Resistivity Structure from Magneto-Telluric Soundings

2 New Zealand Geothermal Workshop

RESISTIVITY STRUCTURE FROM MAGNETO-TELLURIC SOUNDINGS

C.J. BROMLEY

Wairakei Research Centre, Institute of Geological and Nuclear Sciences

SUMMARY - Magneto-telluric resistivity soundings have recently been made at 23 sites between the Tongariro and Tokaanu geothermal fields, in the central Island of New Zealand. The resistivity structure, derived from layered interpretation of the soundings, suggests that these fields are not hydrologically connected. They are separatedby a graben or depression, which has a maximum depth of about 1.2 near Lake Rotoaira, and is filled with Tertiary mudstone, sandstone, siltstone and Quaternary volcanic alluvium. The basement is high resistivity greywacke. Gravity modelling supports this interpretation. Both Tongariro and Tokaanu fields have high-standing,low-resistivity ohm-m) steam condensate aquifers, associated with intense hydrothermal clay alteration. Both fields also have northern structures,and evidence of increasing resistivity at depth where higher temperature alteration may be present

1. INTRODUCTION puzzling, and led to conjecture about whether the high chloride and boron springs at 20 Tongariro and Tokaanu geothermal fields are to the north, might be the surface expression located between Lake Taupo and Mt Ruapehu in of an outflow Tongariro. Such are the central Island of New Zealand. They commonly found around geothermal systems with are both situated in steep terrain associated with elevated volcanic settings in the Philippines and andesite volcanoes (Tongariro - 1960 m and Indonesia. Giggenbach (1996) discredited the - 1300 m). Access by vehicle is very idea of a link between Tongariro and Tokaanu on and this has limited their coverage by geochemical grounds. However, the Tokaanu traditional DC resistivity traversing methods. springs do have geochemical similarities to the Between these geothermal fields, which are about Waihi and Hipaua features to the west. 20 apart, there is another andesite cone Furthermore, the primary upflow for the Tokaanu- and a low-lying area occupied by Lake Waihi system is interpreted by Severne (1998) to Rotoaira (560 m). be closest to Hipaua, based on gas and isotope data. Geophysical studies reported by Hochstein and Bromley and later by Walsh et Resistivity surveys conducted previously in the depicted the resistivity structure of Tongariro as Tokaanu-Waihi area (Banwell, 1965, Reeves and consisting of a low resistivity condensate layer Ingham, 1991, Risk et 1998) confirm that the overlying a more resistive vapourdominated Waihi and Hipaua features reservoir. In the 1979 model, a hot brine belong to a single geothermal field with an area of layer was also inferred to exist at about sea level. at least ohm-m). A video infra-red An underlying high resistivity basement was survey of the thermal features contained within interpreted to indicate unaltered hot rock. The this area is reported in Bromley and Mongillo 1998resistivity model was constrained by only 10 (1991). The parent reservoir fluid for this system shallow penetrating DC soundings and 4 is interpreted to have a chloride content of 2400 soundings, of which two (dating from 1976) had a and temperatures of 240-260°C based on very limited measurement frequency range. Na-K-Mg geothermometry (Severne, 1998). The However, the overall resistivity structure is deep fluids undergo boiling at about 190°C. The considered to be consistent with the hydrological resulting, more concentrated brine, rises and setting of a condensate-capped, to the surface at Tokaanu (370 m asl) resulting in system. discharges with up to 3300 chloride. Steam and gas rise to the surface at Hipaua, and The evidence for magmatic fluids, the isotopic produce acid condensates which mix with characteristics of the discharged steam groundwaters. Mixtures of (Giggenbach, 1996) and the history of volcanic groundwaters and deep chloride fluids feed the eruptions from Tongariro craters also support lower elevation dilute springs at Waihi Village, classification of this field as a volcanic and the bore waters in Tokaanu township. geothennal system. There are no deep boreholes in either the Tongariro have high boron Tokaanu-Waihi or the Tongariro geothermal concentrations, but there are no chloride fluid fields to test these conceptual hydrological discharges around the flanks of Tongariro, despite models. However, other geophysical surveys, steep topographic relief. This was initially including gravity, magnetic, seismic, 119 t microearthquake and resistivity, have contributed compared with about -20 at Lake Rotoaira. to our current understanding of the 2D modelling of the Tama Lakes geological, thermal and structural setting in this profile, when combined with magnetic modelling, area. shows a volcanic