U.S. DEPARTMENT OF THE INTERIOR 

U.S. GEOLOGICAL SURVEY

A Preliminary Gravity Survey of the Kailua-Kona Area, Hawai'i, for Delineation of a

Hydrologic Boundary

by

Jim Kauahikaua1 , Ka'eo Duarte1 , and James Foster1 '2

Open File Report 98-110

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey 

editorial standards or with the North American Stratigraphic Code. Any use of trade, firm, or 

product names is for descriptive purposes only and does not imply endorsement by the U.S. 

Government.

!U.S. Geological Survey, Hawaiian Volcano Observatory, Hawaii National Park, HI 96718. 

2now at University of Hawaii at Manoa, 2525 Correa Rd., Honolulu, HI 96822.
ABSTRACT

Gravity values were measured on ten profiles with the purpose of finding a hydrologic boundary 

responsible for impounding ground water tens of meters above sea level. Small gravity variations 

were found superimposed on the larger Hualalai anomaly and are interpreted, in one profile, to be 

due to a thin, west-dipping mass. Geologically, this may be one or more unusually dense lava 

flows that could be relatively impermeable.

INTRODUCTION

The Kailua-Kona area of west Hawai'i island is currently supplied with ground water from 

several wells drilled into a basal lens within a region extending no more than 8 km from the 

coastline and at elevations less than 400 m. This supply is expected to be augmented soon with 

ground water from several wells drilled recently at elevations above 400 m. Since 1990, these 

inland wells have demonstrated that water-table elevations increase abruptly above the 400-m 

elevation contour and that potentially large, new supplies of ground water are available for the 

Kailua-Kona region (Clark, 1994).
Figure 1: Shaded relief map of the Kailua-Kona area showing Hualalai vents (white polygons), Mauna Loa 

fissures (ticked lines), and water wells (filled circles with diameter proportional to the static water level within 

the well - highest level is about 150 m).
Kona Palisade

Hinalani 

Haokuni

Hienaloli 

Kaumalumal

Donkey 

Konawaena HS

10

Kilometers

Papa Ba

20

Although such a 

discontinuity in 

water table is 

expected when 

ground water, 

flowing from 

inland areas to the 

coast, is 

impounded by 

subsurface dikes 

(e.g., the dikeconfined high-level 

water in 

Macdonald and 

others, 1983), no 

such structures

Figure 2: Map of the Kailua-Kona area showing the locations of gravity were expected 

measurements and the ten gravity profiles (labeled). The corrected gravity values

are contoured at a 10 mGal interval. .. , .

parallel to the

Kailua-Kona coast beneath the 400 m elevation contour. Geologic maps of the area (Moore and 

Clague, 1991) show vent structures of Hualalai, the surface expression of subsurface dikes, 

farther north and east and nearly perpendicular to the coastline (fig. 1). Mauna Loa radial vents 

are also oriented approximately perpendicular to the coastline (Lockwood and others, 1988; Fig.
1).

Two geologic hypotheses have been suggested for the nature of the ground-water barrier 

responsible for the abrupt discontinuity in water-table elevations in Kailua-Kona. The first 

hypothesis requires a previously unknown complex of dikes whose associated vents are 

completely covered by later lavas. Hualalai activity then migrated northward toward its present 

summit. An alternative hypothesis is that the barrier is a normal fault scarp draped by later flows. 

Because permeability of a lava flow is a minimum from the top to the bottom flow surface 

(Takasaki and Valenciano, 1969), the drapery of flows over a near-vertical fault scarp would 

form a low-permeability zone for water flowing horizontally.

