GEOTHERMAL AND GROUND WATER EXPLORATION ON MAUl, HAWAII, BY APPLYING D.C. ELECTRICAL SOUNDINGS ! ATHESIS SUBMITTED TO THE GRADUATE DIVISION OF THE UNIVERSITY OF HAWAII IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE GEOLOGY AND GEOPHYSICS AUGUST 1981 By Mark D. Mattice Thesis Committee: Barry R. Lienert, Chairman Charles E. Helsley Dallas B. Jackson Ralph Moberly ACKNOWLEDGEMENTS Special thanks to James Kauahikaua for teaching me how to do the field work for this study, and for many hours of enlightening discussion on analysis and interpretation. The participation of my Committee Chairman Barry Lienert in all phases of this work, from field work through publication is gratefully appreciated. Also, I wish to thank Tommie Thomas and Vindell Hsu for their help with the field work. The cooperation of Pioneer Mill, Wailuku Sugar Co., Dole Pineapple, and Hana Ranch on whose land the field work was conducted is gratefully acknowledged. This research was supported by a Department of Energy grant, No. DE-AC03-80SF10819 as a phase of geothermal assessment of the State of Hawaii. ABSTRACT Twenty-one Schlumberger resistivity soundings were performed on the island of Maui. Analysis consisted of one-dimensional modeling using an automatic ridge-regression inversion algorithm (Anderson, 1979). The inversion results were compared with available well-log information and geologic maps in order to make geologic interpretations. The soundings were conducted primarily to estimate the depth to and the electrical resistivity of, seawater-saturated basalt for different parts of the island. The resistivity of seawater-saturated basalt on Maui ranges between 3.5 and 60 ohm-meters. The lowest values occurred near Ukumehame canyon, on the south rift zone of West Maui. In this area, which is the site of a warm water (33°C) well, the computed resistivity for seawater-saturated basalt is about 4 ohm-m. Using typical Hawaiian basalt porosity values of 15% to 25%, Archie's Law implies temperatures of between 62° and l7loC at depths below 200 meters in the Ukumehame area. Freshwater piezometric heads were estimated from the sounding data. The largest freshwater head (91 m) was obtained in Keanae valley. The inferred large volume of freshwater is perched on Keanae alluvial valley fill and is observed in a well (WlOO) towards the back of the valley. All other freshwater heads are under 4 m, indicating that the freshwater lens is rather thin near the coast at the areas surveyed. TABLE OF CONTENTS Page ACKNOWLEDGEMENTS . ABSTRACT . . . LIST OF TABLES LIST OF ILLUSTRATIONS I. INTRODUCTION .. II. GEOLOGY OF MAUl 1. West Maui . 2. Haleakala III. REGIONAL GEOPHYSICS OF MAUl IV. METHODS AND EQUIPMENT .. 1. Schlumberger Array . 2. Coastal Correction V. THEORY .. . ... 1. Fundamental Equations 2. Forward Problem 3. Inverse Problem 4. Error Analysis VI. LIMITATIONS. 1. Principle of Equivalence 2. Principle of Suppression 3. Anisotropy .. 4. Heterogeneities 5. Depth of Investigation TABLE OF CONTENTS (continued) Page VII. RESISTIVITY DEPENDENCE ON POROSITY AND TEMPERATURE VIII. RESULTS.. . . . ... 1. Sounding 1 2. Sounding 2 . 3. Soundings 3 &16 4. Sounding 4 5. Sounding 5 . 6. Sounding 6 . 7. Sounding 7 8. Sounding 8 9. Sounding 9 10. Soundings 10 &11 11. Sounding 12 12. Sounding 13 13. Sounding 14 14. Sounding 15 15. Sounding 17 16. Sounding 18 17. Sounding 19 18. Sounding 20 19. Sounding 21 IX. SUMMARY ..... X. DISCUSSION AND CONCLUSIONS REFERENCES CITED .. .. LIST OF TABLES Table Page 1 Possible Solutions for S14 2 Rock Resistivities Near Waialua, Oahu 3 Rock Resistivities Near Pahala, Hawaii 4 Rock Resistivities Measured on Maui 5 Resistivities of Fluid-Bearing Maui Basalt 6 Computed and Observed Heads on Maui 7 Calculated Temperatures Near Ukumehame Canyon LIST OF ILLUSTRATIONS Figure Page Map of Maui (Showing Sounding Locations) 2 Gravity Map of Maui 3 Aeromagnetic Map of Maui 4 Schlumberger Electrode Array 5 Graphical Representation of Correlation Coefficients 6 Principle of Equivalence 7 Principle of Suppression 