sequence of andesite lavas (2.58 and pyroclastics (1.8 overlying 2. GRAVITY greywacke basement at about 500 m asl, but cut by a trench of andesite. However, it was noted Interpretation of residual gravity anomalies that the density differencebetween greywacke and (Stern, 1979) implies the presence of a graben or andesite lava is small. This makes it difficult to depression, with predominantly low reliably determine the basement depth. data density material, linking the caldera of Lake shows the presence across the Tama Lakes profile Taupo with the andesite volcanoes in Tongariro of moderately low resistivities (30 to 60 ohm), in National Park and the sediment-filled Wanganui a layer at about 400 mto 1.5km depth. Although Basin to the south. This link was suggested Zeng (1996) interprets this to indicate fractured by Rev. Richard Taylor in his 1855 It was andesite, saturated with fluid beneath first modelled by of the DSIR NZ the volcanic vents at Lakes, it is also Geophysical Survey, (in Gregg p 93) as a possible that a layer of Tertiary mudstones could narrow graben within greywacke basement be contributing to this moderately low resistivity centred beneath Lake Rotoaira (down to anomaly. and the volcanic centres of Tongariro and Ruapehu (down to about sea level). In the Following a review of the information vicinity of Lake Rotoaira, the 1960 model for the above, a simple two-dimensional used an average density contrast of -0.47 model was constructed using the residual gravity for the infilling Tertiary sediments data (from the GNS database) along a NW-SE within the Taupo-Tongariro graben are profile through Lake Rotoaira The end points of predominantly early Pliocene to late Miocene in the profile are located on greywacke. Figure 1 age, with average densities of about 2.2 shows the model, and the fit between observed This is deduced from Hunt (1980) using samples and calculated gravity values. The simplest collected near (south of Mt Ruapehu) and model consists of a about 1.2 deep and near of Tokaanu). 25 wide, of average infill density 2.2 within greywacke basement of The From gravity measurements, Sissons (1981) actual basement interface, rather than smoothly determined in-situ densities of the upper 300 mof dipping, is more likely to be a sequence of steps andesitic rock overlying the Tokaanu tunnel, that along trending normal faults, that straddle passes through flanks of Tihia, north of Lake Lake Rotoaira. Gregg (1960) shows a number of Rotoaira. His best fitting model showed densities these normal faultsthat are of late Quaternary age increasing with depth from 2.14 to about 2.6 indicatingthat the graben has continued to subside at 300 m depth. in a long after deposition of Tertiary sediments. report on the geology of the western diversion Therefore the upper few hundred metres of the tunnels for the Tongariro Power Development, graben is likely to contain recent volcanic showed that a layer of Tertiary marine sediments, alluvium (eg. lahar deposits) or shallow lacustrine exposed to the west, is mantled near Lake sediments, whereas the deeper parts of the graben Otamangakau by about 100 m of Quaternary ring probably contain older beds of marine sediments. plain deposits (alluvium and derived from In the absence of subsurface samples from this Tongariro. A similar or thicker sequence of low graben infill, there is no reliable basis for density volcanic debris is likely to have helped differentiatingbetween these layers solely on the infill depression in the vicinity of Lake basis of density. Rotoaira. Additional useful information on the likely thickness of Tertiary marine sediments STUDIES through this graben comes interpretation of a seismic refraction survey across the southern ring Since 1986, several swarms of micro-earthquakes, plain lahars between Ruapehu and (Sissons with magnitudes typically less than 3.2, have been and Dibble, 1981). Basement greywacke located within the upper crustal rocks of the lies at elevations of about 200 Tokaanu-Waihi geothermal field and on the m m asl). This is overlain by about 500 m northern flanks of Tongariro geothermal field of inferred Tertiary sediments with velocities of et al, 1995, Walsh et al, 1998). The 2.2 to 3 The overlying lahar deposits have average focal depth is about 5 km which is slightly lower velocity (2 and thicken presumably within greywacke basement. These towards Ruapehu. swarms have been linked to the possibility of magma injection into dykes within Recent gravity measurements undertaken by Zeng the brittle crust. Such injection would provide a (1996) across the Tama Lakes saddle between source of heat that might sustain or progressively Tongariro and Ruapehu show a negative residual enlarge these geothermal systems. Some young gravity of -15 mgals at the top of this saddle, volcanic extrusions ka) have occurred

