C.A. Miller, GNS Science, Wairakei Research Centre ... 2013-047.pdf · C.A. Miller, GNS Science,...

© Institute of Geological and Nuclear Sciences Limited, 2013

ISSN 1177-2425 ISBN 978-1-972192-95-5

C.A. Miller, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352 A. Mazot, GNS Science, Wairakei Research Centre, Private Bag 2000, Taupo 3352

BIBLIOGRAPHIC REFERENCE

Miller, C.A.; Mazot, A. 2013. Patterns of hydrothermal activity and groundwater flow at Mount Tongariro revealed through self-potential surveying, GNS Science Report 2013/47. 27 p.

GNS Science Report 2013/47 i

CONTENTS

ABSTRACT ............................................................................................................................. III

KEYWORDS ........................................................................................................................... III

1.0 INTRODUCTION .......................................................................................................... 1

1.1 Self-potential theory....................................................................................................... 3

2.0 SURVEY METHOD ...................................................................................................... 4

2.1 Data quality and errors .................................................................................................. 7

3.0 RESULTS ..................................................................................................................... 8

3.1 Self-potential map .......................................................................................................... 8 3.2 Profile analysis .............................................................................................................. 8

3.2.1 Profile 1 ............................................................................................................. 8 3.2.2 Profile 2 ........................................................................................................... 10 3.2.3 Profile 3 ........................................................................................................... 11

3.3 Self-potential vs elevation “Ce” map ........................................................................... 13

4.0 INTERPRETATION .................................................................................................... 15

4.1 Hydrothermal features ................................................................................................. 15 4.1.1 Red Crater ....................................................................................................... 15 4.1.2 Ketetahi hot springs ......................................................................................... 16

4.2 Groundwater flow ........................................................................................................ 16 4.2.1 Oturere Valley positive SP anomaly ................................................................ 16 4.2.2 Oturere Valley fresh water spring .................................................................... 17 4.2.3 Oturere Valley hydrology ................................................................................. 18

4.3 East Tongariro negative anomaly ................................................................................ 19

5.0 FUTURE WORK ......................................................................................................... 20

6.0 CONCLUSIONS ......................................................................................................... 20

7.0 ACKNOWLEDGEMENTS .......................................................................................... 21

8.0 REFERENCES ........................................................................................................... 21

GNS Science Report 2013/47 ii

FIGURES

Figure 1.1 Oblique Google Earth image looking southwest, showing the Mount Tongariro massif with key features referred to in the text labelled. .......................................................................... 2

Figure 2.1 A) Electrodes and multimeter used in the survey. B) An SP electrode in place during the survey in Oturere Valley. C) SP cable being laid out across Central Crater during the survey. Red Crater is in the background. D) Measurements of soil CO2 flux. .............................. 5

Figure 2.2 SP distribution of Mount Tongariro overlain on shaded relief. ...................................................... 6 Figure 3.1 Profile 1. A) Topography, SP, 10 cm soil temperature, CO2 flux. Black dashed lines mark

zone boundaries labelled with letters. Red dashed line separates Ce profiles in B. B) Ce profiles colour coded by zone with Ce value labelled where calculated. ...................................... 9

Figure 3.2 Profile 2. A) Topography, SP, 10 cm soil temperature, CO2 flux. Black dashed lines mark zone boundaries labelled with letters. Red dashed line separates Ce profiles in B. The shaded part indicates data presented in profile 1. It is shown here again to provide better context for Profile 2. B) Ce profiles colour coded by zone with Ce value labelled where calculated. .................................................................................................................................. 11

Figure 3.3 Profile 3. A) Topography, SP, 10 cm soil temperature, CO2 flux (note log scale). Black dashed lines mark zone boundaries labelled with letters. Red dashed line separates Ce profiles in B. B) Ce profiles colour coded by zone with Ce value labelled where calculated. .................................................................................................................................. 12

Figure 3.4 Ce distribution (coloured dots) plotted with the SP map overlain on a shaded relief map of Mount Tongariro. Red dots represent hydrothermal associated anomalies, blue dots represent groundwater flow anomalies. No Ce was calculated in areas represented by white dots. .................................................................................................................................. 14

Figure 4.1 Modelled (green) SP vs observed (blue) for a vertically charged dipole at a depth of 175 m with a current source of 89A. .................................................................................................. 17

Figure 4.2 Graph of topography (green), water table altitude (blue) and SP (red). Altitude of water table is calculated from Eq. (3). The location of the current source modelled in section 4.2.1 is shown as an orange dot. ................................................................................................ 18

Figure 4.3 Schematic of Oturere Valley hydrothermal and ground water flow. Ce Zones and values (mV/m) are labelled along the top. ............................................................................................. 19

GNS Science Report 2013/47 iii

ABSTRACT

We undertook a large self-potential (SP), survey at Mount Tongariro together with soil CO2 flux and ground temperature measurements. The survey successfully delineated the Ketetahi and Red Crater hydrothermal surface features through an analysis of SP vs topographic gradient. The SP anomalies around hydrothermal areas were subdued, possibly due to low pH fluids, and/or hydrothermally altered and electrically conductive ground. We interpreted a large positive anomaly (+1100 mV) in Oturere Valley as the result of deepening of the ground water aquifer beneath a thick electrically resistive lava flow. Modelling of SP data to determine both water table depth, and the depth to a potential current source, agree with the thickness of the Oturere Valley lava flow as previously modelled from TOPSAR data, and imply that the lava itself is a poor aquifer, with the material underlying the lava hosting the aquifer.

