted to al-most 100 m in height and accretionary mud lapilliwere sent over 6 km from the springs. These erup-tions demonstrate that a limited number of mudvolcanoes do have the capacity to endanger livesand property and may warrant monitoring.

2. Background

According to Grantz et al. (1962) the existenceof mud volcanoes, similarly referred to as salineor mineral springs, in the Copper River Basin ofsouth-central Alaska was ¢rst reported by Allen(1887) and Schrader (1900). Two groups of mudvolcanoes are located in this region: the Drumand Tolsona groups (Fig. 1). The Drum groupis isolated, and genetically distinct, from the Tol-sona group of mud volcanoes which is approxi-mately 55 km west. The Drum group emits warmNaHCO2-rich saline mud and predominantly car-bon dioxide gas and is believed to be linked tosubsurface heating of carbonate sediments by ashallow magma body (Wescott and Turner,

Fig. 1. Location map of Drum mud volcano region, adapted from Sorey et al. (2000). The Drum mud volcanoes (Shrub, UpperKlawasi and Lower Klawasi) reside on the western £ank of Mt. Drum, east of the Copper River and the towns of Glennallenand Copper Center. The Tolsona and Nickel Creek mud volcanoes are situated near the Tazlina River, approximately 50 kmwest of the Drum mud volcanoes.

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M. Patrick et al. / Journal of Volcanology and Geothermal Research 131 (2004) 307^320308

1985; Motyka et al., 1986, 1989), while the Tol-sona group (as well as Nickel Creek mud volcano)erupts cold Na-Ca-rich mud and mostly methanegas and is probably the surface manifestation ofhydrocarbon degassing at depth (Reitsema, 1979).These two locales are representative of the dichot-omous origins of subaerial mud volcanism world-wide: those resulting from magmatic heating andthose from a hydrocarbon source. Mud volcanoeswith plutonic origins can be found in locales in-

cluding Yellowstone (Sheppard et al., 1992), Cal-ifornia and Nevada (White, 1955), and Italy(Chiodini et al., 1996), and those from hydrocar-bon alteration in Central and South America (Ar-nold and Macready, 1956; Humphrey, 1963;Aslan et al., 2001), Azerbaijan (Hovland et al.,1997), New Zealand (Ridd, 1970) and Japan (Chi-gira and Tanaka, 1997). Hydrocarbon mud volca-nism usually results in only weakly heated mud(except in cases where the erupting hydrocarbon-

Fig. 2. Aerial photographs of the active mud pools of Upper Klawasi (top; looking south) and Lower Klawasi (bottom; lookingwest). The Upper Klawasi mud pool is approximately 45 m in diameter, and the Lower Klawasi mud pool is approximately 53m in diameter. Mud upwelling originates at the center of the pools and the £uid is usually con¢ned to the crater, though espe-cially vigorous and sustained outbursts result in mud issuing down the £anks. Photos by R. McGimsey, AVO-USGS, July 1998.

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M. Patrick et al. / Journal of Volcanology and Geothermal Research 131 (2004) 307^320 309

rich mud ignites, as happens in Azerbaijan), andis not amenable to thermal satellite observation.This study focuses on the thermal analyses ofmud volcanoes, and is relevant only to thosewhich erupt signi¢cantly heated mud.

The mud volcanoes studied here include themembers of the Drum group, which includeUpper and Lower Klawasi and Shrub. Upperand Lower Klawasi mud volcanoes measure 90and 45 m in height above the surrounding topog-raphy, have basal diameters of about 1.7 and 2.1km, and summit crater diameters of 45 and 53 m,respectively (Nichols and Yehle, 1961). Both havebeen continuously active since at least the 1950s,with active mud upwelling con¢ned to circularpools within well-de¢ned central craters (Fig. 2;Motyka et al., 1986). In 1999, mud upwelling wasobserved to occur every 20^25 min near the centerof the mud pool (Sorey et al., 2000). Comparisonof a 1938 aerial photo (B. Washburn, in Decaneasand Washburn, 2000, neg. 1785) of Lower Klawa-si with recent ground photographs (R. McGim-sey, personal communication) shows remarkablylittle change in the vicinity of the mud pool over60 years, suggesting that at least Lower Klawasimay be in an extended period of continuous ac-tivity dating back before the 1950s. The Shrubedi¢ce is approximately 95 m above the surround-ing terrain, and erupts mud out of numerous, iso-lated small pools (6 5 m diameter). Largely inac-tive since the 1950s, Shrub’s quiescence abruptlyended with a vigorous eruption in the spring of1997 that continued through 2000 (Sorey et al.,2000). Mud was erupted to a height of 5 m, andlarge amounts of carbon dioxide poured down the£anks killing nearby vegetation and animal life(Richter et al., 1998).