120 I I I I I I I I

E -10

Te Lake Rotoaira

Density Model

Figure 1. Gravity interpretationof a NW-SE profile through Lake Rotoaira, showing a 2D model of low density sediments infilling a

Kakaramea 20

87+

Poutu 21

Ketetahi

2739000 2741 000 2743000 2745000 2747000 2749000 Figure 2. Location map of magneto-telluric soundings in the Tongariro-Tokaanu area, labelled with rotational invariant apparent resistivities at 8Hz frequency. Lines show cross-section locations.

121 adjacent to Tokaanu and In the Tokaanu low resistivity area (5 ohm-m), a as well as at Tongariro (“e crater was suitable minimum distance (600 m) is achievable, historically active between 1869 and 1896). So it however, on the flanks of the andesite volcanoes is possible that magma injection in basement (50 to 250 ohm-m) minimum distances of 2 to fractures is still occurring. The 5 km can be much harder to achieve. For seismic velocity structure in this area is poorly example, six of the new soundings (Figure 2) known. A model used for determining earthquake that are located within 2 km of live power lines hypocentres, that assumes high velocity around Lake Rotoaira, have anomalous 50 greywacke (6 underlying about 2 km of resistivities (2 to 30 times higher than values at low velocity sedimentary and volcanic formations adjacent frequencies, for the E-field dipole (3.5 is consistent with the graben model oriented parallel to the source). described above. 4.1 New MTSoundings 4. MAGNETO-TELLURICRESISTIVITY New tensor magneto-telluric resistivity soundings Early magneto-telluric soundings made in the have been conducted over the past two summers vicinity of Tongariro were limited to low in the Tongariro-Tokaanu area. Locations of the frequency (long period), deep penetration 23 sites are shown in Figure 2. A range measurements. For example, a sounding by of 8 to 0.02 was recorded, allowing a Marriot located a few kilometres west of theoretical penetration depth range (for 20 Lake showed increasing apparent m) of about 20 m to 10 km. In practice, however, resistivities (30 to 150 ohm-m) at periods of 2 to weak natural signal strength, particularly in the 300 seconds. This is consistent with high frequency bands 0.1 to 1 and 1 resistivity greywacke underlying a thick surface to 3 limits the penetration depth range. layer of moderate resistivity (30 ohm-m). Where data is of poorer quality (low signal to soundings reported by Hochstein and Bromley noise ratio) the resistivity models are (1979) on Tongariro were also limited to long less well constrained. Sometimes, longer site period data seconds) and showed occupation times helped to improve data quality increasing resistivities at depths of several (by and averaging). Most sites were kilometres. However, resolution of the resistivity occupied for about three hours. structure in the upper 2 km was hindered by the lack of higher frequency data. Measurements were made with orthogonal horizontal of electric field dipoles (Ex, Ey) Later soundings conducted at Tokaanu by Reeves and magnetic coils Hy, The data and covered a wider frequency acquisition was accomplished with a Zonge spectrum (1 to 0.1 but were to the 32 receiver. This receiver uses cascade low lying north-eastern edge of this field. These decimation, and averaging of fourier soundings were considered suitable for transformed cross and auto-power spectra of the penetration depths of about 100 to 1000 m. and harmonics, to obtain amplitude and dimensional modelling of the 13 soundings phase measurements of the electric and magnetic showed a sharp eastern resistivity boundary fields. Using robust processing mode, data are (possibly fault controlled) in the vicinity of the accepted or rejected according to coherency and Tokaanu hydro-electric power station, and a outlier limit tests: typically a coherency limit of deeper north eastern boundary near the Tongariro 0.7 and outlier rejection limit of two times the River. Within the geothermal field, resistivitiesof median. For all stations, a notch filter to attenuate about 3 ohm-m are underlain by higher 50 (and its odd harmonic frequencies) was resistivities. applied to the incoming signals. This prevents saturation at the gains needed to record natural 50 magneto-telluric resistivity measurements, source data. A signal pre-conditioner (SC-8) also with an uncontrolled artificial source (power pre-amplifiesthe electric field voltages, and filters lines), have also been made at various locations at out and SP drift. Resistivitiesand Tokaanu and Tongariro et al, 1998). phase differences are calculated using standard Unfortunately, many of the sites are affected by Cagniard and plotted against frequency proximity to the power line source (creating for interpretation in terms of layered resistivity “near-field” effects). They also have linearly models. polarised electric and magnetic fields, tensor analysis meaningless. The orientations of One dimensional modelling of all soundings is the resulting scalar resistivities (maximum E field undertaken an iterating best-fit algorithm to direction) can be influenced by both the source minimise the RMS residuals between observed orientation and any local resistivity anisotropy or and calculated resistivities, and phase differences. topography. A guide for the minimum distance to The iteration is weighted towards the average the nearest source to avoid effects is values with the lowest