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A study of the geoelectrical structure of the central part of Piton de la Fournaise volcano (Réunion, Indian Ocean) was made using direct current electrical (DC) and transient electromagnetic soundings (TEM). Piton de la Fournaise is a highly active oceanic basaltic shield and has been active for more than half a million years. Joint interpretation of the DC and TEM data allows us to obtain reliable 1D models of the resistivity distribution. The depth of investigation is of the order of 1.5 km but varies with the resistivity pattern encountered at each sounding. Two-dimensional resistivity cross sections were constructed by interpolation between the soundings of the 1D interpreted models. Conductors with resistivities less than 100 ohm-m are present at depth beneath all of the soundings and are located high in the volcanic edifice at elevations between 2000 and 1200 m. The deepest conductor has a resistivity less than 20 ohm-m for soundings located inside the Enclos and less than 60-100 ohm-m for soundings outside the Enclos. From the resistivity distributions, two zones are distinguished: (a) the central zone of the Enclos; and (b) the outer zone beyond the Enclos. Beneath the highly active summit area, the conductor rises to within a few hundred meters of the surface. This bulge coincides with a 2000-mV self-potential anomaly. Low-resistivity zones are inferred to show the presence of a hydrothermal system where alteration by steam and hot water has lowered the resistivity of the rocks. Farther from the summit, but inside the Enclos, the depth to the conductive layers increases to approximately 1 km and is inferred to be a deepening of the hydrothermally altered zone. Outside of the Enclos, the nature of the deep, conductive layers is not established. The observed resistivities suggest the presence of hydrated minerals, which could be found in landslide breccias, in hydrothermally altered zones, or in thick pyroclastic layers. Such formations often create perched water tables. The known occurrence of large eastward-moving landslides in the evolution of Piton de la Fournaise strongly suggests that large volumes of breccias should exist in the interior of the volcano; however, extensive breccia deposits are not observed at the bottom of the deep valleys that incise the volcano to elevations lower than those determined for the top of the conductors. The presence of the center of Piton de la Fournaise beneath the Plaine des Sables area during earlier volcanic stages (ca. 0.5 to 0.150 Ma) may have resulted in broad hydrothermal alteration of this zone. However, this interpretation cannot account for the low resistivities in peripheral zones. It is not presently possible to discriminate between these general interpretations. In addition, the nature of the deep conductors may be different in each zone. Whatever the geologic nature of these conductive layers, their presence indicates a major change of lithology at depth, unexpected for a shield volcano such as Piton de la Fournaise.

Forty‐five resistivity soundings, using Schlumberger and equatorial dipole electrode configurations, were made on the islands of Oahu and Hawaii to determine the applicability of direct current resistivity methods for locating freshwater aquifers in the State of Hawaii. The soundings were made on the northwestern part of the island of Oahu near the town of Waialua and on the island of Hawaii on the “saddle” area near Pohakuloa and Humuula. Interpretation of 32 sounding curves obtained on the island of Oahu indicates that it is possible to correlate five stratigraphic units underlain by a vesicular basalt basement and that the determination of the approximate depth to the fresh‐water‐saline‐water interface within the basalt is feasible. Two of these Schlumberger soundings with electrode spacings [Formula: see text] reaching 6000 ft yielded sounding curves of the maximum and minimum types whose terminal branches asymptotically approach a resistivity of about 30 ohm‐m, which is believed to be the true resistivity of basalt saturated with sea water. Near the town of Waialua the aquifer is a coral zone as well as parts of the weathered vesicular basalt basement. On the island of Hawaii, near Pohakuloa, an exploratory well drilled in basalt to a depth of 1001 ft (prior to the resistivity survey) proved to be dry. Interpretation of thirteen deep soundings made with Schlumberger and equatorial arrays suggests that the minimum depth to a conductive layer, which may represent basalt saturated with fresh water, is about 2700 ft below land surface. The groundwater appears to be dike impounded.

Between February 22 and 28, 1969, about20x 10 6 m 3 ofbasaltic lava covered more than 6 km 2 of the upper east rift zone of Kilauea Volcano, probably the largest recorded eruption on the upper rift to that time. The eruption broke out along a discontinuous, 11-km-long fissure zone extending downrift from near Aloi Crater. A 70-m-deep lava lake formed in Alae Crater, near the site of the most prolonged activity. The lava is olivine-poor tholeiite; its hybrid chemistry can be explained by mixing of magma of 1967-68 summit vintage with that erupted during the early part of the 196~ 71 Mauna Ulu eruption and with lesser amounts of differentiated magma like that erupted in October 1968. The eruption followed a 4-month-long period of summit and east rift tumescence and was a•ccompanied by moderate summit deflation and severe uplift and dilation near the eruption site. The centers of horizontal and vertical deformation in the summit area migrated laterally before and during the eruption in response to the filling and emptying of a complex reservoir system. Comparison of depths to the summit reservoir derived from five theoretical models and a graphic approach shows poor agreement, although all agree on depths ofless than 5 km. The pattern of seismicity, timing of deformation and eruptive events, and chemistry of the lava imply the presence of a shallow magma reservoir on the east rift near Makaopuhi Crater. Seismic data suggest the existence of a nearly unobstructed magma conduit within the rift zone long before the eruption. Dikes that fed magma to the surface issued from this conduit, not directly from the summit reservoir.

Seismicity was concentrated in the summit and eruption areas during each eruption. Earthquakes and harmonic tremor at the summit were probably related both to the mechanics of subsidence and the transfer of magma. Earthquakes in the eruption area were mainly associated with ground rupture as magma forced its way to the surface; once dikes were emplaced, earthquakes gave way to tremor, caused by both subsurface transfer of magma and surface fountaining. The presence of a magma reservmr on the east rift zone in the vicinity of Makaopuhi Crater is inferred from seismic, petrographic, and chemical evidence.

When Mauna Loa showed signs of unrest in the spring of 1974, this volcano, the largest on earth, had not erupted since 1950. This unrest, marked by increased seismic activity and geodetic changes, led to a small summit eruption in July 1975, a type of eruption which in the past had typically been followed by flank activity within a few years. Based on this historic pattern and on the continuing inflation, a forecast was published that called for a major northeast rift zone eruption “between 2,800 and 3,000 meters elevation… sometime before the summer of 1978” [Lockwood et al, 1976]. The forecast proved correct in all details except for timing, which proved in error by almost 6 years!

Direct current resistivity and time domain electromagnetic techniques were used to study the electrical structure of the Long Valley geothermal area, A resistivity map was compiled from 375 total field resistivity measurements. Two significant zones of low resistivity were detected, one near Casa Diablo Hot Springs and one surrounding the Cashbaugh Ranch—Whitmore Hot Springs area. These anomalies and other parts of the caldera were investigated in detail with 49 Schlumberger dc soundings and 13 transient electromagnetic soundings. An extensive conductive zone of 1‐ to 10‐Ωm resistivity was found to be the cause of the total field resistivity lows. Drill hole information indicates that the shallow parts of the conductive zone in the eastern part of the caldera contain water of only 73°C and consist of highly zeolitized tuffs and ashes in the places that were tested. A deeper zone near Whitmore Hot Springs is somewhat more promising in potential for hot water, but owing to the extensive alteration prevalent in the caldera the presence of hot water cannot be definitely assumed. The resistivity results indicate that most of the past hydrothermal activity, and probably most of the present activity, is controlled by fracture systems related to regional Sierran faulting.

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