3. Thermal considerations

A Landsat 7 ETM+ image over the Wrangellarea (Path: 65; Row: 17; ID: 7065017000027450)was acquired and analyzed. The acquisition dateof September 30, 2000 is bene¢cial for our studydue to lower background temperatures increasingthe thermal contrast of the hot mud while stilllacking deep snow that would a¡ect spectral ob-

servations. The ETM+ sensor records eight spec-tral bands, ranging from visible to thermal infra-red (Table 1). Band 6 data were used to analyzeheat £ux, as this band’s wavelength range (10.4^12.5 Wm) encompasses the wavelengths of peakemittance for objects in the temperature rangebetween 341 and 5‡C, according to Wein’s dis-placement law, and is the most appropriate bandfor observing typical mud temperatures (10^40‡C). The next closest band in the spectrum isBand 7 (2.08^2.35 Wm). Although it has the bene-¢t of a smaller pixel size (30 m), its wavelength issensitive only to higher-temperature thermal emit-tance, above about 150‡C (Rothery et al., 1988).The ETM+ on Landsat 7 includes a higher-reso-lution Band 6 (V60 m pixel size), improving onthe Thematic Mapper (Landsats 4 and 5) Band 6pixel size of V120 m. As will be shown, this high-er-resolution thermal IR sensor on ETM+ a¡ordsus the ¢rst opportunity for reliable thermal mon-itoring of mud volcano temperature regimes onthis scale.

The Band 6 data contain anomalously hightemperature pixels (thermal anomalies) at thesummits of Lower and Upper Klawasi mud vol-canoes (Fig. 3), but show nothing anomalous atShrub. Upper and Lower Klawasi are more orless continuously active and have a single, largepool of heated mud. Shrub issues mud from nu-merous small pools distributed around the edi¢cethat are often obscured by thick overlying vegeta-tion (R. McGimsey, personal communication).Although the Shrub mud has been measured tobe somewhat warmer than the Klawasi muds, thedistribution and small pool sizes may make ther-

Table 1Landsat 7 ETM+ spectral bands

Band Wavelength Pixel size EM region(Wm) (m)

1 0.45^0.52 30 Visible2 0.52^0.60 30 Visible3 0.63^0.69 30 Visible4 0.76^0.90 30 Near IR5 1.55^1.75 30 SWIR6 10.4^12.5 60 Mid IR7 2.08^2.35 30 SWIR8 0.50^0.90 15 Visible

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mal detection of Shrub mud activity more di⁄cultand explain the lack of a clear thermal anomaly inthis particular image. The thermal anomalies aretwo and three pixels in size at Upper and LowerKlawasi mud volcanoes, respectively. Coregistra-tion of the Band 6 image with the 15 m pixel sizepanchromatic image (Band 8) indicates that theseelevated pixels are centered around the summitcraters, where the active mud pools are located(Figs. 2 and 3). Pixel-integrated temperatures hov-er just a few degrees above background, makingidenti¢cation somewhat challenging.