In order to obtain some new information about this ground-water configuration and the structures 

that confine it, we obtained 727 additional gravity measurements in the Kailua-Kona area. The 

use of gravity measurements for the investigation of subsurface ground-water configurations is 

unusual in Hawai'i and was undertaken for several reasons. Electrical geophysical techniques, 

used in previous studies (Kauahikaua and Jackson, 1983; Kauahikaua and others, 1985) to map 

the fresh water-sea water contact by the change in electrical resistivity, can not penetrate deeply 

enough beneath a highly developed area such as Kailua-Kona because of the many forms of 

electromagnetic interference now present. On the other hand, development poses no special 

problems for gravity surveying. Gravity variations are produced by subsurface density variations; 

because the change in water table elevation is nearly 150 m in places and the average void space 

in Hawaiian lava flows is 20-30 per cent (Strange and others, 1965, based on gravity
measurements), a discontinuity in water saturation of the subsurface rocks would likely produce a 

similarly abrupt change in surface gravity of as much as two mGals. In addition, dike complexes 

have been mapped with gravity measurements on other Hawaiian volcanoes because dikes are 

generally denser than lava flows (Kinoshita and others, 1963). An isolated dike complex should 

be detectable as a local gravity high. To distinguish the two hypothesized structures, subsurface 

dikes should produce a larger gravity anomaly than flows draping a fault scarp; however, the 

gravity anomaly produced by the impounded water alone should be significantly larger than the 

gravity effect of either barrier structure.

PREVIOUS WORK

A reconnaissance gravity map was completed in the early 1960s by Kinoshita and others (1963). 

It showed that Hualalai is the only one of the five subaerial volcanoes that make up the island of 

Hawai'i that does not have a gravity high associated with its summit. Instead, the Hualalai 

summit lies 12 km to the north of a low-amplitude gravity nose that runs parallel to the coast and 

about 8 km inland. The authors speculated that Hualalai may lie "on the north rift zone of an 

older volcano buried by Mauna Loa lavas" (Kinoshita and others, 1963). Later seismic refraction 

work (summarized in Hill and Zucca, 1987), interpreted with combined onshore and offshore 

gravity data, did not conflict with the earlier speculation: "The fairly narrow zone of elevated Pwave velocity beneath the coast is consistent with but not required by the seismic-refraction data. 

A comparable zone of elevated density is required, however, by the elongate gravity high that 

extends southward along the coast from Hualalai" (Hill and Zucca, 1987, p. 910). The most
detailed geologic mapping to date found no vent structures over the Hualalai gravity anomaly 

(see Figure 1; Moore and Clague, 1991; Lockwood and others, 1988; updated and compiled in 

Wolfe and Morris, 1996). Hildenbrand and others (1993) note magnetic lows parallel to the 

surface rift zones of Hualalai, possibly due to thick accumulations of trachyte.

Kauahikaua and others (1985) reported the results of five deep Schlumberger soundings, 

obtained at 210 m elevation, along with several very shallow electromagnetic soundings. They 

concluded that the area between the Donkey Mill and Konawaena HS profiles (fig. 2) appears to 

have the thickest basal ground-water lens at an elevation of 200 m. These soundings were 

interpreted as if the earth under them were horizontally layered, but we now know that they are 

less than 2 km west of the high-level water body. Hence the interpretation of a thicker basal lens 

may have been due to the lateral detection of the high-level water body and not simply to the 

ground-water conditions beneath the sounding. So far, the distribution of wells intersecting highlevel ground water and those intersecting basal ground water reveal the existence of this 

hydrologic barrier better than other data.

METHODS

Accurate measure of gravitational acceleration relative to a point of known absolute gravitational 

acceleration was obtained using LaCoste-Romberg gravity meters G-615 and G-721. Two gravity 

benchmarks were established in the Kailua-Kona area from very accurate gravity benchmarks on 

Kilauea Volcano that are measured frequently relative to the ISGN71 standard in Hilo, Hawai'i.
All subsequent gravity measurements were made relative to either or both of these Kailua-Kona 

benchmarks and were corrected for tides and instrument drift. Final uncertainty is estimated to be 

less than 0.05 mGals for points measured once and 0.03 mGals for points measured two or more 

times.
Kona EW Profiles

Complete Bouguer Values

$ 

E

270

260

250

240

230

-55 -50 -45 

Easting, km

-40

Figure 3: A composite plot of the reduced gravity data projected onto east-west profiles. The distance in 

parentheses after some of the profile names indicates a horizontal offset for the data from that profile for 

improved legibility.

