The concurrent measurement of self-potential (SP) and soil CO2 flux (FCO2s) in volcanic sys- tems may be an important tool to monitor intrusive activity and understand interaction between magmatic and groundwater systems. However, quantitative relationships between these parameters must be established to apply them toward understanding processes operating at depth. Power-law scaling exponents calculated for SP and FCO2s measured along a fault on the flanks of Masaya volcano, Nicaragua indicate a nonlinear relationship between these parameters. Scaling exponents suggest that there is a declining increase in SP with a given increase in FCO2s, until a threshold (log FCO2s ≈ 2.5 g m−2d−1) above which SP remains constant with increasing FCO2s. Implications for subsurface processes that may influence SP at Masaya are discussed

Connor, Charles B.

Hilley, G. E.

Lewicki, J. L.

The concurrent measurement of self-potential (SP) and soil CO{sub 2} flux (F{sub s}{sup CO2}) in volcanic systems may be an important tool to monitor intrusive activity and understand interaction between magmatic and groundwater systems. However, quantitative relationships between these parameters must be established to apply them toward understanding processes operating at depth. Power-law scaling exponents calculated for SP and F{sub s}{sup CO2} measured along a fault on the flanks of Masaya volcano, Nicaragua indicate a nonlinear relationship between these parameters. Scaling exponents suggest that there is a declining increase in SP with a given increase in F{sub s}{sup CO2}, until a threshold (log F{sub s}{sup CO2} {approx} 2.5 g m{sup -2}d{sup -1}) above which SP remains constant with increasing F{sub s}{sup CO2}. Implications for subsurface processes that may influence SP at Masaya are discussed

Conner, C.