To extract thermal information the image datavalues were converted to temperature after adjust-

ing for atmospheric and emissivity e¡ects. Theimage digital numbers were converted to at-satel-lite radiance using the date-appropriate calibra-tion parameters ¢le provided by the EROS datacenter (http://edcwww.cr.usgs.gov/). Pixel-inte-grated ground surface radiance was determinedafter correcting the at-satellite radiance for atmo-spheric transmissivity, upwelling atmospheric ra-diation, and surface emissivity. Transmissivitywas calculated using MODTRAN4 (Berk et al.,1999) with a subarctic winter atmosphere model,and was approximately 0.96 for Band 6. Upwell-ing atmospheric radiance was also calculated us-ing MODTRAN4, and the average value between

Fig. 3. Close-ups of the September 30, 2000 Landsat 7 ETM+ image showing Lower Klawasi and Upper Klawasi mud volca-noes. On the left are the Band 8 panchromatic views (V15 m pixel size) of Lower Klawasi (top) and Upper Klawasi (bottom).On the right are the Band 6 thermal images (V60 m pixel size) of the equivalent region (white is warm). Note the Band 6 im-agery has been signi¢cantly enhanced to show contrast.

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edcwww.cr.usgs.gov/

10.4 and 12.5 Wm was about 0.2 W/m2/str/Wm,making it responsible for a change of about 2‡Cat 0‡C. An average mud emissivity in the 10.4^12.5-Wm window was assumed based upon there£ectance data in Salisbury and D’Aria (1992a).Emissivity was garnered from re£ectance by as-suming Kircho¡’s Law, in which emissivity isthe complement of re£ectance (Oppenheimer,1998). Modeling the erupted mud as a mixtureof soil and water, one can restrict the possibleemissivities to a narrow range of values. Mostsoils have an emissivity between 0.97 and 0.99,while pure water is close to 0.99, so we mightassume an emissivity close to 0.98. Saturation ofthe soil (typically between 9 and 14 wt% water),however, results in emissivity being e¡ectivelyidentical to that of water (Salisbury and D’Aria,1992a,b). Since the mud observed at the Klawasimud volcanoes is very much water-laden, a mudemissivity of 0.99 was assumed. As the areaaround the mud volcanoes is thickly covered inpines, the background emissivity of conifers, mea-sured by Salisbury and D’Aria (1992a) at 0.98,was used. Finally, pixel-integrated ground temper-ature was calculated by inserting the ground sur-face radiance into the Planck function.

Errors in radiance were introduced from thee¡ects of the positioning of the pixel footprints,also referred to as the instantaneous ¢eld of view(IFOV). Problems originate from two aspects.First, the commonly accepted IFOV of 60U60m is only an approximation of the actual spatialresponse. For the TM sensor, whose Band 6 hasan accepted IFOV of 120U120 m, Markham(1985) used prelaunch speci¢cations to determinean e¡ective IFOV of 124 m (along track)U141 m(cross track). This disparity might also be ex-pected for Band 6 on ETM+, and would directlya¡ect radiance calculations. For example, mea-surement areas would overlap between adjacentpixels if the e¡ective IFOV exceeds the groundsample distance. For lack of a re¢ned value forthe e¡ective IFOV of Band 6 on ETM+, however,we defer to the accepted value. Second, geo-graphic resampling may also a¡ect the results byconvolving pixel radiance values. Nearest neigh-bor resampling is the preferred method as itavoids alteration of the parent radiance values

(but changes relative locations and occasionallyrepeats pixels), whereas cubic convolution canmanipulate the data values. Due to availabilitycircumstances, however, we were limited to a cu-bic convolution resampled image. It is importantto recognize, therefore, this potential limitation inthe data.

3.1. Area of active mud discharge using assumedtemperature

In order to determine the area of active muddischarge, the total radiance of each anomalouspixel was modeled as the area-weighted sum oftwo components: hot mud and cooler back-ground. The radiance equation is as follows:

R6 ¼ pL6ðThÞ þ ð13pÞL6ðTbÞ

in which R6 is the Band 6 pixel-integrated radi-ance, p is the fractional area of mud in the pixel,L6(Th) is the radiance of the hot mud in Band 6,and L6(Tb) is the radiance of the cool backgroundin Band 6. In order to solve for p, we assumedboth the temperature of the hot mud (Th) andthat of the background (Tb). The backgroundtemperature is estimated from pixels away fromthe hot area, which ranged over approximately2‡C. For volcanic hot targets, usually hundredsof degrees Celsius over background, this minorvariance would not be an issue. With the mudbeing just tens of degrees above background,however, this uncertainty in the background issigni¢cant and the calculations were performedover the range of observed background temper-atures. The two-component radiance equationwas then solved for area using the range of mudtemperatures measured in the ¢eld during the last40 years. For the Klawasi mud volcanoes, themud temperature ranged between 13 and 31‡Cbased on measurements taken between 1961 and1999 (Nichols and Yehle, 1961; Motyka et al.,1986; Sorey et al., 2000). Using the IFOV areaof 3600 m2 (60 mU60 m), the upper and lowerbounds for mud discharge area at each assumedtemperature were calculated (Fig. 4). The UpperKlawasi mud discharge area was calculated to bebetween 293 and 1570 m2 (Fig. 4A) correspondingto maximum and minimum ¢eld-measured mud