We interpreted a large negative (-600 mV) SP anomaly mapped on the NE flank of North Crater as a zone of down-flowing meteoric fluids, outside of the hydrothermal system. A mapped fault coincides with the boundary between the hydrothermal and groundwater flow at Ketetahi and may indicate some structural influence controlling the extent of the hydrothermal system.

The implication of subdued SP anomalies in areas of surface hydrothermal activity as resulting from low pH fluids and hydrothermally altered ground, is that they represent areas of weakened ground which may be prone to future flank collapse, especially in areas of steep topography. Magmatic intrusion could cause flank collapse in a weakened part of the volcanic edifice by raising the pore pressure of surrounding rocks and promoting their ability to fail. Such a mechanism may have caused the landslide coincident with the 2012 Te Maari eruption. Therefore monitoring of SP changes over time may provide a method of determining changes in the hydrothermal system prior to an eruption or further flank collapse.

KEYWORDS

Self-potential, CO2, Tongariro, Hydrothermal, Groundwater, Aquifer, Flank Collapse.

GNS Science Report 2013/47 1

1.0 INTRODUCTION

Mount Tongariro (Figure 1.1) is a composite volcano of overlapping vents that has been active for around 275 ka with 6 main periods of cone building (Lee et al., 2011). The current cone building phase has produced Mount Ngauruhoe in the last 6.5 ka (Moebis et al., 2011). Deposits include extensive lava flows, pyroclastic deposits, and sector collapse deposits. The massif has been heavily modified by glaciation and is dissected on either side by numerous NE-SW trending young (active) normal faults, down thrown towards the centre of the massif. Holocene eruptions have occurred from Red Crater, Te Maari Crater and Ngauruhoe (outside the survey area). Lava flows from Red Crater have flowed into the glacially eroded Oturere Valley within the last 4 ka and lava flows from Te Maari may be as recent as 500 years old. Te Maari was active in the late 1890s from Upper Te Maari Crater, producing thin pyroclastic deposits and explosion pits.

Mount Tongariro hosts an active hydrothermal system manifesting as hot springs, steaming ground and fumaroles at Ketetahi, Red Crater, Central Crater and Te Maari. The temperatures of the hot springs and fumaroles are mostly at boiling point and pH of fluids is in the range 2-6 (GeoNet unpublished data). Areas of altered ground exist around most of the surface hydrothermal features as a result of the presence of acidic fluids and high temperatures.

Mount Tongariro erupted twice in 2012 after about 125 years of dormancy. The August 6th eruption occurred after 4 weeks of unrest and was likely related to dyke intrusion (Hurst et al., 2013 in prep.) within the volcano. It is likely that the intrusion increased pore pressure in the overlying hydrothermal system which initiated a landslide down the western flank (Jolly et al., 2013). This in turn uncapped the hydrothermal system allowing it to rapidly depressurise, triggering the explosion at Te Maari.

Previous geophysical work on Mount Tongariro has been limited. Hill et al. (2012) recently undertook an extensive 3D magnetotelluric survey of the massif and mapped deep conductive bodies beneath Tongariro and Ngauruhoe. Cassidy et al. (2009) completed a gravity and magnetic profile south of Mt Ngauruhoe which delineated faulting and the gross distribution of volcanic products. Walsh et al. (1998) reviewed geophysical data, mostly DC resistivity soundings by Hochstein and Bromley (1979) and concluded the hydrothermal system is vapour dominated and capped by a coherent c. 200-300 m thick condensate layer. The condensate layer is hosted in hydrothermally altered rocks with low resistivity. They estimated a total heat discharge of c. 58 MW of which most (44 MW) is released by the Ketetahi fumarole field.

In response to the eruptions we undertook a combined self-potential (SP), soil CO2 and ground temperature survey to learn more about the hydrothermal system, its distribution and fluid flow patterns.


Figure 1.1 Oblique Google Earth image looking southwest, showing the Mount Tongariro massif with key features referred to in the text labelled.


1.1 SELF-POTENTIAL THEORY

Self-potential (SP) surveying is one of the earliest geophysical techniques performed on volcanoes (Zablocki, 1976). SP is a geophysical parameter that is directly generated from subsurface water flow, which produces electrical potentials at the earth’s surface. Measuring the voltage difference between pairs of non-polarising electrodes placed on the surface allows interpretation of fluid flow beneath the surface.

SP anomalies are generated by several mechanisms (Aizawa, 2008) with the electrokinetic (EK) effect being the dominant contributor (Corwin and Hooper, 1979). The thermoelectric effect can also generate potentials at the surface from thermal gradients, however this effect is generally smaller in amplitude than the EK effect (Jardani et al., 2008, Corwin and Hooper, 1979). Electrokinetic effect results from the separation of charged surfaces on rocks as a result of fluid flowing within the rocks. Charge separation results in a measurable potential difference. The degree of charge separation that occurs is controlled by the magnitude and polarity of the zeta potential of the rock. Fluid flow carries positive charges through electrokinetic coupling if the zeta potential is negative, as shown by most laboratory measurements (Hase, 2003). Therefore to a first order approximation (assuming a negative zeta potential and ignoring topography), upward flowing fluids will generate a positive SP anomaly and downward flowing fluids will generate a negative SP anomaly.

Hase (2003) also showed that zeta potential is dependent on pH and can become positive for lower pH fluids. His studies on rocks from Aso Caldera showed zeta potential became positive at pH 4.5 to 5.5 and was more positive for samples having a relatively low SiO2 content and an abundance of elements having high iso-electric points. Therefore an upwelling, low pH fluid could be expected to produce smaller values of SP than the same upwelling fluid with a higher pH. At Tongariro the SP signal may be caused by both fresh water flow and acidic hydrothermal liquids.