UNT Digital Library

The scaling relationship between self-potential and fluid flow on Masayavolcano, NicaraguaJ. L. LewickiEarth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USAG. E. HilleyDepartment of Earth and Planetary Sciences, University of California, Berkeley, CA, USA.C. ConnorDepartment of Geology, University of South Florida, Tampa, FL, USAABSTRACT: The concurrent measurement of self-potential (SP) and soil CO2 flux (FCO2s ) in volcanic sys-tems may be an important tool to monitor intrusive activity and understand interaction between magmatic andgroundwater systems. However, quantitative relationships between these parameters must be established to ap-ply them toward understanding processes operating at depth. Power-law scaling exponents calculated for SPandFCO2smeasured along a fault on the flanks of Masaya volcano, Nicaragu indicate a nonlinear relationshipbetween these parameters. Scaling exponents suggest that there is a declining increase in SP with a given in-crease inFCO2s, until a threshold (logFCO2s≈ 2.5 g m−2d−1) above which SP remains constant with increasingFCO2s. Implications for subsurface processes that may influence SP at Masaya are discussed.1 INTRODUCTIONSP anomalies have been observed within many vol-canic and hydrothermal regions, and SP variationshave been linked to changes in volcanic activity andhydrothermal fluid flow (Corwin and Hoover 1979;Fitterman and Corwin 1982; Massenet and Pham1985; Hashimoto and Tanaka 1995; Zlotnicki et al.2003). Monitoring these anomalies on active vol-canoes may therefore be a tool to forecast erup-tions and understand interaction between magmaticand groundwater systems. SP anomalies observed involcanic systems have been primarily attributed toelectrokinetic (EK) processes involving transport ofcharge contained in the electrical diffuse layer at thepore surface by electrolyte solution flow (Andersonand Johnson 1976; Massenet and Pham 1985; Revilet al. 1999; Ishido and Pritchett 1999) or rapid fluiddisruption (RFD), a process whereby charge separa-tion can be produced by rapid disruption or vapor-ization of liquid water by high heat and/or gas flow(Johnston et al. 2001). While EK potentials are pro-duced by capillary flow of aqueous solutions, RFDmay produce positive SP anomalies in thermal areasfar above the groundwater table as charge is carriedby water droplet and/or vapor flow (Johnston et al.2001).Masaya craterSantiago craterSan Pedro craterComalito cinder coneStudy Site (A)Nindiri craterN1 km90Wû 86W 82W12ûN16NûCosta   RicaHondurasNicaraguaStudy AreaC4C1C3C2(A)Figure 1: Areal photograph showing study site loca-tion adjacent to Comalito cinder cone on the flanksof Masaya volcano. Inset (A) shows locations of SPandFCO2stransects C1-C4, arealFCO2smeasurements(dots), and inferred normal fault (dashed line).12.22.42.62.833.23.43.63.844.2Log CO2 flux    (g m-2 d-1)C2C4C1 C35                25               45               65020406080100NDistance (m)Distance (m)Figure 2: Interpolated image map of logFCO2smea-sured adjacent to Comalito cinder cone (Lewicki etal. 2003). Dashed line and dots show locations of in-ferred normal fault andFCO2smeasurements, respec-tively. Also shown are locations of transects C1-C4.Lewicki et al. 2003 investigated the spatial relation-ship between SP,FCO2s, and soil temperature (Ts) andthe mechanism(s) that may produce SP anomalies onthe flanks of Masaya volcano, Nicaragua (Figure 1).FCO2sandTs up to 5.0 x 104 g m−2d−1 and 80oC, re-spectively, and positive SP anomalies were measuredwithin an area surrounding an inferred normal fault,adjacent to Comalito cinder cone (Figure 2). Lewickiet al. 2003 showed that SP,FCO2s, andTs were spa-tially correlated and that trends inFCO2sandTs wereconsistent with advective transport of heat and CO2with steam along a fault zone. Also, a model calcu-lation (Babu 2003) showed that polarization must beshallow (<10 m) to produce the high horizontal SPgradients and short wave lengths observed at the studysite. Based on the presence of mechanisms to produceRFD (i.e., subsurface temperatures above liquid boil-ing points, high gas flow rates), a thick unsaturatedzone (100 to 150 m), and relatively high resistivities,Lewicki et al. 2003 suggested that SP anomalies atMasaya are mainly produced by RFD.While clear spatial correlation was observed be-tween SP andFCO2s, Lewicki et al. 2003 did not inves-tigate the quantitative relationship between these pa-rameters. Such a link is important to establish to betterunderstand the influence of fluid flow processes oper-0246-1000100200(c) C3024-1000100200300400(a) C16 0246 (b) C2-200-1000100200300-113-200-1000100200300(d) C40 