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temperatures of 31 and 13‡C, respectively, whichequates to a circular area between 19.3 and 44.7m in diameter (Fig. 4B). The mud discharge areasolutions are inversely related to the assumed mudtemperature as higher-temperature mud requiresless area than lower-temperature mud to producethe observed radiance. These dimensions are gen-erally within the bounds of the measured summitcrater diameter of 45 m (Nichols and Yehle,1961). An active mud discharge area smallerthan the crater diameter is entirely consistentwith ¢eld observations, in which the active mudroiling was observed to cover approximately halfthe area of the crater in 1999, surrounded by

£oating organic debris (Sorey et al., 2000). ForLower Klawasi the mud discharge area rangesbetween 1110 and 3520 m2 (Fig. 4A), again cor-responding to the maximum and minimum ¢eld-measured mud temperatures of 31 and 13‡C, re-spectively. This equates to a circular area between38 and 67 m in diameter (Fig. 4B), and is alsoconsistent with the measured summit crater diam-eter of 53 m. Lower temperatures result in areaswhich are slightly larger than the measured craterdiameter, and this situation is still realistic sinceLower Klawasi contains a large break in its craterwall through which mud issues down the £anks.

3.2. Temperature of mud using assumed area

If the area of active mud discharge is known orassumed and the temperature is unknown, a sim-ilar but reversed solution can be applied. In thiscase, the temperature of the hot mud can besolved for using the measured total area of theactive summit crater as a constraint. Since thearea of active mud discharge may not necessarilycompletely ¢ll the crater, temperature was solvedfor using a range of assumed fractional areas.Again, a two-component model of the surfacewas used and applied to the radiance equation,but with this reverse approach the radiance ofthe anomaly as a whole must be used instead ofa pixel-by-pixel approach as was done with solv-ing for fractional area. This di¡erent approachwas necessary because while the temperature ofthe hot component was assumed to be constantand uniform among the anomalous pixels, thefractional area will di¡er for each pixel, requiringus to simulate a larger pixel to encompass theentire crater area in order to solve the radianceequation. With this method, demonstrated byHarris et al. (1999) with lava lakes, the radianceover the entire anomaly area is integrated accord-ing to:

Rint ¼1n

Xn

i¼1

Ri

where Rint is the integrated anomaly radiance, n isthe number of pixels in the anomaly, and Ri is theradiance of an individual pixel in the anomaly.This technique has the bene¢t of encompassing

Fig. 4. Results from thermal calculations for Upper andLower Klawasi mud volcanoes. The range of possible mudareas (A) and the corresponding diameters (B) for a range ofassumed mud temperatures.

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the entire hot source, so that where the total areaof the hot target is known (as in the case of a lavalake or a mud crater) this area can be used tobetter constrain the other unknowns. A majordrawback of merging pixels is that it presents thepossibility of ‘double-counting’ if IFOVs overlap.

3.3. Heat £ux calculations

From this temperature and area informationthe total surface heat loss from the mud poolswas estimated. Numerous studies have addressedheat loss from water bodies (e.g. Weisman andBrutsaert, 1973; Ryan et al., 1974; Sill, 1983;Adams et al., 1990) with a few addressing vol-canic crater lakes (Hurst and Dibble, 1981; Ste-venson, 1992; Oppenheimer, 1993, 1996, 1997).Following Ohba et al. (1994) and Oppenheimer(1996), the total heat budget for a body of watercan be expressed by:

x total ¼ x ppt þx vol þx seep þx evap þx sensþ

x rad þx sun þx sky

where xppt, xvol, xseep, xevap, xsens, xrad, xsun,xsky are the energy £uxes (in Watts) from mete-oric in£ow, magmatic/hydrothermal £uids, seep-age, evaporation, conduction of sensible heatinto the atmosphere, emitted radiation, incomingsolar radiation, and incoming radiation from theatmosphere, respectively (with terms on the rightbeing positive if heat is removed). For this study,xvol would represent the heat carried in by the

mud. And for simplicity, meteoric water in£uxand seepage are assumed to be negligible com-pared with the mud in£ux, thereby removing thexppt and xseep terms. The outgoing net heat £uxfrom the surface of the mud pools is then:

x surf ¼ x evap þx sens þx rad þx sun þx sky

For the bene¢t of comparison with other stud-ies, these total heat £uxes are calculated in termsof their heat £ux density (q), in units of W/m2.The total surface heat £ux (x) is garnered simplyby multiplying the heat £ux density by the totalarea of the mud pool. Note that the surface heat£ux does not encompass heat conducted into theedi¢ce, which may also be a signi¢cant heat lossmechanism.

Both evaporation and sensible heat loss arecontrolled by the convection of air above themud pool surface, and this may be either free(buoyancy-driven) or forced (wind-driven) (Ryanet al., 1974; Adams et al., 1990). Evaporative andsensible heat losses, under either free or forcedconvective conditions, were calculated using theformula of Ryan et al. (1974):

qevap þ qcond ¼

½V ðT sv3TavÞ1=3 þ boW 2�½es3e2 þ CðT s3TaÞ�

where the variables are listed in Table 2.Outgoing radiation is computed easily from the

Stefan-Boltzmann equation:

qrad ¼ OcT4s

Table 2Evaporative and sensible heat £ux variables from Ryan et al. (1974)

Variable Value Units Description

V 2.7 W m32 mbar31 (‡C)31=3 constantbo 3.2 W m32 mbar31 (m/s)31 constantW2 0^3 m/s windspeed at 2 m heightes mbar vapor pressure of water at Ts

e2 mbar vapor pressure of water at 2 m heightC 0.61 mbar (‡C)31 constantTs 13^31 ‡C water surface temperatureTa 0 ‡C air temperatureTsv ‡C virtual water surface temperatureTav ‡C virtual air temperatureTv ‡C T/(130.78e/P), where P is atmospheric pressureP 900 mbar atmospheric pressure

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where O is emissivity, c is the Stefan-Boltzmannconstant (5.67051U1038 W/m2/K4), and Ts is thewater surface temperature. Incoming atmosphericlong-wave radiation (qsky) can also be calculatedin this way, using the assumed background tem-perature for air temperature and assuming abroadband atmospheric emissivity of 0.7, whichis an average based upon the analysis by Hender-son-Sellers (1986) of several methods (e.g. Swin-bank, 1963; Satterland, 1979; Idso, 1981) avail-able to calculate this parameter.

The incoming solar (qsun) short-wave irradia-tion at the surface was calculated using MOD-TRAN4, considering the solar zenith angle(65.3‡) found in the acquisition parameters ofthe Landsat image. MODTRAN4 accounts forshort-wave absorption in the atmosphere due toclouds (not a factor in this clear scene), aerosols,water vapor and ozone. To calculate solar irradi-ance at the ground surface, the atmospheric ab-sorption factor is multiplied by the solar irradi-ance at the top of the atmosphere, which is thesolar constant at approximately 1368 W/m2 (Will-son and Hudson, 1991). Using MODTRAN4with a subarctic winter model atmosphere and adefault rural aerosol model with 23 km visibility,the solar irradiance at the ground surface is esti-mated at 656 W/m2 for surfaces normal to thesun’s rays. This equates to an atmospheric reduc-tion by about 52%. For our particular solar zenithangle, this irradiance spread over a horizontal sur-face was calculated by:

656 W=m2 cosð65:3�Þ ¼ 274 W=m2

It should be stated that the model results arehighly sensitive to the aerosol model used; for arural aerosol model with 5 km visibility, the solarirradiance over a horizontal ground surface is just111 W/m2.