standard deviations, and is four times the skin depth where D = 503 also repeated numerous times fi-om different SQRT (Zonge and Hughes, 1988). starting models to determine the overall 122 in the optimum layered model (Monte structure at depth is complicatedby the 2D effects Carlo method). described above, however there is a suggestion of a resistive basement (approximately 100 ohm-m) Two-dimensional cross-sections are compiled at about -1800 m beneath North Crater and from the layered models. These are shown in rising to the north. A small outflow of diluting Figures 3 and 4. Full 2D inversion of these cross- condensate fluids (15 ohm-m) is interpreted to sections is not justified because the sounding occur downslope of Ketetahi, but these fluids locations, being designed for regional coverage, never reach the surface, and appear to diminish are not spaced closely enough to properly towards the base of the volcano. constrain any inversion of resistivity boundary structures. However, soundings located close to In the depression occupied by Lake the boundaries inferred fi-om the cross-sections often sounding interpretations suggest a sequence of reveal evidence of 2D or 3D effects (diverging lithologies, based on their probable resistivity resistivity curves at low frequencies). In these signature: (a) near-surface andesite (150- cases, the layered interpretation at depth, is treated 300 Quaternary volcanic cautiously. and Tertiary (40-50 (c) Tertiary mudstones (23-28 (d) greywacke Static-shift distortion effects (significant parallel basement (500-750 These resistivity values displacement of Ex or Ey oriented resistivity are consistent with many in-situ measurements curves) were observed at six stations, and made on such lithologies. divergence of curves at low frequency at four stations. Shallow DC resistivity measurements Although no on depth were applied in of 25 m or 50 m) were made at many of the the resistivity modelling process, the similarity in sites, using the orthogonal Ex and Ey dipoles. the level of the interpreted greywacke basement These generally showed isotropic resistivity near fiom soundingsand from gravity modelling is the surface. There was no correlation between quite convincing (see Figures 1, 3 and 4). Both shallow DC resistivity distortions and either static models independently show basement at about shift or curve divergence effects, suggesting that -600 m centred in a graben beneath Lake the distortion effects originate from lateral Rotoaira. The inferred Tertiary mudstone layer resistivity inhomogeneity outside the array, or at that sits on top of basement has a relatively depths greater 25 m. For example, soundings uniform resistivity ohm-m) and a thickness located at the centre of South Crater of up to 1 Interpretation of this layer as and Central Crater, on flat terrain, both showed mudstone is supported by several in-situ strong divergence at frequencies below 2 (see resistivity measurements on mudstone to Fig. 5). The lowest resistivities, in each case, were the south of Tongariro National Park. These oriented in the Ex direction (110”). This could be include values of 22 and 23 ohm-m interpreted to indicate the presence of a strong 2D soundings at and in Dawson et resistivity anomaly, at least 500 m the 1982). soundings, and oriented approximately NNE. A likely candidate structure is the linear vent which Interpretation of the five new soundings probably underlies the hydrothermally active Red between Kakaramea, Tihia and Hipaua suggests Crater and Emerald Lakes, on the SE side of that the Tokaanu geothermal system extends Central Crater. west than was implied from the previous shallow resistivity measurements et 4.2 MT Resistivity Interpretation 1998). A low resistivity anomaly (4 ohm-m) encompasses Kakaramea, borders the Tihia The cross-sections in Figures 3 and 4 illustrate the saddle, and reaches an elevation (about interpretation of most of the soundings in this similar to the highest thermal features identified study. Figure 2 shows the location of the two by Bromley and Mongillo (1991). This suggests section lines, Tongariro to Tokaanu, and the presence of a perched layer of condensate from to Poutu Falls. Also plotted on fluid and hydrothermal alteration, as at Figure 2 are the apparent resistivities (rotational The “condensate” layer of low invariant) at 8 frequency for each station. This resistivity dips steeply to the north. At elevations represents an effective penetration depth (for 30 of about 400 the low resistivities are probably ohm-m) of about 700 caused by deep chloride water mixing with steam- heated groundwater, and outflowing to the springs A low resistivity layer of 2 to 5 ohm-m occurs at Waihi Village beside Lake Taupo (360 m). within Tongariro, between Red Crater and Similar low resistivity (3 ohm-m, from Reeves Ketetahi fumarole areas. The top of this layer is at and Ingham, 1991) occurs near the Tokaanu high about elevation, matching Ketetahi acid chloride springs to the east. A resistive basement sulphate springs. This layer is probably caused by (nominally 100 ohm-m) occurs beneath the two acid-condensate fluids, with associated intense soundings near Hipaua, but this is based on hydrothermal alteration, perched above a vapour relatively noisy data at low frequency. Additional dominated geothermal system. The resistivity soundings, with better data at frequencies 123 North