20 40 60 80 100 120 140Distance (m)SW NE5Self-potential (mV)Self-potential (mV)Log CO2 flux (g m-2 d-1)Log CO2 flux (g m-2 d-1)Self-potential (mV)Self-potential (mV)Log CO2 flux (g m-2 d-1)Log CO2 flux (g m-2 d-1)Figure 3: Plots of logFCO2s(dots) and SP (open cir-cles) versus distance along transects (a) C1, (b) C2,(c) C3, and (d) C4 (modified from Lewicki et al.2003).ating at depth on SP anomalies measured at the sur-face. Here, we establish power-law scaling relation-ships for measured values and discuss the subsurfaceprocesses that may influence SP at Masaya.2 DATA ANALYSISScaling relationships between SP andFCO2swere cal-culated by linear regression of the log-transformeddata. These relationships were determined separatelyfor each traverse (C1-C4) because SP measurementsalong each traverse were referenced to the back-ground base station at the southwest end of each tra-verse (Lewicki et al. 2003). To remove backgroundeffects not attributable to processes within the faultzone, we subtractedFCO2smeasured at each back-ground base station from all measurements alongeach traverse. These values are reported as “adjusted”FCO2s. Since SP represents the electric potential dif-ference between the base station and a given measure-ment along the traverse, adjustment was unnecessary.2Log self-potential (mV)-2 -1 0 1 2 3 4 5-0.500.511..53Log adjusted CO2 flux (g m-2 d-1)2.52Figure 4: Plot of log SP versus log adjustedFCO2sfortransects C1 (dots), C2 (circles), C3 (triangles) andC4 (stars). Best-fit lines determined by linear regres-sion are shown for log adjustedFCO2s≤ 2.5 g m−2d−1and log adjustedFCO2s> 2.5 g m−2d−1.3 RESULTSSelf-potential and logFCO2smeasured along transectsC1-C4 (Figure 3) are moderately positively correlated(correlation coefficient,C = 0.67, 0.71, 0.59, and 0.70for transects C1, C2, C3, and C4, respectively). A plotof log SP versus log adjustedFCO2sfor transects C1-C4 (Figure 4) shows a sharp change in correlation ofthese parameters at logFCO2s≈ 2.5 g m−2d−1. LogSP and log adjustedFCO2sare moderately positivelycorrelated (C = 0.44 to 0.56 for C1-C4) at logFCO2s≤ 2.5 g m−2d−1 and poorly correlated (C = -0.25 to0.08 for C1-C4) at logFCO2s> 2.5 g m−2d−1.Linear regression of log-transformed values of SPand adjustedFCO2swas used to calculate the power-law scaling for SP versus adjustedFCO2s:SP = b(FCO2s)a (1)(Figure 4). In the log-transformed space, the regressedslope yields the exponent value (a) and the interceptyields the scaling constant (b). Table 1 shows calcu-lated power-law coefficients for transects C1-C4.4 DISCUSSION AND CONCLUSIONSWe assume that rapid disruption (D) and/or vaporiza-tion (V) of groundwater by high gas and/or heat flowderived from a cooling intrusion is primarily responsi-ble for the observed SP anomalies at Masaya (Lewickiet al. 2003). The magnitude of a given SP anomalyhere should be proportional to the sum of the fluxesof positively charged water molecules produced bythese processes (FH2Ot= FH2OV+ FH2OD). Scaling ex-ponents suggest that there is a declining increase inSP with a given increase inFCO2s, until a threshold atTable 1: Parametersa and b calculated for best-fitlines to log SP versus log adjustedFCO2s, log SP ver-sus log adjustedTs, and log adjustedTs versus logadjustedFCO2s. The mean (µ) and standard error (σ)of a andb are shown.SP = b(FCO2s )atraverse logFCO2s ≤ 2.5 logFCO2s > 2.5a b a bC1 0.30 1.5 -2.9e−2 2.5C2 0.50 0.98 5.2e−2 2.1C3 0.44 1.0 -1.0e−2 2.2C4 0.50 1.1 -6.8e−3 2.3µ 0.44 1.2 1.4e−3 2.3σ 9.2e−2 0.26 3.5e−2 0.16log FCO2s≈ 2.5 g m−2d−1 above which SP remainsrelatively constant with increasingFCO2s.Lavas and scoria at the Masaya study site areporous and highly fractured. We observe the highestrates of CO2 (and steam) degassing along linear trendsthat correspond to the location/trend of the inferrednormal fault (Figure 2). Elevated logFCO2svalues(> 2.5 g m−2d−1) along transects C1-C4 also corre-late with fault/fracture locations (Figure 3). These ob-servations indicate distinct gas transport mechanisms,where the highest fluxes are likely associated withadvective transport along fractures and the moderateto low fluxes are associated with diffusive transportthrough the porous matrix. Relative toFCO2s, positiveSP anomalies show little variability over short spatialscales (Figure 3) and do not reflect the presence orabsence of fractures. Hence, at elevatedFCO2svalues,log SP becomes poorly correlated with log adjustedFCO2s.One explanation for the observed scaling relation-ship between SP and adjustedFCO2smay be thepresence of two sources of CO2 degassing at depth.Based on carbon isotopic compositions of soil