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[1] Abstract: We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic field anomaly within a 10 Â 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to $3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization ($10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.

SUMMARY Airborne laser altimetry has proved useful in recent years in examining volcanic landforms and glacial ice sheets. With the advent of accurate differential GPS aircraft tracking, we believe that airborne lasers will also prove useful in monitoring timevarying topographic uplift, with rates as low as several centimetres of vertical motion per year. The Long Valley, California, caldera provides an excellent testing ground for this new technology. The region has a history of extensive volcanism, and its central dome has recently been undergoing resurgent uplift of up to 4 cm per year. In September 1993 we conducted three aircraft topographic surveys over the caldera and resurgent dome, utilizing a NASA T39 jet aircraft outfitted with a nadir‐profiling altimetric laser (ATLAS), two P‐code GPS receivers, a Litton LTN92 inertial unit for attitude, and aerial cameras. In addition, we operated two base‐station GPS receivers for post‐flight differential navigation and conducted a kinematic automobile survey of roads crossing the dome. The aircraft flew at a mean altitude of 500 m above ground, and at speeds of 80‐100 m s‐1. The laser had a divergence of 1.7 mrad, and output 50 pulses per second, yielding footprints of 0.9 m diameter separated by about 2 m along track. Precision flying yielded multiple profiles along nearly identical paths, including crossing profiles over the resurgent dome, off the dome, and along a nearby highway. The surveys included daily flights over Mono Lake for roll and pitch bias calibrations, and over the well‐surveyed Lake Crowley to provide an independent check of estimated elevations. Much progress has been made in developing analysis procedures for the laser timing and attitude corrections. Crossover points from repeated profiles over the dome and the lake indicate that although some profiles contain relative bias errors of up to 10 cm, most are substantially more accurate. The accuracy of the measurements will increase as the various sources of error are better defined and dealt with. Even with the present analysis level, the distribution of crossovers near the centre of the dome is roughly Gaussian, with a mean of 2.6 cm and a standard deviation of 11 cm. The standard error of the mean is low (1 cm), due to the high number of crossover points, which holds promise for measuring systematic changes in the dome height from year to year. Also, comparison of laser heights of Lake Crowley to tidal gauge heights yields only a 1–4 cm difference in absolute height. These encouraging results serve to confirm further the concept of using aircraft laser surveys for geodetic tectonic monitoring.

A scalar magnetic anomaly map for South America and adjacent marine areas was prepared from MAGSAT data. Preparation of the map posed problems, notably in separating external field and crustal anomalies, and in reducing data to a common altitude. External fields are manifested in a long‐wavelength ring current effect, a medium‐wavelength equatorial electrojet, and short‐wavelength noise. The ring current is corrected through use of a standard ring current equation, augmented by further wavelength filtering. Because the electrojet is confined primarily to dusk profiles, its effect is minimized by drawing the data set from dawn profiles only. The noise is reduced by selecting profiles from “quiet” periods [Formula: see text]. The optimum filter cutoff level is determined by a statistical regression technique to be approximately 50 degrees wavelength. When wavelengths longer than this are rejected, resultant profiles from redundant satellite orbits correlate well, with differences due primarily to satellite altitude. These differences are eliminated by normalizing the data to a constant altitude by the equivalent point‐source inversion technique. The resultant map differs significantly from the original 2 degree averaged version, most notably in low geomagnetic latitudes. A version of this map reduced to the pole shows correlations with several regional South American and Caribbean tectonic features.

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