Abstract. New results are presented from the teleseismic component of the Jemez Tomography Experiment conducted across Valles caldera in northern New Mexico. We invert 4872 relative P wave arrival times recorded on 50 portable stations to determine velocity structure to depths of 40 km. The three principle features of our model for Valles caldera are: (1) near-surface low velocities of-17% beneath the Toledo embayment and the Valle Grande, (2) midcrustal low velocities of-23% in an ellipsoidal volume underneath the northwest quadrant of the caldera, and (3) a broad zone of low velocities (-15%) in the lower crust or upper mantle. Crust shallower than 20 km is generally fast to the northwest of the caldera and slow to the southeast. Near-surface low velocities are interpreted as thick deposits of Bandelier tuff and postcaldera volcaniclastic rocks. Lateral variation in the thickness of these deposits supports increased caldera collapse to the southeast, beneath the Valle Grande. We interpret the midcrustal low-velocity zone to contain a minimum melt fraction of 10%. While we cannot rule out the possibility that this zone is the remnant 1.2 Ma Bandelier magma chamber, the eruption history and geochemistry of the volcanic rocks erupted in Valles caldera following the Bandelier tuff make it more likely that magma results from a new pulse of intrusion, indicating that melt flux into the upper crust beneath Valles caldera continues. The low-velocity zone near the crust-mantle boundary is consistent with either partial melt in the lower crust or mafic rocks without partial melt in the upper mantle. In either case, this low-velocity anomaly indicates that underplating by mantle-derived melts has occurred.
Extraction of dense, nonaqueous-phase liquid (DNAPL) contaminants trapped in groundwater aquifers is a major problem in environmental remediation because existing field techniques, such as pump and treat, have limited effectiveness. We present new laboratory experimental evidence that low-frequency (100 Hz or less) stress waves could increase the mobility and transport of DNAPL by an order of magnitude or more during pump and treat. This unproven technology could decrease the time and cost of groundwater remediation efforts and reduce the final amount of contaminant trapped in an aquifer. A sand core was confined in a specialized flow apparatus and contaminated with a small amount of trichloroethylene (TCE). Steady-state water flow was then initiated through the core, and effluent TCE concentrations were measured using gas chromatography (GC). At various stages of water flow, axial stress cycling at 25-100 Hz was applied by mechanical coupling of a magnetostrictive actuator to the sand core's outlet face. During most of the stimulated runs pure-phase TCE droplets were produced in the effluent and the effective TCE concentration increased by as much as a factor of 20. Numerous flow runs were performed to investigate the effects of stimulation frequency, amplitude, and duration, as well as water flow rate and background effluent TCE concentration. Calibrated accelerometer and pore pressure measurements allowed determination of the stimulation threshold parameters required to achieve enhanced TCE production. The physical mechanisms responsible for the observed phenomenon are not fully understood. Further research is required before this promising new technology can be applied reliably to contaminated groundwater aquifers.
The measurement of small changes in elastic wave velocity and attenuation is important to a broad range of problems, such as earthquake prediction and early detection of rock failure in mines. Previous authors proposed a method for estimating small temporal velocity changes in the earth’s crust by analyzing progressive relative phase delays between the scattered waves of two signals generated by nearly identical earthquake sources, called doublets, recorded at different times at the same receivers. Several improvements have been made to the original method and are presented here. The reliability of measured velocity changes has been increased by using active, repeatable sources instead of natural earthquakes. The robustness of the analysis technique has been improved by eliminating unnecessary intermediate phase regression steps and thus reducing the sensitivity to spurious data. Finally, the phase-delay algorithm has been extended to allow measurement of small attenuation changes from relative amplitude decay rates. Using ultrasonic source and receiver transducers embedded in Plexiglas test samples, velocity changes as small as 0.01%, caused by ambient temperature variations in the Plexiglas, have been measured. Changes in attenuation on the order of 10%, due to permanent damage induced in one of the samples, have also been measured.
Laboratory experiments were conducted to investigate the feasibility of using ultrasonic energy to reduce formation damage caused by fines and mud solids. Cores were damaged with drilling muds in a dynamic filtration cell. Damage caused by fines migration was simulated with freshwater injection. The damaged cores were then treated with ultrasonic energy at various frequencies and intensities. The experiments were conducted with fully brine-saturated cores and cores partially saturated with decane. Permeability was monitored as a function of ultrasonic treatment time during backflow for three different sections of each core. Permeability increase, the depth of treatment, and the ultrasonic energy requirements were investigated for both sandstone and limestone cores. The results showed that permeability increased by a factor of three to seven after ultrasonic treatment for cores that were damaged by mud solids and fines migration. Treatment was successful for frequencies of approximately 20 to 80 kHz and acoustic intensities of approximately 20 to 250 W/m 2 . For these wavefield parameters, the effective depth of treatment for reducing fines damage was approximately 2.5 in. Damage caused by mud infiltration penetrated only the first 2.5 in. ofthe cores, and thus, ultrasonic treatment had no effect on deeper sections.
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