CO2at Masaya, this CO2 is ultimately derived from deepmagmatic/marine carbonate sources (Lewicki et al.2003). However, CO2 exsolved from the cooling in-trusion may either be dissolved in and subsequentlydegas from the groundwater system due to changes insubsurface temperatures, or rise directly to the sur-face. In the former case, we would expect a posi-tive (although not necessarily linear) correlation be-tween CO2 flux and SP, as areas of high heat flowwill cause both CO2 degassing from the groundwaterand vaporization of groundwater yieldingFH2OV. Thecharged water vapor and CO2 may then rise throughthe porous medium, creating relatively broad surfaceanomalies in SP andFCO2s. In the later case, CO2(and steam) degassed from the intrusion may rise di-rectly to the surface along highly permeable fractures3with minimal interaction with groundwater, produc-ing the highest soil gas fluxes. If relatively lowFH2OVis consequently produced during this process, then wewould not expect SP to be correlated withFCO2soverthe highFCO2srange. It is also unlikely that steamexsolved from the high-temperature intrusion wouldproduce an SP anomaly, due to the absence ofFH2OVproduction at temperatures> ∼400oC (Blanchard1964). Therefore, the scaling relationships observedin Figure 4 may be related to two CO2 flux popula-tions, a low-flux population associated with ground-water degassing/vaporization and positive correlationof SP andFCO2s, and a high-flux population associ-ated with direct gas flow from depth along fracturesand poor SP-FCO2scorrelation.The scaling relationship observed between SP andadjustedFCO2smay alternatively be explained by theinfluence of CO2 flow on the SP anomaly by fluiddisruption. With RFD, we would expect positive cor-relation between bulk gas flux from depth and SP;however, there may be a non-linear relationship be-tween this gas flux and the number of water moleculesripped from the fluid (FH2OD). This may explain thescaling relationship between adjustedFCO2sand SPat adjustedFCO2s≤ 2.5 g m−2d−1. Above thisFCO2sthreshold, we observe a plateau in SP with adjustedFCO2swhich may be due to local removal and deple-tion of the fluid source for RFD from the rock, leadingto a plateau in SP versus adjustedFCO2sflux.While intriguing trends are observed in SP andFCO2sdata at Masaya, a range of processes may ex-plain these trends. To use concurrent measurements ofSP andFCO2sto better understand processes operatingat depth in the volcanic system, additional field, labo-ratory, and numerical experiments are required to de-fine the relationships between these parameters undera range of hydrogeologic and fluid/heat source condi-tions.REFERENCESAnderson, L. A. and G. R. Johnson (1976). Appli-cation of the self-potential method to geother-mal exploration in Long Valley, California.J.Geophys. Res. 81, 1527–1532.Babu, R. (2003). Relationship of gravity, magnetic,and self-potential anomalies and their appli-cation to mineral exploration.Geophysics 68,181–184.Blanchard, D. C. (1964). Charge separation fromsaline drops on hot surfaces.Nature 201, 1164–1166.Corwin, R. F. and D. B. Hoover (1979). The self-potential method in geothermal exploration.Geophysics 44, 226–245.Fitterman, D. V. and R. F. Corwin (1982).Inversion of self-potential data from theCerro-Prieto geothermal field, Mexico.Geo-physics 47, 938–945.Hashimoto, T. and Y. Tanaka (1995). A largeself-potential anomaly on Unzen volcano,Shimabara peninsula, Kyushu island, Japan.Geophys. Res. Lett. 22, 191–194.Ishido, T. and J. W. Pritchett (1999). Numericalsimulation of electrokinetic potentials associ-ated with subsurface fluid flow.J. Geophys.Res. 104, 15,247–15,259.Johnston, M. J. S., J. D. Byerlee, and D. Lock-ner (2001). Rapid fluid disruption: A source forself-potential anomalies on volcanoes.J. Geo-phys. Res. 106, 4327–4335.Lewicki, J. L., C. Connor, K. St-Amand, J. Stix,and W. Spinner (2003). Self-potential, soilco2 flux, and temperature on Masaya volcano,Nicaragua.Geophys. Res. Lett. 30, 1817.Massenet, F. and V. N. Pham (1985). Mapping andsurveillance of active fissure zones on a vol-cano by the self-potential method, Etna, Sicily.J. Volcanol. Geotherm. Res. 24, 315–338.Revil, A., H. Schwaeger, L. M. C. III, andP. D. Manhardt (1999). Streaming potentialin porous media 2. Theory and applicationto geothermal systems.J Geophys. Res. 104,20,033–20,048.Zlotnicki, J., Y. Sasai, P. Yvetot, Y. Nishida,M. Uyeshima, F. Fauquet, H. Utada, Y. Taka-hashi, and G. Donnadieu (2003). Resistivityand self-potential changes associated with vol-canic activity: The July 8, 2000 Miyake-jimaeruption (Japan).Earth Planet. Sci. Lett. 205,139–154.4

