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J. H. Healy

Publications and source records attributed to J. H. Healy.

At least 37 records · Page 2Linked to original sources
USGS1983

Saudi Arabian seismic deep-refraction profiles; final project report

In February 1978 a seismic deep-refraction profile was recorded by the U.S. Geological Survey along a 1000-km line across the Arabian Shield in western Saudi Arabia. The line begins in Mesozoic cover rocks near Riyadh on the Arabian Platform, leads southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan (Tihamat-Asir), and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, including 19 in the Farasan Islands. Six shot points were used: five on land, with most charges placed below the water table in drill holes, and one at sea, with charges placed on the sea floor and detonated from a ship. Slightly more than 61 metric tons of explosives were used in 19 discrete firings. Seismic energy was recorded by 100 newly-developed portable seismic stations deployed in approximately 200 km-long arrays for each firing. Each station consisted of a standard 2-Hz vertical component geophone coupled to a self-contained analog recording instrument equipped with a magnetic-tape cassette. In this final report, we fully document the field and data-processing procedures and present the final seismogram data set as both a digital magnetic tape and as record sections for each shot point. Record sections include a normalized set of seismograms, reduced at 6 km/s, and a true-amplitude set, reduced at 8 km/s, which have been adjusted for amplifier gain, individual shot size, and distance from the shot point. Appendices give recorder station and shot information, digital data set descriptions, computer program listings, arrival times used in the interpretation, and a bibliography of reports published as a result of this project. We used two-dimensional ray-tracing techniques in the data analysis, and our interpretation is based primarily on horizontally layered models. The Arabian Shield is composed, to first-order, of two layers, each about 20 km thick, with average velocities of 6.3 km/s and 7.0 km/s, respectively. At the western shield margin the crust thins to less than 20 km total thickness, beyond which the Red Sea shelf and coastal plain are interpreted to be underlain by oceanic crust. A major crustal lateral velocity inhomogeneity northeast of Sabhah in the Shammar Tectonic Province is interpreted as the suture zone of two crustal blocks of different composition. Several high-velocity anomalies in the upper crust correlate with mapped gneissic dome structures. Two intra-crustal reflectors at13 km depth are interpreted as the tops of mafic intrusives. The Mohorovicic discontinuity beneath the shield varies from 43 km depth in the northeast with 8.2 km/s mantle velocity to 38 km depth in the southwest with 8.0 km/s mantle velocity. Two velocity discontinuities are identified in the upper mantle, at 59 and 70 km depth. We suggest further work, including refined analyses of the data employing filtering and synthetic seismogram techniques, as well as consideration of attenuation properties. Extension of the seismic refraction profile to the Arabian Gulf and some short profiles perpendicular to the existing profile would be fruitful areas for future field work.

Open-File Report↗
USGS1983

Crustal structure of southwestern Saudi Arabia

The southwestern Arabian Shield is composed of uplifted Proterozoic metamorphic and plutonic rocks. The Shield is bordered on the southwest by Cenozoic sedimentary and igneous rocks of the Red Sea paar and on the east by the Arabian Platform, an area of basin sedimentation throughout Phanerozoic time. The Shield appears to have been formed by successive episodes of island arc volcanism and sea-floor spreading, followed by several cycles of compressive tectonism and metamorphism. An interpretation and synthesis of a deep-refraction seismic profile from the Riyadh area to the Farasan Islands, and regional gravity, aeromagnetic, heat flow, and surface geologic data have yielded a self-consistent regional-scale model of the crust and upper mantle for this area. The model consists of two 20 km-thick layers of crust with an average compressional wave velocity in the upper crust of about 6.3 km/s and an average velocity in the lower. crust of about 7.0 km/s. This crust thins abruptly to less than 20 km near the southwestern end of the profile where Precambrian outcrops abut the Cenozoic rocks and to 8 km beneath the Farasan Islands. The data over the coastal plain and Red Sea shelf areas are fit satisfactorily by an oceanic crustal model. A major lateral velocity inhomogeneity in the crust is inferred about 25 km northeast of Sabhah and is supported by surface geologic evidence. The major velocity discontinuities occur at about the same depth across the entire Shield and are interpreted to indicate horizontal metamorphic stratification of the Precambrian crust. Several lateral inhomogenities in both the upper and lower .crust of the . Shield are interpreted, to indicate bulk compositional variations. The subcrustal portion of the model is composed of a hot, low-density lithosphere beneath the Red Sea which is systematically cooler and denser to the northeast. This model provides a mechanism which explains the observed topographic uplift, regional gravity pattern, heat flow, and mantle compressional wave velocities. Such a lithosphere could be produced by upwelling of hot asthenosphere beneath the Red Sea which then flows laterally beneath the lithosphere of the Arabian Plate.