The results of these equations are shown in Fig.5, for the range of previously assumed tempera-tures and calculated areas. The upper and lowerbounds of the results for each temperature areshown, due to the variation in background tem-perature. The heat £ux density is shown in Fig.5A for windspeeds of 0 and 3 m/s, and the totalabove-background surface heat £ux is shown inFig. 5B (wind= 0 m/s) and Fig. 5C (wind= 3 m/s).

Fig. 5. Heat £ux density (A) and corresponding total heat£ux, for windspeeds of 0 m/s (B) and 3 m/s (C) from Klawa-si mud pools. The lines show the respective upper and lowerbounds for the calculations, a result of solving over a rangeof background temperatures.

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For Lower Klawasi, the net surface heat loss forthe mud pool ranges between 31.04U105 and6.94U105 W for a windspeed of 0 m/s and3.34U105 and 14.8U105 W for a windspeed of3 m/s. The negative values simply indicate thatthe particular assumed mud temperature is insuf-¢cient to provide heat above the atmospheric andsolar radiation, and these are not realistic solu-tions as the thermal anomaly itself indicates asurplus of heat being provided to the environ-ment. For Upper Klawasi, the net surface heatloss for the mud pool ranges between 0.03U105

and 3.74 U105 W for a windspeed of 0 m/s and1.25U105 and 7.77U105 W for a windspeed of3 m/s.

These values can be put into context by consid-ering the total heat potential of the erupting mud.Measured eruption rates at Upper and LowerKlawasi range between 8 and 110 l/min (Table3), and if the speci¢c heat of water is assumedthe total heat supplied to the surface by themud enthalpy can be calculated. These valuesrange between 7.07U103 W (for 8 l/min at13‡C) and 2.37 U105 W (for 110 l/min at 31‡C),and represent an upper limit to the surface heat£ux (Fig. 5A,B) as some of this mud enthalpy iscertainly conducted into the edi¢ce. For a wind-speed of 0 m/s, there is generally good overlapbetween the heat £ux solutions and the maximumheat £ux derived from the ¢eld-measured eruption

rates. Possible Upper Klawasi heat £ux valuesspan the entire range of assumed temperatureswhile Lower Klawasi solutions are limited to anarrow range between about 15 and 17‡C. Heat£ux results for an assumed windspeed of 3 m/s areseveral times higher than the maximum heat £uxderived from ¢eld measurements, suggesting theselevels of heat £ux are unrealistic. The broad rangeof results for the surface heat £ux estimates is adirect function of the uncertainty in most of theinput variables. With the exception of the summitcrater dimensions, the uncertainty is high fornearly all the variables due to the paucity ofknowledge both on the mud volcanoes themselvesand on the local conditions at the time of imageacquisition. Error may also be introduced by theimage resampling method and IFOV overlap, de-scribed earlier. This attempt at estimating heat£ux serves more to present a viable method thanto predict values suitable for comparison with¢eld-based numbers. Hopefully, future work onthe Klawasi mud volcanoes will establish moreaccurate values for these properties, and providean opportunity to validate or re¢ne the approachpresented here.

3.4. Detector sensitivities

Detection of Klawasi mud activity using space-borne sensors is challenging and possible only

Table 3Field observations of Klawasi mud volcanoes

Temperature Discharge rate Reference(C) (l/min)

Upper Klawasi1954 31 Nichols and Yehle (1961)1960 31 8^19 Nichols and Yehle (1961)1981 13 Motyka et al. (1986)1982 17 110 Motyka et al. (1986)1985 19 Motyka et al. (1986)1998 29^31 Sorey et al. (2000)1999 23^26 Sorey et al. (2000)

Lower Klawasi1956 28 Nichols and Yehle (1961)1960 20^22 19^38 Nichols and Yehle (1961)1981 20 Motyka et al. (1986)1982 20 110 Motyka et al. (1986)1985 22 Motyka et al. (1986)