The scaling relationship between self-potential and fluid flow on Masaya volcano, Nicaragua

Scholar Commons - University of South Florida

The Scaling Relationship between Self-Potential and Fluid Flow on Masaya Volcano, Nicaragua

USFSP Digital Archive

Lewicki, J.L.

Hilley, G.E.

eScholarship - University of California

Lawrence Berkeley National LaboratoryLawrence Berkeley National LaboratoryTitleThe scaling relationship between self-potential and fluid flow on Masaya volcano, NicaraguaPermalinkhttps://escholarship.org/uc/item/6r9547jpAuthorsLewicki, J.L.Hilley, G.E.Conner, C.Publication Date2003-11-11eScholarship.org Powered by the California Digital LibraryUniversity of CaliforniaThe scaling relationship between self-potential and fluid flow on Masayavolcano, NicaraguaJ. L. LewickiEarth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA, USAG. E. HilleyDepartment of Earth and Planetary Sciences, University of California, Berkeley, CA, USA.C. ConnorDepartment of Geology, University of South Florida, Tampa, FL, USAABSTRACT: The concurrent measurement of self-potential (SP) and soil CO2 flux (FCO2s ) in volcanic sys-tems may be an important tool to monitor intrusive activity and understand interaction between magmatic andgroundwater systems. However, quantitative relationships between these parameters must be established to ap-ply them toward understanding processes operating at depth. Power-law scaling exponents calculated for SPandFCO2smeasured along a fault on the flanks of Masaya volcano, Nicaragu indicate a nonlinear relationshipbetween these parameters. Scaling exponents suggest that there is a declining increase in SP with a given in-crease inFCO2s, until a threshold (logFCO2s≈ 2.5 g m−2d−1) above which SP remains constant with increasingFCO2s. Implications for subsurface processes that may influence SP at Masaya are discussed.1 INTRODUCTIONSP anomalies have been observed within many vol-canic and hydrothermal regions, and SP variationshave been linked to changes in volcanic activity andhydrothermal fluid flow (Corwin and Hoover 1979;Fitterman and Corwin 1982; Massenet and Pham1985; Hashimoto and Tanaka 1995; Zlotnicki et al.2003). Monitoring these anomalies on active vol-canoes may therefore be a tool to forecast erup-tions and understand interaction between magmaticand groundwater systems. SP anomalies observed involcanic systems have been primarily attributed toelectrokinetic (EK) processes involving transport ofcharge contained in the electrical diffuse layer at thepore surface by electrolyte solution flow (Andersonand Johnson 1976; Massenet and Pham 1985; Revilet al. 1999; Ishido and Pritchett 1999) or rapid fluiddisruption (RFD), a process whereby charge separa-tion can be produced by rapid disruption or vapor-ization of liquid water by high heat and/or gas flow(Johnston et al. 2001). While EK potentials are pro-duced by capillary flow of aqueous solutions, RFDmay produce positive SP anomalies in thermal areasfar above the groundwater table as charge is carriedby water droplet and/or vapor flow (Johnston et al.2001).Masaya craterSantiago craterSan Pedro craterComalito cinder coneStudy Site (A)Nindiri craterN1 km90Wû 86W 82W12ûN16NûCosta   RicaHondurasNicaraguaStudy AreaC4C1C3C2(A)Figure 1: Areal photograph showing study site loca-tion adjacent to Comalito cinder cone on the flanksof Masaya volcano. Inset (A) shows locations of SPandFCO2stransects C1-C4, arealFCO2smeasurements(dots), and inferred normal fault (dashed line).12.22.42.62.833.23.43.63.844.2Log CO2 flux    (g m-2 d-1)C2C4C1 C35                25               45               65020406080100NDistance (m)Distance (m)Figure 2: Interpolated image map of logFCO2smea-sured adjacent to Comalito cinder cone (Lewicki etal. 2003). Dashed line and dots show locations of in-ferred normal fault andFCO2smeasurements, respec-tively. Also shown are locations of transects C1-C4.Lewicki et al. 2003 investigated the spatial relation-ship between SP,FCO2s, and soil temperature (Ts) andthe mechanism(s) that may produce SP anomalies onthe flanks of Masaya volcano, Nicaragua (Figure 1).FCO2sandTs up to 5.0 x 104 g m−2d−1 and 80oC, re-spectively, and positive SP anomalies were measuredwithin an area surrounding an inferred normal fault,adjacent to Comalito cinder cone (Figure 2). Lewickiet al. 2003 showed that SP,FCO2s, andTs were spa-tially correlated and that trends inFCO2sandTs wereconsistent with advective