SP signal generation is influenced by topography so interpretation involves viewing SP as a function of elevation gradient. On volcanoes rainfall infiltrates more or less easily to different geological beds depending on their permeability. The downward flow is generally stopped by underlying impermeable layers (Fournier, 1989). The topography of geological beds governs the gravitational flow downward along subhorizontal layers giving rise to springs at the interface of permeable and impermeable lava flows (thick lava flow beneath a scoria bed for instance). When a geological barrier such as an impermeable fault or crater boundary prevents the downward flow, an aquifer is formed. Such a scheme explains the so-called “topographic effect” where the potential increases when the altitude of the topography decreases (Zlotnicki and Nishida, 2003). This negative relationship of SP/elevation is expressed in mV/m.

SP changes may also be caused by a variation in the source or heterogeneities in the resistivity of the medium between the source and the surface (Ishido, 2004), (Minsley et al., 2007). Aizawa et al. (2009) concluded that SP signals may be much smaller in areas of hydrothermally altered rock because of the high conductivity of these rock types. Therefore the lack of an SP anomaly may be related to alteration of rocks and so still implies hydrothermal activity.

SP measurements have previously been made in hydrothermal systems in New Zealand (Kohpina, 1985; Mayhew, 1982) as well as small scale surveys at White Island volcano (Hashimoto et al., 2004). However this is the first survey to cover a large area over a volcanic complex in New Zealand.


2.0 SURVEY METHOD

We took SP measurements with a pair of non-polarising Cu/CuSO4 electrodes connected by a 500 m insulated copper cable to a high impedance (10 MΩ) multimeter (Figure 2.1). We took readings every 50 m (Figure 2.2) using a handheld GPS for positioning; locations are accurate to within 5 m. The measurement spacing was a trade-off between adequate resolution and the speed of the survey. 50 m spacing was chosen as the time available was limited and the desired outcome of the survey was a broad overview of the volcano rather than a detailed assessment of individual features. At each measurement point we dug a shallow (10-20 cm) hole and inserted the electrode, ensuring good ground contact with the base of the electrode. We filled the hole after taking the measurement.

We used the same reference electrode over the length of the cable and connected the base station reference electrode to the negative terminal of the multimeter and the roving electrode to the positive terminal. To reduce the effects of electrode polarisation we swapped the base and roving electrodes at the end of every cable length i.e., the last roving electrode became the new base electrode. We also measured the circuit contact resistance at each profile point.

At the start of a new cable length we repeated the last 1 or 2 measurements of the previous cable length to check repeatability between the end of one cable and the start of another. We also took measurements with the two electrodes side by side at the start of each cable length to detect any electrode drift during the course of the survey.

We surveyed in closed loop so that errors generated along the profile (instrumental drift and temporal changes) can be quantified from the accuracy of the loop closure and distributed along the loop in post processing. We chose Blue Lake as the survey zero potential point as it is the largest body of free standing water on the volcano and therefore represents a large iso-potential surface making it suitable for a base location. All measurements are relative to that location. We were able to survey 2-3 km of ground each day, depending on terrain conditions.

We surveyed two loops, in March and April 2013 each taking 3-4 days to complete (Figure 2.2). The survey occurred at the end of a dry summer so SP changes due to recent rainfall are not expected to be significant. One loop traversed from Blue Lake across Central Crater, over Red Crater and down into southern part of the Oturere Valley. It then traversed north across the valley to a hiking track before heading back up to Central Crater and Blue Lake, via the Emerald Lakes area. The second loop started at Central Crater (at a point common to the first loop), traversed over North Crater, and down to the west of Ketetahi Springs before crossing below the base of the springs and following the Tongariro Alpine Crossing track back up to Central Crater. In total, we took 338 SP measurements, along 16.34 km of profile.

In addition to the SP measurements, we took co-located soil CO2 flux and ground temperature measurements at 10 cm depth, for the Red Crater, Oturere Valley loop (parts of Profiles 1 and 2 and all of Profile 3). We took temperature measurements with a thermocouple TX10 (Yokogawa Electric Corporation, Tokyo, Japan) at a depth of 10 cm. We measured Soil CO2 fluxes by the accumulation chamber technique using a portable non-dispersive infrared system (WS-LI820-CO2: West Systems S.r.l., Pontedera (PI), Italy) as described in detail in Chiodini et al. (1998).


Soil CO2 flux (FCO2, g m-2 d-1) was calculated as follows:

𝐹𝐶𝑂2 = 𝑘 𝑉𝐴 𝑇0

𝑇 𝑃

𝑃0 𝑑𝑐

𝑑𝑡, (1)

where dc/dt is the change in concentration with time (ppm s-1); k is a constant (155.87 m-3) to convert ppm s-1 to g m-2 d-1; P is the measured pressure (kPa); T is the measured temperature (K); V is the volume of the chamber; and A is the area of the base of the chamber. The T0 and P0 are 298 K and 101.3 kPa, respectively.

A

B

C

D

Figure 2.1 A) Electrodes and multimeter used in the survey. B) An SP electrode in place during the survey in Oturere Valley. C) SP cable being laid out across Central Crater during the survey. Red Crater is in the background. D) Measurements of soil CO2 flux.


Figure 2.2 SP distribution of Mount Tongariro overlain on shaded relief.


2.1 DATA QUALITY AND ERRORS

We made repeat measurements in several ways to assess electrode drift, repeatability and loop closure errors. We measured electrode drift at the beginning of each cable length (500 m) by placing the electrodes side by side in the same hole. The average drift of all side by side measurements was 1.6 mV. We also checked the drift by placing the two electrodes side by side in a bath of CuSO4 solution. In this case drift was always 0.1 mV or less. We corrected the drift by distributing a linear correction to each measurement made between the drift check points.