25 28

from layered models

Figure 3. Layered resistivity interpretations from soundingsalong Tongariro-Tokaanucross-section.

NW Otukou Lake Rotoaira Poutu SE

volcanics

mudstone 28 500

KEY 750 -Scale H -1000 1 km MT sounding location

from layered models

Figure 4. Layered resistivity interpretations for soundings along a NW-SE cross-section through Lake Rotoaira. Compare this with 10 of gravity cross-sections in Figure 1.

124 below 1 are needed in this area to improve the be explained as lateral resistivity anodes resistivity model of this part of the Tokaanu beneath the hydrothermally active Red Crater geothermal field. It is possible that the higher Higher resistivity layers observed in resistivity, at about sea level or deeper, is caused some Tongariro soundings at depths below sea by a transition to higher temperature alteration level may be caused by higher temperature (chlorite, illite, quartz) within a productive alteration (chlorite, illite, quartz) or low geothermal reservoir. This is mantled by a low permeability, unaltered hot rock. resistivity cap of intense clay alteration (smectite, kaolinite, etc). Such a model is commonly The Tokaanu geothermal system also has a high observed in explored geothermal fields hosted in standing, low resistivity layer ohm-m) volcanic lithologies. interpreted to indicate steam condensate fluids and clay alteration particularly beneath the eastern Finally, despite the probable existence of a flanks of Kakaramea and the northern side of connecting graben, is clear from Figure 3 that Tihia. This western extension of the resistivity there is no direct connection between the low anomaly has increased the known area of the field resistivity anomalies of Tongariro and Tokaanu to about 30 Underlying higher resistivities geothermal fields. Geophysical data therefore in some of the Tokaanu soundings may also supports the geochemical evidence that the two indicate higher temperature conditions (less fields are conductive clays). Additional are recommended in the south western sector of the 5. CONCLUSIONS Tokaanu geothermal field to better elucidate these resistivity structures, and help refine the Magneto-telluric resistivity soundings, recently conceptual model of the reservoir. conducted in the Tongariro-Tokaanu area, have been interpreted to indicate that these two 6. ACKNOWLEDGEMENTS geothermal fields are not hydrologically connected. There is no continuous low resistivity Stewart Bennie and Duncan Graham are thanked layer ohm-m) between them, so it is inferred for their untiring dedication during the difficult that the high chloride fluids at Tokaanu do not fieldwork, particularly on Tongariro. originatefi-om Tongariro. Charlotte Severne is also thanked for her assistance at some of the sounding sites, and for The two fields are underlain by a NNE trending productive discussions regarding interpretations graben, which is centred beneath Lake Rotoaira, of geophysical surveys at Tokaanu. This paper where it is about 1.2 thick. Basement is was by the NZ Foundation for Research interpreted to consist of high resistivity Scienceand Technology, Contract greywacke (500-750 ohm-m), and the graben infill mostly consists of a thick sequence of REFERENCES Tertiary marine sediments (predominantly mudstone, with an average resistivity of 25 ohm- Banwell, C.J., (1965). Tokaanu Geophysics. In: m). Overlying layers of sandstone, siltstone and Thompson, B.N., L.D., Quaternary volcanic alluvium (lahars) have a New Volcanology - Central resistivity of about 50 ohm-m. The infill material Region. DSIR Information Series 50: 152-155. is adequately modelled using an average density of 2.2 Bromley, C.J., Mongillo, M. (1991). video survey of Tokaanu-Waihi geothermal field. The Tertiary sediments are probably Late Proc Geothermal Workshop, p. 27-32. Miocene to early Pliocene in age and may extend south beneath Tongariro, to beyond Bromley, C.J., (1996). Tensor Ruapehu. It is suggested that a of these Resistivity Survey of Ruapehu. of NZ mudstones, 500-1000 m thick, may be Geophysical Society Symposium, Wairakei, 1996. contributing to the moderately low resistivities beneath Tama Lakes and to a low velocity, low Dawson, A.W., Bennie, S.L. (1982). resistivity structure beneath the southern flanks of Report on resistivity at sites on Railway. Ruapehu (Bromley,1996). Unpublished report, Geophysics Div. DSIR.