transport of heat and CO2with steam along a fault zone. Also, a model calcu-lation (Babu 2003) showed that polarization must beshallow (<10 m) to produce the high horizontal SPgradients and short wave lengths observed at the studysite. Based on the presence of mechanisms to produceRFD (i.e., subsurface temperatures above liquid boil-ing points, high gas flow rates), a thick unsaturatedzone (100 to 150 m), and relatively high resistivities,Lewicki et al. 2003 suggested that SP anomalies atMasaya are mainly produced by RFD.While clear spatial correlation was observed be-tween SP andFCO2s, Lewicki et al. 2003 did not inves-tigate the quantitative relationship between these pa-rameters. Such a link is important to establish to betterunderstand the influence of fluid flow processes oper-0246-1000100200(c) C3024-1000100200300400(a) C16 0246 (b) C2-200-1000100200300-113-200-1000100200300(d) C40 20 40 60 80 100 120 140Distance (m)SW NE5Self-potential (mV)Self-potential (mV)Log CO2 flux (g m-2 d-1)Log CO2 flux (g m-2 d-1)Self-potential (mV)Self-potential (mV)Log CO2 flux (g m-2 d-1)Log CO2 flux (g m-2 d-1)Figure 3: Plots of logFCO2s(dots) and SP (open cir-cles) versus distance along transects (a) C1, (b) C2,(c) C3, and (d) C4 (modified from Lewicki et al.2003).ating at depth on SP anomalies measured at the sur-face. Here, we establish power-law scaling relation-ships for measured values and discuss the subsurfaceprocesses that may influence SP at Masaya.2 DATA ANALYSISScaling relationships between SP andFCO2swere cal-culated by linear regression of the log-transformeddata. These relationships were determined separatelyfor each traverse (C1-C4) because SP measurementsalong each traverse were referenced to the back-ground base station at the southwest end of each tra-verse (Lewicki et al. 2003). To remove backgroundeffects not attributable to processes within the faultzone, we subtractedFCO2smeasured at each back-ground base station from all measurements alongeach traverse. These values are reported as “adjusted”FCO2s. Since SP represents the electric potential dif-ference between the base station and a given measure-ment along the traverse, adjustment was unnecessary.2Log self-potential (mV)-2 -1 0 1 2 3 4 5-0.500.511..53Log adjusted CO2 flux (g m-2 d-1)2.52Figure 4: Plot of log SP versus log adjustedFCO2sfortransects C1 (dots), C2 (circles), C3 (triangles) andC4 (stars). Best-fit lines determined by linear regres-sion are shown for log adjustedFCO2s≤ 2.5 g m−2d−1and log adjustedFCO2s> 2.5 g m−2d−1.3 RESULTSSelf-potential and logFCO2smeasured along transectsC1-C4 (Figure 3) are moderately positively correlated(correlation coefficient,C = 0.67, 0.71, 0.59, and 0.70for transects C1, C2, C3, and C4, respectively). A plotof log SP versus log adjustedFCO2sfor transects C1-C4 (Figure 4) shows a sharp change in correlation ofthese parameters at logFCO2s≈ 2.5 g m−2d−1. LogSP and log adjustedFCO2sare moderately positivelycorrelated (C = 0.44 to 0.56 for C1-C4) at logFCO2s≤ 2.5 g m−2d−1 and poorly correlated (C = -0.25 to0.08 for C1-C4) at logFCO2s> 2.5 g m−2d−1.Linear regression of log-transformed values of SPand adjustedFCO2swas used to calculate the power-law scaling for SP versus adjustedFCO2s:SP = b(FCO2s)a (1)(Figure 4). In the log-transformed space, the regressedslope yields the exponent value (a) and the interceptyields the scaling constant (b). Table 1 shows calcu-lated power-law coefficients for transects C1-C4.4 DISCUSSION AND CONCLUSIONSWe assume that rapid disruption (D) and/or vaporiza-tion (V) of groundwater by high gas and/or heat flowderived from a cooling intrusion is primarily responsi-ble for the observed SP anomalies at Masaya (Lewickiet al. 2003). The magnitude of a given SP anomalyhere should be proportional to the sum of the fluxesof positively charged water molecules produced bythese processes (FH2Ot= FH2OV+ FH2OD). Scaling ex-ponents suggest that there is a declining increase inSP with a given increase inFCO2s, until a threshold atTable 1: Parametersa and b calculated for best-fitlines to log SP versus log adjustedFCO2s, log SP ver-sus log adjustedTs, and log adjustedTs versus logadjustedFCO2s. The mean (µ) and standard error (σ)of a andb are shown.SP = b(FCO2s )atraverse logFCO2s ≤ 2.5 logFCO2s > 2.5a b a bC1 0.30 1.5 -2.9e−2 2.5C2 0.50 0.98 5.2e−2 2.1C3 0.44 1.0 -1.0e−2 2.2C4 0.50 1.1 -6.8e−3 2.3µ 0.44 1.2 1.4e−3 2.3σ 9.2e−2 0.26 3.5e−2 0.16log FCO2s≈ 2.5 g m−2d−1 