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Five Schiumberger and 24 transient electromagnetic soundings were successfully obtained on Kaho'olawe, Hawaii for the purpose of assessing the island's ground-water potential. The soundings were intended to determine the depth to a conductor at or below sea level which is thought to represent seawater-saturated basalts. The most common type of fresh-water occurrence in Hawaii, the fresh-water lens, would be revealed in such soundings as a conductor which is significantly deeper than sea level. The soundings in this study show that the conductor is anomalously deep beneath the central eastern portion of Kaho'olawe. It is impossible to derive a unique interpretation for the geometry and salinity of the fresh-water lens interpreted above the conductor, but it is possible to show that select simple lens geometries would yield a similar pattern of sounding results. Introduction "Kaho'olawe is a single shield volcano with a caldera, almost 5 kilometers in diameter, at its present eastern end" (Macdonald and others, 1983). Kaho'olawe (Figure 1) is better known among long-time Hawaii residents as the 'Target Isle" because of the Navy's persistent bombing, artillery, and land assault practice on the island. The island has been a center of controversy because it is close to Maui, a fast-growing residential and tourist center, and because native Hawaiian groups have demanded that the bombing be stopped and the island returned to the Hawaiian public. Recently, the state of Hawai'i has provided limited funds to study erosion control and assess the surface and ground-water resources of the island. This report summarizes the results of geophysical soundings obtained during three trips to the island in 1988 to aid the ground-water assessment of Kaho'olawe. Electrical geophysical methods are ideal for this type of ground-water assessment because they respond only to the resistivities of earth materials which are largely controlled by degree of weathering, the salinity and degree of saturation by fluids. Because of the significantly lower resistivity of salt water compared to fresh water, these techniques have the potential to discriminate between basalt that is unsaturated or saturated with fresh water and basalt that is saturated with salt water. If we assume that most fresh ground water in Hawai'i is in the from of a lens floating on salt water, and that geophysical techniques, while not allowing detection of fresh water directly, can determine how deep below sea level salt water can be found, then geophysical techniques can estimate the thickness of the fresh water lens (Swartz, 1937). Previous Studies Because access to the island is limited by the military, few geophysical studies have been done on Kaho'olawe. The geology and ground-water potential were studied by Stearns (1940). His conclusions on ground water were: "There are several sites where a drilled well might tap water in the dike complex, but the most promising place at the lowest elevation, is at an altitude of 800 feet in Ahupu Gulch, 1 mile southwest of Moaula cone. A well at this place should encounter the north margin of the southwest rift zone, but the dikes may not be close enough to confine water. A well at an altitude of 1,200 feet half a mile east of this site would penetrate even more certainly the dike complex. The height of the water table at these sites is unknown, but it may not be far above sea level because of small recharge. Resistivity station A indicated a water table only about 1.0 foot above mean sea level not far away." The results of four resistivity soundings were included in that Bulletin. They were all located on the northern flank of the island and did not indicate any large fresh-water aquifers (Stearns, 1940). A reconnaissance gravity survey (Furumoto, 1965) showed the highest Bouguer anomalies were in the southern half of the island. Because gravity highs often indicate dike zones on other Hawaiian islands (Strange and others, 1965), and dike zones often host large fresh-water bodies (Takasaki and Mink, 1985), this data suggested that exploration of the southern portion of the island might be more successful. Field and Interpretation Procedures Five Schlumberger soundings were obtained on the island at the locations shown in Figure 2. 