The largest correction made to gravity measurements compensates for the known effects of 

elevation. Precise elevations were obtained in several ways for this study. The elevations of 

several sites along public roads were obtained from State and County Highways department
surveys and were reported with errors less than 0.1", or 0.3 cm. The elevation of municipal water 

storage tanks and valves were obtained from the County Department of Water Supply with 

similar reported errors. All locatable USGS benchmarks were used. In addition, ten east-west 

profiles across the 400 m contour were surveyed with a Pentax PTS-V electronic total station 

surveying instrument. Elevations are repeatable to better than 5 cm.

10
Kona Gravity Residuals

10

8

6

es . 

0 4

S

0

-2

 1"*. . 

 Kona Palisades 

o-o Hinalani

*-* Hao Kuni 

H I- Keopu 

A- A Hienaloli 

D-D Kaumalumalu

*-* Donkey Mill 

V-v Konawaena HS

* KKRC 

o-o Papa Bay

-55 -50 -45 -40 

Easting, km

Figure 4: A composite plot of gravity residuals obtained by subtracting a best-fit line from each of the profiles 

plotted in figure 3. The residuals are offset from each other vertically for legibility.

Elevations for several of the most remote sites, where total-station surveying was not practical, 

were obtained using Global Positioning Satellite (GPS) equipment with an accuracy of better 

than 1 meter relative to the WGS84 ellipsoid. Gravity data reduction procedures require an 

elevation relative to the geoid (an orthometric elevation) and not an elevation relative to the

11
WGS84 ellipsoid. The GPS elevations were converted to orthometric elevations using the 

following procedure: The height of the geoid relative to the WGS84 ellipsoid was estimated at 

the few sites where both an orthometric and ellipsoid elevation had been measured. Orthometric 

elevations were then calculated at each of our gravity sites by interpolating the geoid height and 

subtracting that interpolated value from the ellipsoid elevation.

All gravity values discussed in this paper have also been corrected for the effect of topography 

and bathymetry (Chase and others, 1980) from 1 to 150 km away from each measurement site 

using standard terrain-correction procedures. The complete corrected map of gravity variations, 

shown in figure 2 along with the actual measurement locations, is similar in general shape to the 

map produced earlier by Kinoshita and others (1963).

GRAVITY PROFILES

Table l:Parameters of best-fit lines for Alon§ each of the ten profiles located in figure 2, 

Complete bouguer anomaly profiles.

gravity increases from west to east as expected Profile intercept gradient

Kona Palisades 355.2 2.3

Hinalani 401.6 3.2

Hao Kuni 387.4 2.9

Keopu 461.2 4.4

Hienaloli 449.2 4

Kaumalumalu 404.5 3.2

Donkey Mill 505 5.3

Konawaena HS 416.5 3.5

KKRC 365.2 2.4

Papa Bay 366.2 __ 3.1

from examination of the reconnaissance map by 

Kinoshita and others (1963). Our increased 

density of data allows a better estimation of the 

spatial gradients, determined to be 2 to 5 mGals 

per km (fig. 3), and a better definition of this 

anomaly's shape (fig. 2). Some profiles in Figure

12
3 are smoother than others due to lower measurement error. Nonetheless, the final reduced data 

show similar variations in each profile. For example, the eastern parts of the Hinalani and Kona 

Palisades profiles have a similar "notch", and the western parts of the Keopu, Hienaloli, and 

Kaumalumalu profiles have a similar "hip". Only after removing these regional trends, can 

smaller, more local variations be seen. As an initial attempt, a least-squares best-fit line was 

drawn through each of the complete Bouguer profile data sets. Then this line was subtracted from 

the respective data set. The residuals obtained are plotted together in figure 4 and the parameters 

of the best-fit lines are given in table 1. All further discussion is directed toward these residuals.