above which SP remainsrelatively constant with increasingFCO2s.Lavas and scoria at the Masaya study site areporous and highly fractured. We observe the highestrates of CO2 (and steam) degassing along linear trendsthat correspond to the location/trend of the inferrednormal fault (Figure 2). Elevated logFCO2svalues(> 2.5 g m−2d−1) along transects C1-C4 also corre-late with fault/fracture locations (Figure 3). These ob-servations indicate distinct gas transport mechanisms,where the highest fluxes are likely associated withadvective transport along fractures and the moderateto low fluxes are associated with diffusive transportthrough the porous matrix. Relative toFCO2s, positiveSP anomalies show little variability over short spatialscales (Figure 3) and do not reflect the presence orabsence of fractures. Hence, at elevatedFCO2svalues,log SP becomes poorly correlated with log adjustedFCO2s.One explanation for the observed scaling relation-ship between SP and adjustedFCO2smay be thepresence of two sources of CO2 degassing at depth.Based on carbon isotopic compositions of soil CO2at Masaya, this CO2 is ultimately derived from deepmagmatic/marine carbonate sources (Lewicki et al.2003). However, CO2 exsolved from the cooling in-trusion may either be dissolved in and subsequentlydegas from the groundwater system due to changes insubsurface temperatures, or rise directly to the sur-face. In the former case, we would expect a posi-tive (although not necessarily linear) correlation be-tween CO2 flux and SP, as areas of high heat flowwill cause both CO2 degassing from the groundwaterand vaporization of groundwater yieldingFH2OV. Thecharged water vapor and CO2 may then rise throughthe porous medium, creating relatively broad surfaceanomalies in SP andFCO2s. In the later case, CO2(and steam) degassed from the intrusion may rise di-rectly to the surface along highly permeable fractures3with minimal interaction with groundwater, produc-ing the highest soil gas fluxes. If relatively lowFH2OVis consequently produced during this process, then wewould not expect SP to be correlated withFCO2soverthe highFCO2srange. It is also unlikely that steamexsolved from the high-temperature intrusion wouldproduce an SP anomaly, due to the absence ofFH2OVproduction at temperatures> ∼400oC (Blanchard1964). Therefore, the scaling relationships observedin Figure 4 may be related to two CO2 flux popula-tions, a low-flux population associated with ground-water degassing/vaporization and positive correlationof SP andFCO2s, and a high-flux population associ-ated with direct gas flow from depth along fracturesand poor SP-FCO2scorrelation.The scaling relationship observed between SP andadjustedFCO2smay alternatively be explained by theinfluence of CO2 flow on the SP anomaly by fluiddisruption. With RFD, we would expect positive cor-relation between bulk gas flux from depth and SP;however, there may be a non-linear relationship be-tween this gas flux and the number of water moleculesripped from the fluid (FH2OD). This may explain thescaling relationship between adjustedFCO2sand SPat adjustedFCO2s≤ 2.5 g m−2d−1. Above thisFCO2sthreshold, we observe a plateau in SP with adjustedFCO2swhich may be due to local removal and deple-tion of the fluid source for RFD from the rock, leadingto a plateau in SP versus adjustedFCO2sflux.While intriguing trends are observed in SP andFCO2sdata at Masaya, a range of processes may ex-plain these trends. To use concurrent measurements ofSP andFCO2sto better understand processes operatingat depth in the volcanic system, additional field, labo-ratory, and numerical experiments are required to de-fine the relationships between these parameters undera range of hydrogeologic and fluid/heat source condi-tions.REFERENCESAnderson, L. A. and G. R. Johnson (1976). Appli-cation of the self-potential method to geother-mal exploration in Long Valley, California.J.Geophys. Res. 81, 1527–1532.Babu, R. (2003). Relationship of gravity, magnetic,and self-potential anomalies and their appli-cation to mineral exploration.Geophysics 68,181–184.Blanchard, D. C. (1964). 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The scaling relationship between self-potential and fluid flow on 
Masaya volcano, Nicaragua

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The Scaling Relationship between Self-Potential and Fluid Flow on Masaya Volcano, Nicaragua

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