We assessed the repeatability of measurements by back tracking from the start of a new cable length to the last reading on the previous cable length and re taking that measurement. Errors in repeatability were on average 4.2 mV.

Each loop started and ended at the same point, therefore potentials along the profile should sum to zero along the loop. The difference between the potential measured at the start of the profile and that made at the end of the profile allows a loop closure correction to be made. This correction was distributed evenly along the loop to each measurement point. The Oturere Loop closure error was 3.3 mV and the North Crater Loop closure error was 13.4 mV.

Drift and loop closure errors result in corrections of <1 mV being made to individual measurement points along each loop.

The 10 cm depth of the soil surveys was not deep enough to avoid diurnal temperature variations. Repeat temperature measurements taken in the morning compared to those taken in the previous afternoon show a change of up to 4°C. Temperature fluctuations along profile of < 4°C are therefore not considered to be significant.


3.0 RESULTS

Results are presented in both map (section 3.1) and profile (section 3.2) form.

3.1 SELF-POTENTIAL MAP

Figure 2.2 shows the total SP field. The SP field varies from +1100 mV to -700 mV which is within a range typically seen at large volcanoes (Aizawa, 2008, 2004; Finizola et al., 2004, 2003). Two large, long-wavelength anomalies are mapped with several smaller, shorter-wavelength anomalies superimposed. The Oturere Valley shows a large, long-wavelength positive SP anomaly up to 1100 mV, over 1 km. The extent of this anomaly is not fully resolved by the survey. A long wavelength negative anomaly (-700 mV over 1 km) is mapped along the Tongariro Alpine Crossing track on the eastern slope of North Crater. This anomaly extends west to the summit of North Crater.

A weaker negative anomaly (-100 to -200 mV) is mapped over the Ketetahi hot springs area. A weak positive anomaly (+200 mV) is seen on the south flanks of North Crater. SP in the Red Crater area is close to zero.

3.2 PROFILE ANALYSIS

We divided the survey area into 3 profiles for detailed analysis and took two approaches to analysing the SP profiles. Firstly we review long and short wavelength SP anomalies and compare them to CO2 flux and soil temperature data where available. General trends are analysed and anomalies of only a few points are ignored as they probably relate to near surface variations which aren’t of interest to this study.

Secondly we undertook an analysis of SP with elevation. Comparison of SP with elevation allows more detailed analysis and the opportunity to identify hydrothermal zones. Jackson and Kauahikaua (1987) defined a correlation coefficient “Ce” as the ratio of SP over elevation with units of mV/m. The Ce can be used to distinguish zones of up flow, where a positive correlation between SP and elevation is expected, from gravity driven down-flow, where a negative SP – elevation gradient is expected (assuming a negative zeta potential).

Topographic changes account for changes in SP caused by changes in distance to the source, or variation in the source, i.e., fluids moving towards the surface or away from the surface. Tongariro volcano has large flat areas where the Ce calculation is not appropriate so we only calculate the Ce where there is sufficient change in topography to give sensible results. Indeed, in flat topographic areas little gravitational driven fluid flow is to be expected. Ce is presented initially on the profiles and later the Ce distribution is discussed further in section 3.3.

3.2.1 Profile 1

Profile 1 (Figure 3.1a) extends from Oturere Valley, past Emerald Lakes, over North Crater, and down to the west of Ketetahi hot springs (refer Figure 2.2). The profile shows many localised anomalies of several hundred millivolts, and apart from the first 500 m of profile, has a generally flat overall appearance. The profile is divided into 7 zones (A – G) based on large scale changes of elevation or SP so that some of the heterogeneities in the profile can be analysed in more detail. Figure 3.1b shows the Ce for each zone.


Figure 3.1 Profile 1. A) Topography, SP, 10 cm soil temperature, CO2 flux. Black dashed lines mark zone

boundaries labelled with letters. Red dashed line separates Ce profiles in B. B) Ce profiles colour coded by zone with Ce value labelled where calculated.


The start of the profile (Zone A) covers the large positive anomaly shown in Figure 2.2. This +800 mV anomaly is in a relatively flat lying area where there is not expected to be any topographic effect so it is not relevant to calculate Ce. We discuss mechanisms which may cause this large anomaly in section 4. Soil temperatures are between 4°C and 8°C in Zone A and show no significant variation. No CO2 flux is measured.

Zone B covers the rise in topography to Central Crater and shows a negative Ce of -1.7 mV/m. Soil temperatures vary from 8°C at the start of the zone, to 5°C at Central Crater and show no significant variation. A single CO2 flux anomaly of 35 g/m2/day is measured adjacent to an area of hot ground below Emerald Lakes.

Zone C crosses the flat Central Crater so we don’t calculate Ce, but a short wavelength (-400 mV) V shaped anomaly is mapped 300 m before the end of Zone C, at the base of the North Crater cone. Soil temperature and CO2 flux measurements extend only part way along this zone; temperatures vary from 8°C to 4°C and 2-3 g/m2/day CO2 flux is measured at 4 points around 2500 m distance.

Zone D covers the ascent to North Crater and produces a negative Ce (-0.5 mV/m). The data is quite noisy however so some care is required in its interpretation.

Zone E crosses the flat North Crater where a broad V shaped SP anomaly (500 mV peak to trough) is mapped. The apex of the anomaly is coincident with a prominent drainage feature and mapped fault line (GNS Science Active Faults Database) that crosses North Crater. The topography in Zone 5 is too flat to permit a Ce calculation.

Zone F covers the descent from North Crater to the base of Ketetahi hot springs and has a positive Ce (0.7 mV/m). The SP profile is mostly flat (-200 mV) adjacent to the hot springs.