Of the ten residual profiles, five (Kona 

Palisades, Hinalani, Hienaloli, Kaumalumalu, 

and Konawaena HS) have a similar shape, with 

a sharp-sided gravity low in the center of the 

profile and with flanking highs on both sides 

(fig. 4). The others show broadly similar 

variations, but do not have the whole shape. The 

location of this subtle variation in an otherwise 

strong east-west gradient is consistently 

between those wells that clearly intercept highlevel ground water and those that intercept basal

Figure 5: Detail map showing the actual profile data ground water. Figure 5 shows that a high-level 

and well locations within a 1 km grid. Static water

level is in meters above sea level. Heavy line indicates well (static water levei of 147.7 meters above 

axis of residual gravity low.

13
sea level) is located at an easting of approximately -43.5 km, east of the low in profile 

Konawaena HS residuals (fig. 4), and that a basal well (static water level of 1.2 meters above sea 

level) is located at an easting of approximately -45.5, near

the west edge of the residual low. The 

Donkey Mill profile is 2 km north of a 

battery of three high-level wells at easting - 

45, within the highest residual values; the 

basal wells are west of easting -46.5. Figure 

6 shows the same relationship between 

high-level and basal wells along the Kona 

Palisades and Hinalani profiles; the highlevel wells are east of the residual low while 

the basal wells are west. The Keopu and 

Hienaloli profiles also are near high-level 

wells that project east of the lowest

residuals plotted in Figure 5.

Figure 6: Detail map showing the actual profile data 

and well locations within a 1 km grid. Static water level 

is in meters above sea level. Heavy lines indicate axis of 

residual gravity low.

DISCUSSION AND CONCLUSION

The exact shape of the residual is less important for this analysis than the fact that several of the

14
profiles have a similar shape that can be emphasized by subtracting a best-fit line. Removing the 

best-fitting linear trend from the data and comparing the residuals from different profiles for 

similar features is a simplistic, way of removing the effect of the larger gravity anomaly running 

along the coast (see figure 2) and comparing any remaining local anomalies. These local 

anomalies could be due to buried structures that may be responsible for the large difference in 

static water level observed in this part of the island.

15
Extended Kaumalumalu Gravity Profile

276 i^i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i i

270-

_ 260- 

CO

250-

240

Easting, km

-2.0-

A better way to 

remove the 

regional anomaly 

from the profile 

data is to model it 

using a larger data 

set. The new 

gravity values were 

obtained across the 

Hualalai anomaly 

in only three places 

- near the 

Kaumalumalu, 

Donkey Mill, and

Konawaena

Figure 7: Comparison plot of model response and data. Model shape and data 

locations are shown in the lower portion. Density contrasts are shown for the model. HS/KKRC profiles.

The latter profile is significantly affected by the mass of Mauna Loa so we will proceed with the 

regional analysis using an extended data profile that includes Kaumalumalu profile.

Figure 7, the result of this effort, shows that the fit between model response (line) and data 

(pluses) in the upper part is rather good. Modeling was done using the computer program SAKI

C .0- -

Q_

<i>

to

2.0-

0.245 g/cc

16
(Webring, 1985). Much of the data scatter results from including data within 3 km off the profile. 

The model itself is shown in cross-section in the lower portion of Figure 7 along with the data 

locations (pluses). The model mass is 21.5 km wide at a depth of 5 km below sea level, 8 km 

wide at sea level, and has a top 500 m above sea level. S AKI can compute the responses of 

terminated bodies, so the masses in this model were terminated 5 km north of the profile and 

several tens of kilometers south of the profile. The density of the mass is just above 2.8 g/cc; 

below sea level, the reduction density of 2.6 g/cc plus the model density contrast of 0.245 

represents a real density of 2.845 g/cc. Above sea level, the reduction density of 2.3 g/cc plus the 

model density contrast of 0.518 represents a real density of 2.818 g/cc. The model mass is similar 

in shape to the core of the summit and rift zones of Kilauea volcano (Kauahikaua and others, 

1996).