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with high spatial resolution instruments due to therelatively low temperature contrast and typicalsmall areas of active mud upwelling. In fact, theprecursors to Landsat 7 ETM+ were either inca-pable or only minimally capable of thermal detec-tion of mud volcanism on this scale. Advancedvery high-resolution radiometer (AVHRR) datahave proven very useful in the analysis of silicatevolcanism because of the high temporal resolution(up to eight images daily over the Drum mudvolcanoes). Unfortunately, the large IFOV size(about 1.1 km2 at nadir) renders the AVHRRentirely incapable of detecting mud volcanismon this scale. We calculate the NOAA-12AVHRR Band 4 (10.5^11.5 Wm) IFOV area usingan IFOV size of 1.41 milliradians (Kidwell, 1991),an elliptical footprint, and the simple geometricapproach of Cahoon et al. (1992) for calculationof IFOV dimension as a function of scan angle(0^55.4‡) on a curved Earth. Assuming an ellipti-cal IFOV footprint and mud temperature of 31‡Cat 1172 m2 (from Lower Klawasi calculation), abackground temperature of 0‡C, as well as assum-ing that the hot mud discharge area is entirelywithin the pixel, we calculate that the Klawasimud would raise the AVHRR Band 4 pixel-inte-grated temperature just 0.031‡C at nadir and0.0027‡C at a scan angle of 55‡. Both of thesevalues are well below visible detection limits, donot exceed the noise-equivalent vT of 6 0.12 K,and even elude the 10-bit quantization scheme ofthe AVHRR data (i.e. there would be no digitizedresponse from the sensor).

The TM sensor aboard Landsats 4 and 5 car-ried a Band 6 detector with V120-m pixel size.Adopting the same assumptions for the mud tem-perature and extent as with the AVHRR cal-culation above, a TM Band 6 pixel-integratedtemperature would be raised only 2.9‡C. This cor-responds to about 5 digital numbers (DNs), orimage grey values, and is close to the thresholdof visible detection considering the possible vari-ability in background DNs. In addition, if anypart of the hot area falls on a pixel boundarythe e¡ect would likely fall below the detectionlimit.

The V60-m pixel size Band 6 sensor on theLandsat 7 ETM+ marks the ¢rst instance in

which we can reliably perform thermal monitor-ing of Klawasi mud volcanism. Using the sameapproach as with the previous calculations of de-tector response for the assumed mud temperatureand area, and again assuming the hot mud is en-tirely contained within the IFOV, we calculatethat such a mud area would result in a pixel-in-tegrated temperature increase of about 11‡C, orabout 33 DNs for the high-gain Band 6 (B6H orB62). Such a di¡erence should be easily detected,and ensures that even in instances where the tar-get is located at a pixel border it would still besomewhat visible.

The ASTER sensor aboard the Terra platform,launched 3 months after Landsat 7, also providesthe possibility of viable thermal detection of mudvolcanism. With the thermal IR bands having apixel size of V90 m and 8-bit quantization, visi-ble detection of the assumed mud area should bepossible. Preliminary examination of an ASTERimage over the Copper River Basin acquired onApril 14, 2001 shows elevated temperatures at thesummits of Upper Klawasi and Shrub mud volca-noes (Lower Klawasi was obscured by clouds;

Fig. 6. ASTER Band 13 image (10.2^10.95 Wm) over Shruband Upper Klawasi mud volcanoes, taken on April 14, 2001.Shrub seems to have two elevated pixels near its summit.Upper Klawasi has one distinct elevated pixel directly overthe summit mud pool.

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Fig. 6). Shrub activity may stand out more clearlyin this ASTER image, as opposed to the Landsat7 ETM+ image where it was absent, for severalreasons. The ASTER scene is a nighttime acqui-sition, and without the e¡ects of solar heatingwarm areas may stand out more easily. Also,the ASTER scene was acquired in April whendeep snow cover was undoubtedly present. Anywarm mud periodically or continuously £owingdown the £anks would keep its immediate areasnow-free, and therefore an anomaly may bemore of a consequence of the mud exposing theground than a direct observation of the mud it-self. Finally, the viewing angle or mud activitymay have been more favorable in the ASTERimage.