Downslope from Ketetahi the SP profile rises and has a negative Ce (-2.0 mV/m) (Zone G).

3.2.2 Profile 2

The overall shape of Profile 2 (Figure 3.2a) is that of a broad “V’ with a SP low (-800 mV) north of Central Crater. Profile 2 shares the same first part as Profile 1 (Zones A – C). The second part of the profile (Zones H – K) extends from the northeast corner of Central Crater (Figure 2.2) and runs down slope to the east of Ketetahi hot springs, following the Tongariro Alpine Crossing track most of the way. Figure 3.2b shows the Ce calculated on Zones H – K.

Zone H starts from the highest topographic point on the profile. A very short wavelength SP peak is mapped in Zone H but the overall trend is for decreasing SP, resulting in a positive Ce (5.5 mV/m).

The start of Zone I marks a reversal in the SP trend, where SP starts to increase from a -800 mV minimum resulting in a negative Ce (-4.3 mV/m). Together Zones H and I form a broad V shaped anomaly.

Zone J covers the area adjacent to Ketetahi hot springs and shows a mostly flat SP profile (~200 mV) across its length. The flat SP with decreasing elevation produces a neutral Ce (0.1 mV/m).

Zone K starts below Ketetahi hot springs where the SP profile slowly rises, resulting in a negative Ce (-3.2 mV/m).


Figure 3.2 Profile 2. A) Topography, SP, 10 cm soil temperature, CO2 flux. Black dashed lines mark zone

boundaries labelled with letters. Red dashed line separates Ce profiles in B. The shaded part indicates data presented in profile 1. It is shown here again to provide better context for Profile 2. B) Ce profiles colour coded by zone with Ce value labelled where calculated.

3.2.3 Profile 3

Profile 3 (Figure 3.3a) is a semi-circular loop extending from Blue Lake, over Red Crater and down into the Oturere Valley to the start of Profiles 1 and 2. From 0 to 3700 m the SP profile has a relatively flat appearance but within that area are several low amplitude SP anomalies (± 200 mV). A large positive anomaly (1100 mV) is mapped from 3700 m to the end of the profile and is part of the large anomaly mapped at the start of Profiles 1 and 2.

We divided Profile 3 into 6 zones based on major changes in elevation or SP. Several soil temperature and CO2 flux anomalies are mapped as the profile traverses the hydrothermally active Red Crater.


Zone L traverses from Blue Lake, across Central Crater to the base of Red Crater. SP measurements show little change and because of the flat topography no Ce is calculated (Figure 3.3b). Soil temperatures are around 5°C-6°C and CO2 flux anomalies up to 200 g/m2/day are measured.

Figure 3.3 Profile 3. A) Topography, SP, 10 cm soil temperature, CO2 flux (note log scale). Black dashed lines

mark zone boundaries labelled with letters. Red dashed line separates Ce profiles in B. B) Ce profiles colour coded by zone with Ce value labelled where calculated.


Zone M covers the north flank of Red Crater and has a mostly flat SP profile resulting in a near neutral Ce (-0.3 mV/n). Soil temperatures up to 22°C and CO2 flux anomalies up to 1900 g/m2/day are measured. The highest soil temperatures and strongest soil gas flux anomalies are measured at the top of Red Crater.

Zone N covers the south flank of Red Crater down to South Crater. Similar to Zone M, the SP profile is largely flat resulting in a neutral Ce (0.2 mV/m). Soil temperatures on the south flanks of Red Crater are 5-10°C and CO2 fluxes of 2-6 g/m2/day are measured.

Zone O spans the steep descent into Oturere Valley from South Crater. SP increases from -100 mV to + 200 mV resulting in a negative Ce (-2.1 mV/m). Soil temperatures decrease from 10°C to 5°C and CO2 fluxes of 2-6 g/m2/day are measured.

As the topography in Zone P gradually descends Oturere Valley the SP signal decreases from 200 mV to 0 mV producing a positive Ce (+2.3 mV/m). A fresh water spring occurs at 3500 m along profile; we use this observation to calculate the depth to water table in the Oturere Valley in Section 4. Next to the springs weak CO2 fluxes of 4 g/m2/day are measured. Soil temperatures are around 5°C and CO2 fluxes up to 25 g/m2/day are measured at the start of Zone P.

Zone Q traverses across Oturere Valley and a large SP anomaly (+1100 mV) is mapped. Soil temperatures remain around 5°C and no CO2 flux is measured. We discuss the cause of this anomaly in Section 4.

3.3 SELF-POTENTIAL VS ELEVATION “CE” MAP

Figure 3.4 shows the Ce value distribution in map form. Ce anomalies are colour coded so that red dots represent SP signals of hydrothermal origin and blue dots represent SP signals related to groundwater flow. At Mount Tongariro hydrothermal zones are distinguished by near neutral Ce values between -1 and +1 mV/m whereas ground water flow zones have larger scale (either positive or negative) Ce values.

Three areas of near neutral Ce are identified, two of which correlate with the Ketetahi and Red Crater hydrothermal surface features. The third (Zone D) is on the south flanks of North Crater. No CO2 flux or soil temperature readings were made in this area and there are no hydrothermal surface features so it is unclear if this anomaly relates directly to hydrothermal activity. The SP data are also somewhat noisy in Zone D so we don’t make any further interpretation of this anomaly.

Strong negative Ce anomalies are mapped below Ketetahi hot springs and on the steeper flanks to the south and east of Red Crater and mark out zones of down-flowing groundwater.