Figure 8 shows the result of taking the regional Hualalai model and comparing its response to 

the Kaumalumalu profile data alone. It does a much better job matching most of the variation in 

the profile and is more geologically reasonable. A flattening can still be seen in the data between 

eastings of -46 and -47 km. A better fit is achieved by adding a relatively thin mass dipping 

westward from a point within 100 m below the ground surface (fig. 9). Shallower gradients to the 

west than to the east require the westward dip. The fit between model and data is improved from 

a RMS error of 0.543 for figure 8 to 0.387 for figure 9. The density of this rather small, shallow 

body is also about 2.8 g/cc.

17
Kaumalumalu Gravity Profile

' ' i ' ' ' ' i ' ' ' ' i ' ' ' ' i ' ' ' ' i 

While still not definitive, this analysis suggests that a geologic model for the gravity data

includes a broad, dense core, which may be a volcanic 

dike zone, and a thin body dipping west, which may 

represent unusually dense flows. These flows may be 

so dense that they can impede ground-water that

Easting, km

+++ 4

Figure 8: SAKI plot of extended profile model Eastins' km

with Kaumalumalu profile data. RMS

error=0.543.

would normally flow to the coast, resulting in a

high-level water body like that found here.

Similar features appear in some of the other

profiles: in the Kona Palisades and the Hinalani

profiles at an easting of -52 km, the

Kaumalumalu profile at an easting of -46.5 km,

and the Donkey Mill profile at an easting of -46 km. Two of the profile data sets have two

"flattenings" - Konawaena HS at an easting of -45 and -42.5 kms and KKRC at an easting of -

42 and -40 kms. The Hao Kuni, Keopu, and Hienaloli profiles do not have this feature, although

Kaumalumalu Gravity Profile

Figure 9:SAKI plot identical to Figure 8 with the 

addition of a thin mass dipping to the west. RMS 

error = 0.387

18
they display broad variations in the lower parts of the profiles. We conclude that the profiles are 

either too short (Hao Kuni) or that the local structure does not occur beneath the profiles (Keopu 

and Hienaloli).

Our original goal was to locate the structures that are responsible for impounding the high-level 

water. The secondary goal was to identify the nature of the structures. Based on the data in this 

study, we can say that the western boundary of the high-level water body is characterized by a 

gravity variation that appears as a 1-2 mGal flattening of the limb of the main Hualalai gravity 

anomaly. This characteristic is small relative to the much larger Hualalai anomaly but is 

observable in many of the data profiles obtained for this study. The shape of the anomaly is 

asymmetric favoring interpretation as a dense lava flow, or a sequence of flows dipping westward 

rather than nearly-vertical dikes. These flows may be thick trachytes found in some water wells 

drilled into the northwest rift zone of Hualalai (Clague, 1987; Steve Bowles, oral 

communication, 1997). The western edge of the mass interpreted to be responsible for the larger 

Hualalai anomaly is closely coincident with the hydrologic boundary. This mass must represent 

one or more intrusion complexes; however, the gravity model does not rule out single or sparse 

dikes that may extend beyond the edges of the complex. The apparent absence of a step gravity 

increase over the area of high-level water table suggests that the saturated rocks are fairly dense.

Acknowledgements
Miklius, Kathryn Riesenberg, and Annabelle Pleininger helped with the elevation control. In 

addition, Jennifer Pallon, Louise Timmermans, and Marije van Koolwijk operated the total field 

survey station for the detailed gravity profiles. Louise Timmermans, Jennifer Pallon, and Gary 

Puniwai made many of the gravity measurements We thank the U.S. Geological Survey Water 

Resources Division, Hawaii District, Bill Meyer, chief and Glenn Bauer and Rae Loui of the 

State of Hawaii Department of Land and Natural Resources for providing funding with which to 

initiate this study.

