Allen H. Cogbill

Gravity terrain corrections calculated using digital elevation models

Corrections for terrain effects are required for virtually all gravity measurements acquired in mountainous areas, as well as for high‐precision surveys, even in areas of low relief. Terrain corrections are normally divided into two parts, one part being the correction for terrain relatively close to the gravity station (the “inner‐zone” correction) and the other part being the correction for more distant, say, >2 km, terrain. The latter correction is normally calculated using a machine procedure that accesses a digital‐terrain data set. The corrections for terrain very close to the gravity station are done manually using Hammer’s (1939) procedures or a similar method, are guessed in the field, or simply are neglected. Occasionally, special correction procedures are used for the inner‐zone terrain corrections (e.g., LaFehr et al., 1988); but such instances are uncommon.

A geophysical‐geological transect of the Silent Canyon Caldera Complex, Pahute Mesa, Nevada

Revision of lithological logs for boreholes penetrating the volcanic center at Pahute Mesa, Nevada, has led to a thorough review of the volcanic stratigraphy and geologic structure. We have combined this review with a compilation of old and newly acquired gravity and seismic travel time data, producing a unified interpretation along a northwest to southeast profile. The analysis supports a new interpretation of the Silent Canyon caldera complex. The caldera is found to be more asymmetric than previously suggested, with the southeastern boundary formed by linear, high-angle normal faults and a more gently sloping northwestern boundary. The total thickness of volcanic units within the caldera complex does not appear to exceed 5 km. The shallow structure at Pahute Mesa could have a profound effect on the seismic response for regional and teleseismic signals from this nuclear test site. The Silent Canyon caldera complex is actually a set of nested calderas first filled by thick (>1 km) postcaldera lavas and subsequently buried by outflow sheets of the Timber Mountain caldera to the south. Thick, postcaldera lavas filled a half-graben structure formed west of the West Greeley fault, dropping the tops of the youngest caldera-forming units to depths in excess of 2 km. Therefore the western boundary of the caldera complex is poorly defined. East of the West Greeley fault, two overlapping calderas are defined, and stratigraphic data suggest the presence of even older calderas. The youngest caldera, the calc-alkaline Area 20 caldera, is well defined from drill hole data. The Area 20 caldera overlaps the 13.6 Ma peralkaline Grouse Canyon caldera, which is less well defined, but apparently collapsed in trap-door style along the Almendro fault. For both these calderas, collapse continued after the main caldera-forming eruption, concurrent with the accumulation of thick (>1 km) lavas within the peripheral collapse zones. The geophysical interpretation indicates that the major structural boundary of the caldera complex corresponds to the NNE trending Scrugham Peak and Almendro faults, which offset the pre-Tertiary contact more than 1 km but have less than 200 m offset in rocks of 11 Ma age. Drill hole data show that offsets along these faults increase systematically within older (up to 15 Ma) units, which are commonly rotated eastward in a style similar to units at the surface. Abrupt changes in the subsurface thickness of the caldera-forming units occur across the faults, indicating that these linear features served as caldera boundaries.

Improving Regional Seismic Event Location in China

Abstract — In an effort to improve our ability to locate seismic events in China using only regional data, we have developed empirical propagation path corrections and applied such corrections using traditional location routines. Thus far, we have concentrated on corrections to observed P arrival times for crustal events using travel-time observations available from the USGS Earthquake Data Reports, the International Seismic Centre Bulletin, the preliminary International Data Center Reviewed Event Bulletin, and our own travel-time picks from regional data. Location ground truth for events used in this study ranges from 25 km for well-located teleseimic events, down to 2 km for nuclear explosions located using satellite imagery. We also use eight events for which depth is constrained using several waveform methods. We relocate events using the EvLoc algorithm from a region encompassing much of China (latitude 20°–55°N; longitude 65°–115°E). We observe that travel-time residuals exhibit a distance-dependent bias using IASPEI91 as our base model. To remedy this bias, we have developed a new 1-D model for China, which removes a significant portion of the distance bias. For individual stations having sufficient P-wave residual data, we produce a map of the regional travel-time residuals from all well-located teleseismic events. Residuals are used only if they are smaller than 10 s in absolute value and if the seismic event is located with accuracy better than 25 km. From the residual data, correction surfaces are constructed using modified Bayesian kriging. Modified Bayesian kriging offers us the advantage of providing well-behaved interpolants and their errors, but requires that we have adequate error estimates associated with the travel-time residuals from which they are constructed. For our P-wave residual error estimate, we use the sum of measurement and modeling errors, where measurement error is based on signal-to-noise ratios when available, and on the published catalog estimate otherwise. Our modeling error originates from the variance of travel-time residuals for our 1-D China model. We calculate propagation path correction surfaces for 74 stations in and around China, including six stations from the International Monitoring System. The statistical significance of each correction surface is evaluated using a cross-validation technique. We show relocation results for nuclear tests from the Balapan and Lop Nor test sites, and for earthquakes located using interferometric synthetic aperture radar. These examples show that the use of propagation path correction surfaces in regional relocations eliminates distance bias in the residual curves and significantly improves the accuracy and precision of seismic event locations.