Strong positive Ce anomalies are mapped in Oturere Valley and to the east of North Crater. The Oturere Valley anomaly coincides with a fresh groundwater spring whilst the North Crater anomaly is part of a long wavelength V shaped SP anomaly and the positive Ce here represents one limb of the V and is not thought to relate to upwelling groundwater. The profile east of North Crater is not perpendicular to the topographic slope so Ce calculations are likely to be less accurate in this area.


Figure 3.4 Ce distribution (coloured dots) plotted with the SP map overlain on a shaded relief map of Mount

Tongariro. Red dots represent hydrothermal associated anomalies, blue dots represent groundwater flow anomalies. No Ce was calculated in areas represented by white dots.


4.0 INTERPRETATION

The most obvious result from this survey is the lack of a large positive, central (“W” shaped) anomaly under the high standing part of the volcano, as is commonly seen in strato-volcanoes with active hydrothermal systems (Finizola et al., 2006). The irregular shape of Mount Tongariro compared to classic strato-volcano cones may also mean a single symmetric drainage system that would produce the classic W shape SP anomaly is less likely to form. Rather the SP distribution more closely fits the “no anomaly” type of Aizawa (2008) and Aizawa et al. (2009). In their model reduced SP signal associated with active hydrothermal systems is caused by a low resistivity zone of altered rock at the top of the hydrothermal system. The electro-kinetic effect is further reduced by the presence of low pH fluids. Low pH fluids exist at Red Crater and Ketetahi (pH 2-6 GeoNet unpublished data) along with areas of altered ground, hence application of this model to Tongariro seems applicable.

The heterogeneous nature of the SP profiles (lots of small wavelength anomalies, within the largely flat trend) could be the result of the complex cone building history of Tongariro, where the short wavelength anomalies might indicate the locations of lithological boundaries and other structure within the volcano. These structures may be preferential pathways for fluid flow and hence cause localised SP anomalies. The negative Ce anomalies are mostly in areas of steep topography and are probably caused by down-flow of meteoric fluids outside of the hydrothermal areas, producing the SP “topographic effect”.

We divide further discussion of SP anomalies into those associated with hydrothermal surface features or caused by groundwater flow.

4.1 HYDROTHERMAL FEATURES

The hydrothermal surface features (Ketetahi and Red Crater) are only discerned in the SP data when the Ce is calculated removing the effects of topography. These features map as near neutral Ce anomalies (-1 to +1 mV/m), compared to the stronger Ce anomalies seen in areas associated with groundwater flow. The near neutral Ce values are caused by only a small change in SP over a large change in altitude. The cause of the small SP signal is discussed below.

4.1.1 Red Crater

Red Crater hosts a range of surface features; mostly steaming ground and weak fumaroles with temperatures of around 92°C (GeoNet unpublished data) and occasional small springs. The SP signal over Red Crater is weak and several explanations are put forward for this. Steam is unable to produce SP (Tyrand and Marsden Jr, 1985), and therefore a vapour dominated reservoir has less ability to generate SP from the electro-kinetic effect. Secondly the condensate layer is likely to be low pH, for example Emerald Lakes have a pH of 3-4 (GeoNet unpublished data). This may reduce the zeta potential of the rocks so that there is no strong SP signal even if there is significant fluid flow. Thirdly, the ground in the surface feature areas contains a lot of clay minerals which will lower the overall resistivity of the subsurface and again reduce the SP signal. A combination of these factors may account for the low SP anomaly at Red Crater. By analysis of the SP vs elevation gradient the extent of the hydrothermal activity can be determined. The hydrothermal area mapped by SP is 1.1 km long and a minimum of 250 m wide (red hatched area on Figure 3.4), covering a minimum area of 0.13 km2, and sits above an altitude of 1650 m. The rocks in this area are likely to be weakened from hydrothermal activity and may pose a landslide threat on the steeper slopes.


4.1.2 Ketetahi hot springs

The Ketetahi hot springs were the largest hydrothermal features on Mount Tongariro prior to the Te Maari eruptions of 2012. The springs have a pH of 2-3 while the large “black cauldron” hot pool has a pH of around 6 (GeoNet unpublished data). Ketetahi fumaroles are around 120°C. Similar to Red Crater, the SP anomaly over Ketetahi is indistinct when viewed on its own. When the SP vs elevation gradient is calculated the hydrothermal area is discerned from the positive gradient around the springs. The same reasons as applied to Red Crater are likely for the subdued SP anomaly around Ketetahi. Low pH of fluids, vapour dominated reservoir beneath and altered, low resistivity ground. Walsh et al. (1998) conducted DC-resistivity soundings in the Ketetahi area and modelled a 10 Ωm surface layer, which would have the effect of reducing the SP generated in this area. The SP profiles traversed the edges of the surface features so a stronger SP anomaly may have been obtained had a line been run through the central part of the area.

Based on SP vs elevation gradient the Ketetahi hydrothermal area is located between 1850 m and 1400 m above sea level and extends to a minimum width of 700 m, covering a minimum area of 0.57 km2 (Figure 3.1 red hatched area). The fault cutting across North Crater appears to mark the south eastern boundary of the hydrothermal system, separating the hydrothermal zone from the groundwater flow to the south. This boundary is seen in both the raw SP map (Figure 2.2) and in the Ce map (Figure 3.1).

The rocks in the Ketetahi area are likely to be altered and weakened from hydrothermal activity and may pose a landslide threat on the steeper slopes.

4.2 GROUNDWATER FLOW

Two anomalies are mapped in the Oturere Valley; a large positive SP anomaly and a positive Ce anomaly. Both these anomalies relate to the groundwater flow in the valley and are discussed in more detail below.