References

Chase, T.E., Miller, C.P., Seekins, B.A., Normark, W.R., Gutmacher, C.E., Wilde, P. and Young,

J.D., 1980, Topography of the southern Hawaiian Islands, U.S. Geological Survey OpenFile Report 81-120, 3 sheets. 

Clague, D.A., 1987, Hawaiian xenolith populations, magma supply rates, and development of

magma chambers, Bull. Volcanol, v. 49, p. 577-587.

Clark, H., 1994, It's taps forKona's water woes, The Honolulu Advertiser, 6 April, 1994, p. 1. 

Hildenbrand, T., Rosenbaum, J.G., and Kauahikaua, J.P., 1993, Aeromagnetic study of the Island

of Hawaii, JGR, v. 98, p. 4099-4119. 

Hill, D.P., and Zucca, J.J., 1987, Geophysical constraints on the structure of Kilauea and Mauna

Loa volcanoes and some implications for seismomagmatic processes, U.S. Geological

Survey Professional Paper 1350, p. 903-934. 

Kauahikaua, J., and Jackson, D.B., 1983, Ground water exploration in the South Kohala District

using vertical electrical sounding, U.S. Geological Survey Open-File Report 83-67, 20 p. 

Kauahikaua, J., Takasaki, K., and Jackson, D.B., 1985, An evaluation of ground water resources

in Kailua-Kona, Hawai'i using electric geophysical measurements, U.S. Geological

Survey Open-File Report 85-33, 21 p. 

Kauahikaua, J., Foster, J., Timmermans, L., and Pallon, J., 1993, A West Mauna Loa structure

revealed by static groundwater levels and geophysical evidence, Hawai'i [abs],

Supplement to Eos, v. 74, No. 43, p. 641. 

Kauahikaua, J., Hildenbrand, T., and Rosenbaum, J., 1996, Insights on the structure of Kilauea

Volcano, Hawai'i from gravity and magnetic interpretations, Eos, Transactions, American

Geophysical Union, v. 77, p. F797. 

Kinoshita, W.T., Krivoy, H.L., Mabey, D.R., and MacDonald, R.R., 1963, Gravity survey of the

island of Hawai'i, U.S. Geological Survey Professional Paper 475C, p. Cl 14-C116. 

Lockwood, J.P., Lipman, P.W., Petersen, L.D., and Warshauer, F.R., 1988, Generalized ages of

surface lava flows of Mauna Loa Volcano, Hawaii, U.S.G.S. Miscellaneous

Investigations Series Map 1-1908, 1:250,000, 1 sheet. 

Macdonald, G.A., Abbott, A.T., and Peterson, F.L., 1983, Volcanoes in the Sea - the geology of

Hawaii, University of Hawaii Press: Honolulu, 517 p. 

Moore, R.B. and Clague, D.A., 1991, Geologic map of Hualalai Volcano, Hawaii, U.S.

Geological Survey Miscellaneous Investigations Series Map 1-2213, 1:50,000 scale, 2

sheets. 

Strange, W.E., Woollard, G.P., and Rose, J.C., 1965, An analysis of the gravity field over the

Hawaiian Islands in terms of crustal structure, Pacific Science, v. 19, p. 381-389.

20
Takasaki, K.J., and Valenciano, S., 1969, Water in the Kahuku Area, Oahu, Hawaii, Geological

Survey Water-Supply Paper 1874, 59 p. 

Webring, M, 1985, SAKI: a Fortran program for generalized linear inversion of gravity and

magnetic profiles, USGS Open-File Report 85-122, 108 p. 

Wolfe, E.W., and Morris, J. (compilers), 1996, Geologic map of the Island of Hawaii, U.S.

Geological Survey Miscellaneous Investigations Series 1-2524-A, scale 1:100,000, 3

sheets.

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