3 The data were obtained using a SYSCAL R2 resistivity meter. AB/2 distances ranged from 9.1 to 1524 m (30-5000 ft) and MN/2 distances ranged from 1.8 to 61 m (6-200 ft). Field procedures were identical to those reported in Kauahikaua and Jackson 0988). The data were preliminarily interpreted with an automatic inversion program written by Zohdy and Bisdorf (1989) followed by a detailed inversion as described in Kauahikaua and Jackson (1988) using program MARQDCLAGT on an Hewlett-Packard 9000/320 microcomputer. The computer interpretations are included in Appendix A. Twenty-four transient electromagnetic soundings were obtained on Kaho'olawe (locations shown in Figure 2) using the SIROTEM II electronic switcher and digital receiver. The SIROTEM box was connected to two square wire loops each 152.4 m (500 ft) on a side in the coincident offset configuration. In this configuration, the two loops are offset along a diagonal by approximately 6-10 m. One loop functions as the transmitter while the other functions as the receiver. The general application of transient electromagnetic techniques to ground-water problems is described by Fitterman and Stewart (1986). It is necessary to use the offset coincident configuration (displaced loops configuration; Buseili, 1982) in Hawai'i because of the problem of superparamagnetism in the soil (Buseili, 1982). The 4 to 7 Amps of current carried by the transmitter loop is sufficient to temporarily magnetize the soil. Under normal operation, the receiver records the decay of ground currents after the transmitter current is turned off; however, on superparamagnetic soil, the receiver also records voltages induced by the soil becoming demagnetized. Offsetting the receiver from the transmitter reduces the superparamagnetic signal from the soil immediately adjacent to the transmitter wire, but does not significantly reduce the signal from the decaying ground currents. It is impossible to completely remove the superparamagnetic signal, so its effects must always be kept in mind during interpretation of transient electromagnetic soundings in Hawai'i. The superparamagnetic signal is an undesirable artifact that resembles the signal from a deep conductor with the depth to it increasing with loop size. The most straight-forward way of dealing with this problem, besides using the offset-coincident or central-induction configuration, is to determine the apparent conductor depth due to the superparamagnetic signal alone for each loop size used and report it as a maximum penetration depth. This is done in practice by obtaining SIROTEM soundings over a known, highly-resistive section with different sized loops in a coincident offset configuration. The obtained data should be all superparamagnetic signal which may be inverted as real sounding data. For the 152.4 m loops used on Kaho'olawe, the greatest penetration depth is about 700 m. The SIROTEM data were interpreted for the best-fitting layered earth model by computer program NLSTCO (Anderson, 1982) running on a VAX 11/750 located at the Hawaiian Volcano Observatory. The one-dimensional, horizontally-layered interpretations are shown in Appendix B and are summarized in Table I. The apparent-resistivity-versus-time data plots each had a maximum apparent resistivity at a time midway within the 0.8 to 164 ms measurement range. In seven of the sounding plots, the apparent resistivities increased too steeply at early times to be fitted by any horizontally-layered earth model. Most of these soundings were located along a north-south line shown by asterisks in Figure 4. These data could be fit adequately by using only the data near the maximum and at later times. Data from soundings to the west were fit well with a layered-earth model. The value of the apparent resistivity maximum was lower in all soundings to the east of this line. The effect of the non-zero transmitter turn-off time was not taken into account for the SIROTEM data. Fitterman and Anderson (1987) state that this will give first-layer resistivities which are too small. Because the object of this study was not to determine accurate resistivities of the materials involved, but only to determine their geometry, this effect is not significant. SIROTEM soundings were located at each of the four Schlumberger sounding locations in order to compare interpretations. Figure 3 shows the results of the two types of soundings at the four coincident sounding locations. In all four comparisons, the SIROTEM sounding data resolves only one layer at the surface over a thick, more resistive layer whereas the Schlumberger sounding data resolves two or three layers over the thick, resistive layer. SIROTEM's poor resolution at shallow depth is a consequence of the range of measurement times. Had that range extended into earlier times, shallow resolution would have been enhanced. Figure 3a and 3b show pairs of sounding interpretations that agree reasonably well as to the estimated depth to basement conductor. The interpretation for Schlumberger sounding #01 in Figure 3c did not resolve a basement conductor to a depth of at least 900 m whereas the interpretation for SIROTEM sounding #11 at the same site required a basement conductor at about 600-700 m below the surface. Note that this is the same depth range that we expect the SIROTEM data to be contaminated by superparamagnetic responses from the soil. Therefore, we may conclude that the SIROTEM interpretation is not valid below about 600 m from the surface and that, to that depth, both SIROTEM and Schlumberger sounding interpretations did not require a basement conductor. Figure 3d compares SIROTEM sounding #16 and Schlumberger sounding #03 in t

An assessment of the ground-water resources of Kaho'olawe, Hawaii using transient electromagnetic and Schlumberger soundings