4.2.1 Oturere Valley positive SP anomaly

The main SP feature of Oturere Valley is a large (+1100 mV), mid-wavelength SP anomaly (Zone Q Figure 3.3). This anomaly is well away from the main hydrothermal surface features in an area of flat topography, and there is no evidence from CO2 flux or ground temperature measurements that hydrothermal fluids are upwelling near the surface which could cause such a large anomaly. The anomaly appears to be confined laterally to the width of Oturere Valley, although its full extent down valley is not established.

We discuss some possible causes of the SP anomaly using data from previous studies. Walsh et al. (1998) proposed a condensate outflow layer at around 300 m depth, to account for the presence of a 20 Ωm basement layer modelled under this part of the Oturere Valley. Their proposed outflow layer extends from the high standing part of the volcano (Red Crater, Central Crater etc.), under Oturere Valley, and also extends under Ketetahi to the north. Such an extensive outflow would be expected to generate a large long wavelength SP anomaly on both sides of the volcano which is not supported by the SP data collected.

Cassidy et al. (2009) modelled a similar deep, low resistivity layer in the Tama Lakes Saddle (to the south of Mt Ngauruhoe) that correlated with lower density and non-magnetic rocks modelled from gravity and aeromagnetic data. They interpreted these anomalies to be caused by mixed volcanic deposits overlying Tertiary sediments. We prefer the explanation of Cassidy et al. (2009) to that of Walsh et al. (1998) to account for the regionally extensive, deep low resistivity layer mapped in both the Oturere Valley and the Tama Lakes Saddle, as there is no evidence for a wide spread, deep condensate outflow from our SP data.


We now discuss some alternative explanations for the SP anomaly, focusing on a more localised source. We propose that the Oturere Valley SP anomaly is caused by localised fluid flow in the Oturere Valley.

To investigate the presence of an aquifer beneath Oturere Valley, we modelled a current source (representing an aquifer) using the method of Telford et al. (1990) for a vertical charged dipole rod, Eq. (2), Figure 4.1. While the use of a vertical dipole rod is probably not the most appropriate source it does allow a first order calculation of the depth to the potential source.

V = q 1

𝑥2+𝑧12− 1

𝑥2+𝑧22 (2)

Where V is the potential produced, Z1 is depth to the top of rod, Z2 is the depth to base of rod, x is the distance along profile, and q is the charge of the rod.

Figure 4.1 Modelled (green) SP vs observed (blue) for a vertically charged dipole at a depth of 175 m with a

current source of 89A.

In the model the length of the rod is made several times greater than the depth to the top of the rod, so that the rod appears as a monopole from the surface (to replicate the single positive anomaly observed). We determined a best fit model using a vertical rod with a depth to the top of 175 m and a current source of 89 A. The range of possible models (using combinations of depth and varying source strengths) is from depths of ~150 m (67 A) to ~225 m (125 A).

4.2.2 Oturere Valley fresh water spring

The positive Ce feature (+2.3 mV/m) in Profile 3 – Zone P is away from hydrothermal surface features, but does correlate with the location of a fresh water spring observed in the field and a small CO2 flux anomaly. The spring provides a known depth to the water table from which the depth of the water table in the nearby area can be calculated, based on the relationship between vadose zone thickness and SP. We use the method of Zlotnicki and Nishida (2003) who show that if the altitude of point P is h, the altitude H of the equipotential surface between the vadose zone and the water saturated zone (i.e., the water table) is:

𝐻 = ℎ − 𝐶∆𝑉(𝑃,𝑂) − 𝐸(𝑂) (3)

Where O is the base station location and P the measurement point. C is a constant (~10 mV/m). ΔV is the SP value between the base and the measurement point. E is the known depth to the water table (1 m in this case as the base electrode was approximately 1 m above the spring).


Figure 4.2 shows the results of the depth to water table calculation in the Oturere Valley.

Figure 4.2 Graph of topography (green), water table altitude (blue) and SP (red). Altitude of water table is

calculated from Eq. (3). The location of the current source modelled in section 4.2.1 is shown as an orange dot.

The current source model and water table depth calculations are discussed in the following section to give an overall view of the hydrology of Oturere Valley.

4.2.3 Oturere Valley hydrology

As the topography descends from Red Crater into Oturere Valley the SP signal increases resulting in negative Ce (Figure 3.3 Zone O) – typical of down-flowing fluids outside a hydrothermal area. Once the flatter part of Oturere Valley is reached (3000 m along profile) groundwater flow continues remains at a depth of 10-15 m until the aquifer intersects the surface resulting in a spring at 3500 m. The shallowing of the aquifer towards the spring may indicate the presence of an impermeable layer beneath the aquifer that prevents the aquifer from being able to continue to follow the topography down slope underground. The shallow impermeable layer negates the SP topographic effect which would otherwise have produced an increasing SP signal. From 3500 m to 4000 m the water table follows the terrain down slope.

As the profile enters the main part of Oturere Valley (4000 m along profile) and begins to traverse across the axis of the valley, the water table deepens initially to around 30 m and then deepens rapidly to 125 m below the surface as the main body of the lava flow in Oturere Valley is crossed. The deepening of the water table in this area could occur if the lavas are highly fractured and unable to hold water (due to fractures being too wide to sustain capillary action), in which case the water table is possibly representing the base of the lava flow, or a surface that is able to hold larger volumes of water.