The Tongariro geothermal system has an Giggenbach, W.F.(1996). Are Tokaanu chloride extensive high standing (1500 m low waters the outflow from Ketetahi or Hipaua? resistivity ohm-m) layer, caused by intense Proc. NZ Geoth. Workshop, 18: 175-182. hydrothermal clay alteration from a steam condensate cap over a vapour dominated system. Gregg, D.R The Geology of Tongariro A small condensate outflow extends north Survey Bulletin but doesn’t reach the surface. Two- dimensional distortion effects on soundings Hochstein, M.P., Bromley, C.J. (1979). within Tongariro Central and South craters may Resistivity structure of the Tongariro geothermal 125 system. NZ Workshop p 20-29, Severne, C.M., (1998). Recent geochemical 27 investigations of the Tokaanu-Waihi geothermal field. Geothermal Workshop, p. Hochstein, M.P., S., J. (1995). 25 1-258. Earthquake activity beneath the Tokaanu- Waihi Geothermal System, Lake Taupo, NZ. Sissons, B.A. (1981). Densities determined Proc. Geothermal Workshop, p surface and subsurface gravity measurements. Geophysics, V. 46. No. 11, p 1568-1571. Hunt, (1980). Basement of the Wanganui Basin, onshore, interpreted from Sissons, B.A. and Dibble, RR (1981). A seismic gravity data. NZ. Jnl. of Geology and experiment southeast of Ruapehu Geophysics, v.23 :l.p 1-16. volcano. NZ Jnl. of Geology and Geophysics, V.

M.G. (1969). A magneto-telluric investigation of the North Island Volcanic Stem, T.A. (1979). Regional and residual gravity Plateau. Unpublished thesis, Victoria Univ. fields, central North Island, NZ. NZ Jnl. Geology of Wellington, and Geophysics, V. 22, No. 4, p

B.R. (1980). Engineering geology and Walsh, F.D., Hochstein, Bromley, C.J., construction, Western Division tunnels, Tongariro (1998). The Tongariro geothermal system (New Power Development. Unpublished NZ Zealand): Review of data. Proc. Geological Survey Report. NZ Geothermal Workshop, 17-324.

Reeves, R, Ingham, M. (1991). Electrical Zeng, Y. (1996). Geophysical Investigations of structure of the Tokaanu geothermal field Proc. the Subsurface structure of Tongariro Volcanic NZ Geothermal Workshop ,p Centre, New Zealand. Unpublished Thesis, Victoria University of Wellington Risk, G.F., Caldwell, T.G., Bennie, S.L.,Severne, C.M., Bibby, H.M., (1998). Recent resistivity Zonge, K.L., Hughes, L.J. (1988). Controlled measurements at geothermal source audio-frequency magneto-tellurics, in field. Proc. 20* NZ Geoth. Workshop, methods in Applied Geophysics S.E.G.

...... :...... : ...... W ...... 1 ...... 100 ...... a ...... 3 1On ...... , ...... 1 4 16 64 256 10 .

...... 1 1 ...... i...: ...... I I I I I I I I 1 4 16 64

NSAHT DATA 98.2 98

A: 19. 19.2

1/16 1/4 1 4 16 4K

26 1.71

Figure 5. Example of MT resistivity data at Site 19 (Tongariro Central Crater), using a correlation coefficientfilter Curves A and B are oriented magnetic east and north. 126