Time‐lapse crosswell seismic data acquired with a cemented receiver cable have been processed into P‐ and S‐wave tomograms which image heavy oil sand lithofacies and changes as a result of steam injection. Twenty‐seven crosswell surveys were acquired between two wells over a 3.5 month period before, during, and after a 34‐day, 30 MBBL [Formula: see text] steam injection cycle. Interpretation was based on correlations with reservoir data and models, observation well data, and engineering documentation of the production history and steam cycle. Baseline S‐ and P‐wave tomograms image reservoir sand flow units and areas affected by past cyclic steam injection. S‐wave tomograms define lithology and porosity contrasts between the excellent reservoir quality, “high flow” turbidite channel facies and the interbedded “low to moderate flow” bioturbated levee facies. The reservoir dip of approximately 20° is defined by the velocity contrast between lithofacies. P‐wave baseline tomograms image lithology, porosity, structure, and several low velocity zones caused by past steam injection. Previous steam‐heat injection caused the formation of gas which reduced velocities as much as several thousand ft/s (600 m/s), an amount which obscures the velocity contrast between lithofacies and smaller velocity reductions as a result of temperature alone. Time‐lapse and difference P‐wave tomograms document several areas with small decreases in velocity during steam injection and larger decreases after cyclic steam injection. Velocity reductions range from 300 to 900 ft/s (90 to 270 m/s) adjacent to and above injectors located 20 to 50 feet (6 to 15 m) from the tomogram cross‐section. Poisson’s ratio tomograms show a significant decrease (.10) in the same area, and include low values indicative of gas saturation. Continuous injectors located 50 to 350 feet (15 to 100 m) from the survey area also caused a progressive decrease in velocity of the “high flow” channel sands during the time‐lapse survey. Interdisciplinary interpretation indicates that tomograms not only complement other borehole‐derived reservoir characterization and temperature monitoring data but can be used to quantitatively characterize interwell reservoir properties and monitor changes as a result of the thermal recovery process. Monitoring results over 3.5 months confirms that stratification has controlled the flow of steam, in contrast to gravity override. This suggests that tomographic images of reservoir flow‐units and gas‐bearing high temperature zones should be useful for positioning wells and optimizing injection intervals, steam volumes, and producing well completions.

Using recordings from 15 portable instruments and six permanent stations operated during two field investigations of microearthquakes in southern Peru, we determined locations of 888 shallow and intermediate depth earthquakes and 56 fault plane solutions.

In 1981 an 18‐station regional array was operated in southern Peru above the area where the dip of the subducted Nazca plate changes from virtually horizontal in the northwest to about 30° in the southeast. Arrival times of compressional (P) and shear (S) waves from microearthquakes recorded by this array are used in this paper to investigate the three‐dimensional velocity structure of the crust and upper mantle in the region between the coast and the Cordillera Occidental. The results suggest a crustal thickness of about 40 km beneath the coast, increasing to about 70 km beneath the Cordillera Occidental. This change in thickness occurs abruptly in the northwest but is gradual in the southeast. The inverse correlation between the dip of the Moho and the dip of the slab suggests a broad‐scale, causal relation between the two. At the same time, free‐air gravity anomalies suggest that the steepening of the Moho in the northwest may occur only in the region under the array. Because the crust is thicker in the southeast than originally surmised, a suite of earthquakes previously thought to occur in the mantle now appears to be confined to the crust. The association of these earthquakes with a shallow dipping Moho is suggestive of ongoing crustal deformation to the west of the Andes. However, other seismological and geological observations make the role of these earthquakes in crustal deformation unclear. S wave velocities in the mantle between 70‐ and 130‐km depth above the 30° dipping slab are low, suggesting the presence of partially melted asthenosphere that may be responsible for the magmatic activity observed in southern Peru.

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