Open-File Report↗
USGS1979

Seismic refraction profile, Kingdom of Saudi Arabia: field operations, instrumentation, and initial results

In February 1978 a seismic deep-refraction profile was recorded by the USGS along a 1000-km line across the Arabian Shield in western Saudi Arabia. The line begins in Paleozoic and Mesozoic cover rocks near Riyadh on the Arabian Platform, leads southwesterly across three major Precambrian tectonic provinces, traverses Cenozoic rocks of the coastal plain near Jizan (Tihamat Asir), and terminates at the outer edge of the Farasan Bank in the southern Red Sea. More than 500 surveyed recording sites were occupied, including 19 in the Farasan Islands. Six shot points were used--five on land, with charges placed mostly below water table in drill holes, and one at sea, with charges placed on the sea floor and fired from a ship. The total charge consumed was slightly in excess of 61 metric tons in 21 discrete firings. Seismic energy was recorded by means of a set of 100 newly developed portable seismic stations. Each station consists of a standard 2-Hz vertical geophone coupled to a self-contained analog recording instrument equipped with a magnetic-tape cassette. The stations were deployed in groups of 20 by five observer teams, each generally consisting of two scientist-technicians and a surveyor-guide. On the day prior to deployment, the instruments were calibrated and programmed for automatic operation by means of a specially designed device called a hand-held tester. At each of ten pre-selected recording time windows on a designated firing day, the instruments were programmed to turn on, stabilize, record internal calibration signals, record the seismic signals at three levels of amplification, and then deactivate. After the final window in the firing sequence, all instruments were retrieved and their data tapes removed for processing. A specially designed, field tape- dubbing system was utilized at shot point camps to organize and edit data recorded on the cassette tapes. The main functions of this system are to concatenate all data from each shot on any given day onto a single shot tape, and to provide hard copy for monitoring recorder performance so that any problems can be corrected prior to the next deployment. Composite digital record sections were produced from the dubbed tapes for each shot point by a portable processing and plotting system. The heart of this system is a DEC PDP 11VO3 computer, which controls a cassette playback unit identical to those used in the recorders and dubbers, a set of discriminators, a time-code translator, a digitizer, and a digital plotter. The system was used to maintain various informational data sets and to produce tabulations and listings of various sorts during the field operations, in addition to its main task of producing digital record sections. Two master clocks, both set to time signals broadcast by the British Broadcasting Corporation, provided absolute time for the recording operations. One was located on the ship and the other was stationed at a base camp on the mainland. The land-based master clock was used to set three additional master clocks located at the other active shot points a few days in advance of each firing, and these clocks were then used to set the internal clocks in the portable seismic stations via the hand-held tester. A master clock signal was also linked to the firing system at each shot point for determination of the absolute shot instant. It is possible to construct a generalized crustal model from examination of the six shot point composite record sections obtained in the field. Such a model rests upon a number of simplifying assumptions and will almost certainly be modified at a later stage of interpretation. The main assumptions are that the crust consists of two homogeneous isotropic layers having no velocity inversion,, that the Mohorovicic discontinuity is sharp, and that effects of surface inhomogeneities and elevation changes can be ignored. The main characteristics of the tentative model are the following: (1) The thickness of th

Open-File Report↗
USGS1977

Preliminary stress measurements in central California using the hydraulic fracturing technique

Use of the hydraulic fracturing technique for determining in situ stress is reviewed, and stress measurements in wells near the towns of Livermore, San Ardo, and Menlo Park, California are described in detail. In the Livermore well, four measurements at depths between 110 and 155 m indicate that the least principal compressive stress is horizontal and increases from 1.62 to 2.66 MPa. The apparent direction of maximum compression is N 70?? E (??40??). At the San Ardo site the least principal stress is that due to the overburden weight. At depths of 240.2 and 270.7 m the minimum and maximum horizontal stresses are estimated to be 11.4 and 22.5 MPa, and 12.0 (??1.1) and 15.8 (??3.3) MPa, respectively. From an impression of the fracture at 240.2 m, the direction of maximum compression appears to be about N 15?? E. The rock in the Menlo Park well is too highly fractured to yield a reliable measurement of the horizontal stresses. The data indicate, however, that the least principal stress is vertical (due to the overburden weight) to a depth of 250 m. ?? 1977 Birkha??user Verlag.