To investigate the influence of the lava flow on water table depth we refer to the work of Stevens (2002) who used a digital elevation model (DEM) derived from NASA TOPSAR data to estimate the thickness of this lava flow. Their DEM allowed the lava flow volume and depth to be constrained by an estimation of the mean thickness of the flow margins, and from polynomial interpolation of the underlying valley form. Stevens concluded that the thickness of the lava where the SP profile crosses the valley was in the range 126-150 m. Therefore the calculated depth to water table is consistent with being at the base of the lava flow and hence the SP has effectively mapped the pre lava flow topography. As such the SP signal is simply causes by a downward flow of water beneath the permeable lava, resulting in a


negative SP / paleotopography correlation. The valley fill underlying the lava flow is thought to contain glacial till which could be suitable for holding such an aquifer.

The modelled current source depth is therefore broadly similar to the depth of water table calculated independently (section 4.2.2) and also to the thickness of the lava flow calculated from TOPSAR data. Limitations of the model notwithstanding, the depth of the current source may represent the thickest part of the aquifer, whereas the water table depth calculation models the thickness of the unsaturated zone (i.e., the depth to the top of the aquifer). The second measured peak at around +500 m in Figure 4.1 was not modelled but could represent a second SP source or reflect a change in the paleotopography beneath the lava. In comparison, Hase et al. (2005) modelled a current source of 300 A at a depth of ~1.5 km to produce a 800 mV SP anomaly on Aso volcano. A DC resistivity sounding not far from this study area from (Walsh et al., 1998) showed a 250 m thick, 80 Ωm layer at a depth of around 100 m beneath a 5000 Ωm surface layer. This 80 Ωm layer could represent the water table inferred from the SP measurements, whilst the 5000 Ωm layer represents the lava flow.

As discussed in the introduction SP generation is also dependent on the resistivity of rocks. Following ohms law, a resistive rock can produce a higher voltage difference. The thick resistive (5000 Ωm) lava flow that fills Oturere Valley will provide a lateral change in resistivity as the flow is traversed, which will enhance the SP signal generated by the fluid flow beneath the lava.

Figure 4.3 shows a schematic of the inferred hydrology of Oturere Valley.

Figure 4.3 Schematic of Oturere Valley hydrothermal and ground water flow. Ce Zones and values (mV/m) are

labelled along the top.

4.3 EAST TONGARIRO NEGATIVE ANOMALY

We infer the strong negative “V – shaped” anomaly (-600 mV) mapped on Profile 2 on the northeast slope of Tongariro to result from down-flowing meteoric fluids. The slope traversed by the profile is at the head of a large catchment with steep topography. The small V shaped anomaly mapped over North Crater in Profile 1 correlates with a mapped fault (GNS Science Active Faults Database) and may represent an area of enhanced permeability resulting in down flow of fluid (and hence negative SP) around the fault plane. V-shaped anomalies are commonly interpreted to be a manifestation of the topographic effect generated by hydrologic flow without hydrothermal upwelling (Aizawa, 2008). Thus the location of this anomaly would define the edge of the hydrothermal zone.


5.0 FUTURE WORK

This SP survey has delineated the Ketetahi and Red Crater hydrothermal systems when analysis of SP vs elevation is applied and has mapped areas of groundwater flow away from hydrothermal areas. We recommend that the remainder of the Tongariro massif be surveyed to complete the SP mapping. This includes extending the Oturere Valley profile to further investigate the extent of the strong positive anomaly in this area. We recommend an additional profile extending from Central Crater, heading west to the south of North Crater, and looping south into Mangatepopo Valley to cover the west flank of the volcano. If feasible we recommend a survey of the Te Maari area to investigate the nature of the hydrothermal fluid flow around the site of the 2012 eruptions.

Further analytical work will include quantitative modelling of the SP to obtain a fluid flow model once full coverage of the massif is obtained. This model will account for resistivity distribution and we recommend measurements of zeta potential of rocks to further constrain the model.

6.0 CONCLUSIONS

Mount Tongariro lacks a well-defined positive (or W shaped) SP anomaly under the high parts of the volcano that is commonly found on strato volcanoes with active hydrothermal systems, but rather has a relatively flat SP distribution over the central part of the volcano. There is a strong short-wavelength heterogeneity to the SP distribution which may reflect the complex construction history of the Tongariro massif, especially around the North Crater area. Most of the SP anomalies mapped relate to localised groundwater flow rather than to hydrothermal features.

Subtle SP anomalies are mapped around Red Crater and Ketetahi hot springs when SP is compared to elevation. The lack of strong SP anomalies in known hydrothermal areas could be influenced by a number of factors, including the low pH of hydrothermal fluids, and the low resistivity of altered rocks. Vapour movement alone is not sufficient to generate SP and so the anomalies must be generated by movement of fluids within the capping condensate layer. SP v elevation analysis successfully mapped areas that may pose future landslide hazards, from the presence of hydrothermally weakened rock.

SP successfully mapped an area of fresh water discharge in the Oturere Valley. We interpret a large positive anomaly in the Oturere Valley as due to the flow of water beneath a permeable, thick and electrically resistive lava flow, however the full extent of this anomaly needs to be further investigated. We interpret a large negative SP anomaly on the NE flanks of North Crater as down-flowing meteoric fluids, which helps to delineate the edge of the hydrothermal zone. The south eastern boundary of Ketetahi hot springs may be fault controlled as determined by the correlation of the SP distribution and a mapped fault.


7.0 ACKNOWLEDGEMENTS

This study would not have been possible without the support of Harry Keys at the Department of Conservation.

The Iwi groups associated with Tongariro National Park are thanked for their permission.

This report benefited greatly from reviews by Sophie Pearson, Tony Hurst and Lauriane Chardot.

This study was funded by GNS Science program GHZ Innovations in Volcanology, New geophysical techniques.

Special thanks to the hard work put in by Veronica Chiarini and Oliver Beer who assisted with SP, CO2 flux and soil temperature data collection.

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