8 Effect of Heterogeneities on Potential Distribution 9 Effect of Heterogeneities on Apparent Resistivity Curve 10 Two-Layer Curves 11 Sounding 1 12 Sounding 2 13 Soundings 3 and 16 14 Sounding 4 15 Sounding 5 16 Sounding 6 17 Sounding 7 18 Sounding 8 19 Sounding 9 20 Soundings 10 and 11 21 Sounding 12 22 Sounding 13 23 Sounding 14 24 Sounding 15 LIST OF ILLUSTRATIONS (continued) Figure Page 25 Sounding 17 26 Sounding 18 27 Sounding 19 28 Sounding 20 29 Sounding 21 30 Cross-Section Across South Rift Zone of West Maui 31 Cross-Section Across North Rift Zone of Haleaka1a 32 Cross-Section of Ukumehame Area I. INTRODUCTION Groundwater exploration using the direct current electrical method has enjoyed prospering success in Europe, Russia, Japan, New Zealand, India, mainland United States and Hawaii. On the island of Maui two previous studies by J. H. Swartz (1940) and W. M. Adams et al. (1968) have shown excellent determination of the boundary between freshwater saturated rocks and seawater-saturated rocks. Determining depths to the seawater boundary by the D.C.-resistivity method and utilizing the Ghyben-Herzberg buoyancy relation produced accurate estimates of the static piezometric head in the areas of both studies. The head estimates were confirmed by nearby well observations. Extending the D.C.-resistivity technique to explore for anomalous subsurface temperatures has been the subject of much research during the . last 20 years. Results have shown that moderate increases in temperature can decrease resistivities over an order of magnitude (Darknov, 1962). The sensitive dependence of resistivity on temperature has resulted in the widespread use of resistivity methods for geothermal exploration. Presumably, seawater-saturated rock underlies the whole of Maui island. By comparing the resistivity values of the seawater-saturated unit, low resistivity areas which may have geothermal potential can be located. Previous studies in Hawaii (Swartz, 1937; Hussong, 1967; Zohdy and Jackson, 1969) have implied, in most cases, that the boundary at the top of the freshwater table is not electrically distinct. In this study the top of the water table is taken to be the point at which the pore spaces are 100% filled with water. In Hawaii it is well known (i.e. Macdonald and Abbott, 1970) that a transition zone (vadose zone) going from unsaturated to completely saturated pore space exists above the water table. This leads to the concept of critical saturation. The critical saturation represents the minimum saturation for which there is a continuous film of water over all the surfaces in a rock. This film provides a good medium in which electric current may flow, which in turn greatly reduces the resistivity of the rock. Increasing the volume of water, beyond the critical saturation, does not significantly decrease the resistivity. Critical saturation levels generally range from 20% to 80% (Keller and Frischtnecht, 1966). The resistivity of fluid-bearing rocks is highly dependent on the resistivity of the fluid. The resistivity of the fluid is in turn highly dependent on the concentration and nature of dissolved solids. A large range of dissolved solids in freshwater gives a large range of resistiv ities for freshwater-bearing rocks. The nearly constant salinity of seawater gives a small range of resistivities for seawater-saturated rocks. The top of the freshwater table is generally not electrically distinct because there is a diffuse zone of partial saturation; whereas the base of the freshwater table is generally well defined because of a large salinity contrast between freshwater and seawater. Since the base of the freshwater lens is better defined than the top, it is the base which provides the most useful target for groundwater exploration. The nearly constant salinity of seawater makes the resistivity of seawater-saturated rocks mainly dependent on porosity and temperature. This makes the resistivity of the seawater-saturated rock unit the most useful target for geothermal exploration.