Pure and Applied Geophysics PAGEOPH↗
USGS1976

An experiment in earthquake control at Rangely, Colorado

An experiment in an oil field at Rangely, Colorado, has demonstrated the feasibility of earthquake control. Variations in seismicity were produced by controlled variations in the fluid pressure in a seismically active zone. Precise earthquake locations revealed that the earthquakes clustered about a fault trending through a zone of high pore pressure produced by secondary recovery operations. Laboratory measurements of the frictional properties of the reservoir rocks and an in situ stress measurement made near the earthquake zone were used to predict the fluid pressure required to trigger earthquakes on preexisting fractures. Fluid pressure was controlled by alternately injecting and recovering water from wells that penetrated the seismic zone. Fluid pressure was monitored in observation wells, and a computer model of the reservoir was used to infer the fluid pressure distributions in the vicinity of the injection wells. The results of this experiment confirm the predicted effect of fluid pressure on earthquake activity and indicate that earthquakes can be controlled wherever we can control the fluid pressure in a fault zone.

Colorado↗
USGS1973

Structure of the crust in the conterminous United States

On the basis of the newest interpretations of all sufficiently long seismic profiles, a contour map of the Mohorovičić discontinuity for the conterminous United States is compiled. Differences from earlier interpretations are discussed.

conterminous United States↗
USGS1972

Prospects for earthquake prediction and control

The San Andreas fault is viewed, according to the concepts of seafloor spreading and plate tectonics, as a transform fault that separates the Pacific and North American plates and along which relative movements of 2 to 6 cm/year have been taking place. The resulting strain can be released by creep, by earthquakes of moderate size, or (as near San Francisco and Los Angeles) by great earthquakes. Microearthquakes, as mapped by a dense seismograph network in central California, generally coincide with zones of the San Andreas fault system that are creeping. Microearthquakes are few and scattered in zones where elastic energy is being stored. Changes in the rate of strain, as recorded by tiltmeter arrays, have been observed before several earthquakes of about magnitude 4. Changes in fluid pressure may control timing of seismic activity and make it possible to control natural earthquakes by controlling variations in fluid pressure in fault zones. An experiment in earthquake control is underway at the Rangely oil field in Colorado, where the rates of fluid injection and withdrawal in experimental wells are being controlled.

California↗
USGS1970

Earthquakes induced by fluid injection and explosion

Earthquakes generated by fluid injection near Denver, Colorado, are compared with earthquakes triggered by nuclear explosion at the Nevada Test Site. Spatial distributions of the earthquakes in both cases are compatible with the hypothesis that variation of fluid pressure in preexisting fractures controls the time distribution of the seismic events in an “aftershock” sequence. We suggest that the fluid pressure changes may also control the distribution in time and space of natural aftershock sequences and of earthquakes that have been reported near large reservoirs.

Colorado↗
USGS1970

Nuclear explosions and distant earthquakes: A search for correlations

An apparent correlation between nuclear explosions and earthquakes has been reported for the events between September 1961 and September 1966. When data from the events between September 1966 and December 1968 are examined, this correlation disappears. No relationship between the size of the nuclear explosions and the number of distant earthquakes is apparent in the data.

Science↗
USGS1970

LASA anomalies and their relations to crust and upper-mantle structure

Body-wave data from nine teleseisms recorded at the Large Aperture Seismic Array (LASA), Montana, have been analyzed. The events were at different azimuths and distances from LASA. Apparent-velocity measurements using arrival-time data at the subarrays differed by about 30 percent for events in the northwest and southeast directions.Apparent velocities were high near the center of the array for events in the northwest and low for events in the southeast. For events in the other azimuths, the patterns were more complex, but anomalous values were found near the center. Apparent-velocity measurements at subarrays were made using a cross-spectral technique. The resulting values showed patterns similar to those obtained using arrival-time data near the center. Anomalous high and low values were found near the center and also at the E and F subarrays.

Open-File Report↗
USGS1968

A method for estimating the uncertainty of seismic velocities measured by refraction techniques

Time residuals from 7 5-km segments of 18 crustal seismic-refraction profiles in the Basin and Range province are used to investigate the validity of the linear-regression model and to make large sample estimates of the variance in the travel time distributions. A formula for unbiased estimates of velocity uncertainty is derived, assuming a linear trend with distance for the variances of the travel-time distributions. If the recording units are symmetric about the center of the recording interval, this formula is equivalent to the one derived assuming the variances are equal. At the 95-per cent confidence level the chi-squared test implied 84 per cent of the time-residual samples were inconsistent with the hypothesis that their parent populations had Gaussian distributions. If the number of recording locations expceeds 8, confidence limits computed without the Gaussian assumption suggest the departures from normality are not significant for velocity uncertainty estimates. The large sample estimates of the time-residual populations may be applicable to other areas. This evidence motivated the development of a method, requiring very little numerical calculation, for estimating uncertainties in velocities. The method requires, in addition to the large sample estimates of the travel time variances, information on the quality of the data, the location of the recording interval, and the number of recording units. The method is useful for the design of new experiments and independent estimates of uncertainty reported in the literature.

Arizona, California, Idaho, Nevada, Oregon, Utah↗