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A prototype model of a downhole controlled seismic source has been built and field tested. This instrument is a swept‐frequency, penumatically powered seismic source that operates in the 10 to 100 Hz range. This particular version is a vertical‐dipole source and generates vertically polarized shear waves. The instrument is an improved version of a downhole monofrequency source first tested in the crust of Kilauea Lava Lake, Hawaii, in 1981. The present prototype source was field tested recently at the Chevron Oil Field Research facility in La Habra, California. Data from this test, combined with calculations for a full‐scale instrument, indicate that considerable amounts of energy can be coupled into the ground with this type of downhole source. Signals from these prototype sources are easily detectable at distances of 150 m or more. A larger version of the source is currently being designed and fabricated for use in 0.20 m diameter holes, and it will theoretically have a range 30 times greater than the current prototype model. This new seismic source has applications in scientific studies related to the Continental Scientific Drilling Program. The source also has applications in locating magma bodies for the Magma Energy Program and in evaluating oil reservoirs.

s r!This paper identifies and explores the technical requirements and issues associated with remotely monitoring continuous wave (CW) sources with seismic arrays. Potential approaches to this monitoring problem will be suggested and partially evaluated to expose the monitoring challenges which arise when realistic local geologies and cultural noise sources are considered. The selective directionality and the adaptive noise cancellation properties of arrays are required to observe weak signals while suppressing a colored background punctuated with an unknown distribution of point and sometimes distributive sources. The array is also required to characterize the emitters and propagation environment so as to properly focus on the CW sources of interest while suppressing the remaining emitters. The proper application of arrays requires an appreciation of the complexity of propagation in a non-homogeneous earth. The heterogeneity often limits the available spatial coherence and therefore the size of the army. This adversely impacts the array gain and the array's ability to carefully resolve various emitters. Arrays must also contend with multipath induced by the source and the heterogeneous earth. If the array is to focus on an emitter and realize an enhancement in the signal to noise ratio, methods must be sought to coherently add the desired signal components while suppressing interference which may be correlated with the desired signal. The ihpact of these and other issues on army design and processing are described and discussed.

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A three‐component seismometer was emplaced in a borehole in Long Valley caldera for a period of 2 months to record local earthquakes to the south, mostly in the Sierra Nevada. The seismic records were plotted in depth versus time sections to produce “vertical hypocentral profiles” for both the vertical and horizontal components. The data collected and displayed in this way provide good resolution of the crustal structure at depth and avoid the attenuation and complications introduced by the near‐surface caldera fill. The record sections were then modeled with two‐dimensional ray tracing to match the observed travel times. Although nonreversal of the profiles and uncertainties in hypocentral locations introduce ambiguities, the final interpretation reveals two separate low‐velocity bodies which we have tentatively identified as magma beneath the resurgent dome in Long Valley. The first of these is a small (2 km by 6 km) body with a depth to top of approximately 3.7 km beneath the surface at the southern end of the resurgent dome. The other body appears larger and lies at a depth of 5.5 km beneath the northern end of the resurgent dome.