4. Monitoring and future research

This study is meant to demonstrate some of thetechniques that might be useful for monitoringmud volcanism, including estimating the rangeof active mud temperatures and areas as well assurface heat £ux. In practice, for an operationalsetting it may be su⁄cient to simply record andtrack above-background radiance, or equivalentpixel-integrated temperatures, for pixels whichare elevated above background by a certainthreshold in the vicinity of the mud volcano sum-mits. Understanding how this radiant heat £ux isa¡ected by the dynamics of the mud would cer-tainly improve interpretation of the thermal im-agery, and this mud behavior can be better estab-lished with further ¢eld work. For instance, mudover£ow events from the central pools could pro-duce transients in emitted radiance due to the in-crease in surface area of mud. Furthermore therewill be issues relating to signi¢cant short-term£uctuations in emitted radiance due to the timingof the satellite pass relative to the periodic mudupwelling events. As this study examined just asingle image it can only provide conjecture onoperational monitoring approaches, and establish-ing how best to accomplish mud volcano monitor-ing from space will require a robust image setagainst which these techniques and ideas can betested.

Estimating eruption rates from space is natu-rally an enticing possibility. As suggested earlier,equating the surface heat £ux directly to the muderuption rate is not possible because a large por-tion of the mud enthalpy is probably lost throughconduction into the mud volcano edi¢ce. Thisconduction will depend greatly on whether theupwelling mud remains circulating in the mudpool, where it would be highest, or spills outover the £anks and loses most of its heat throughradiation, evaporation and convection. In this lat-ter case, conduction would be at a minimum. Iffuture ¢eld studies can constrain a reasonable val-ue for conductive heat loss to the edi¢ce, thenestimates of eruption rate may be possible fromsatellite or hand-held infrared cameras throughmeasurement of the surface heat £ux. Alterna-tively, if the conduction rate is assumed to berelatively constant or very slowly changing, then£uctuations in surface heat £ux will be propor-tional to changes in eruption rate and can beused as a proxy. The mud eruption rate is partic-ularly important here because it controls the mass£ux of CO2, which is a major hazard for thesetypes of mud volcanoes. Determining a compre-hensive heat budget would also aid greatly inunderstanding the dynamics of mud volcanismat depth, as has been done for silicate volcanoessuch as Stromboli, Italy (McGetchin and Chouet,1979; Harris and Stevenson, 1997). Consideringthe upwelling style at Upper and Lower Klawasi,it is possible that some of the mechanisms char-acteristic of continuously active basaltic systemslike Stromboli, such as periodic gas coalescenceand slug £ow, may be relevant to some degreefor these mud volcanoes as well.

The Drum mud volcanoes are su⁄ciently dis-tant (V15 km) from the closest towns (Glen-nallen, pop. 451, and Copper Center, pop. 449)that no signi¢cant risk is posed by their activity.Our study of their heat emission, therefore, can betaken as an archetype for other, more dangerous,mud volcanoes. This technique is inexpensive,provides data on remote areas and supplementsconventional ground measurements. It may alsoprove an easier means of monitoring isolatedmud volcanoes where ¢eld work can be ham-pered. In areas where mud volcanism could be a

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danger, increased activity might be detected insatellite data and provide a warning of impendinghazard.

5. Conclusions

The new higher-resolution thermal bands onLandsat 7 ETM+, as well as the new ASTERsensor, can be used as a reliable thermal monitor-ing data set for warm mud volcanism. By usingLandsat 7 ETM+ thermal imagery over the Drummud volcanoes their mud temperatures, activemud areas, and heat £ux can be estimated usinga simple two-component radiance model and rel-evant constraints from ¢eld data. High-resolutionsatellite sensors provide easy ‘access’ to mud vol-canoes, and regular acquisition and analysis ofthese data may be e¡ective in detecting increasedactivity and mitigating any potential hazard.

Acknowledgements

We are extremely grateful to R. McGimsey atthe Alaska Volcano Observatory-USGS for hishelpful data and comments. C. Neal, AVO-USGS, provided helpful comments on an earlyversion of the manuscript and S. George, Geo-physical Institute, provided essential data. Thanksalso go to A. Harris and R. Wright, University ofHawaii Manoa, for assistance with heat and mass£ux ideas. Reviews by M. Sorey, USGS-MenloPark, and an anonymous reviewer greatly im-proved the manuscript. This work is supportedby the United States Geological Survey VolcanoHazards Program and by the Geophysical Insti-tute, University of Alaska Fairbanks.

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