Comparison of four moderate-size earthquakes in southern California using seismology and InSAR

Source parameters determined from interferometric synthetic aperture radar (Insar) measurements and from seismic data are compared from four moderate-size (less than M 6) earthquakes in southern California. The goal is to verify approximate detection capabilities of Insar, assess differences in the results, and test how the two results can be reconciled. First, we calculated the expected surface deformation from all earthquakes greater than magnitude 4 in areas with available Insar data (347 events). A search for deformation from the events in the interferograms yielded four possible events with magnitudes less than 6. The search for deformation was based on a visual inspection as well as cross-correlation in two dimensions between the measured signal and the expected signal. A grid-search algorithm was then used to estimate focal mechanism and depth from the Insar data. The results were compared with locations and focal mechanisms from published catalogs. An independent relocation using seismic data was also performed. The seismic locations fell within the area of the expected rupture zone for the three events that show clear surface deformation. Therefore, the technique shows the capability to resolve locations with high accuracy and is applicable worldwide. The depths determined by Insar agree with well-constrained seismic locations determined in a 3D velocity model. Depth control for well-imaged shallow events using Insar data is good, and better than the seismic constraints in some cases. A major difficulty for Insar analysis is the poor temporal coverage of Insar data, which may make it impossible to distinguish deformation due to different earthquakes at the same location.

Candelaria and other left-oblique slip faults of the Candelaria region, Nevada

The region comprising the Candelaria Hills and Excelsior Mountains, Nevada, is structurally anomalous with respect to surrounding tracts of the Basin and Range Province. Its major faults strike east, and some, if not all of them, have undergone significant left slip. Ratios of lateral to vertical displacement components on strands of such faults vary from 2 to 6.5. The region of left-oblique slip faulting seems to constitute the intermediate leg of a gigantic Z pattern of Basin and Range faults. One of these faults, the Candelaria fault system, has been studied closely in pursuit of kinematic details that might characterize the regional faulting. The Candelaria system consists of three en echelon faults of 5 to 10 km trace length joined by connector zones that possess compressional features. The amount of lateral slip transferred from one en echelon fault to another is uncertain; analysis of a complex of faults in one connector zone indicates at least some degree of integrated motion. The same connector zone is the site of a bulge that has an uplift estimated to have been 275 m since 2.8 m.y. B.P. On the basis of fault-plane striae, the extension direction is ∼ N82°W. We propose that the Candelaria fault system and a major asymmetric trough in its northern wall were created in Oligocene time by a regional left-lateral shear system oriented in the extension direction. The connector zones may be kinematic impediments which, at shallow levels at least, have prevented the system from maturing into a single plane fault. The existence of the bulge and the estimated rate of slip on part of the Candelaria fault system suggest that the system is still active.

Uncovering buried volcanoes at Yucca Mountain

The hazard posed by small-volume basaltic volcanism to the proposed Yucca Mountain nuclear waste repository in Nevada has been a topic of scientific debate for well over a decade. In the past few years, debate has focused on the extent and age of buried volcanoes in alluvial-filled basins of the Yucca Mountain region (YMR) and their potential impact on volcanic hazard estimates. To address this issue, the U.S. Department of Energy (DOE) has sponsored an exceptionally high resolution aeromagnetic survey completed in 2004, and a presently ongoing drilling program to characterize the location, age, volume, and chemistry of buried basalt in the YMR (Figure 1). DOE has convened an expert panel that will use this information to update probabilistic volcanic hazard estimates originally obtained by experts in 1996. Partly on the basis of the unknown extent of buried volcanoes, Smith and Keenan [2005] suggested that volcanic hazard estimates might be 1–2 orders of magnitude higher than estimated by the 1996 expert elicitation.

Recent developments in digital gravity data acquisition on land

Land gravity meters have traditionally been analog, with readings carried out and transcribed manually. The precision (theoretically 10–50 μGal) and the observation rate are very dependent on the skill of the observer. However, the advent of digital recording meters, now offered by several commercial firms, has automated much of the work. As a result, land gravity surveying is both faster and more accurate than in the past and can now be applied to problems where it previously was of little help.

Earthquake and volcano clustering via stress transfer at Yucca Mountain, Nevada

The proposed national high-level nuclear waste repository at Yucca Mountain is close to Quaternary cinder cones and faults with Quaternary slip. Volcano eruption and earthquake frequencies are low, with indications of spatial and temporal clustering, making probabilistic assessments difficult. In an effort to identify the most likely intrusion sites, we based a three-dimensional finite-element model on the expectation that faulting and basalt intrusions are sensitive to the magnitude and orientation of the least principal stress in extensional terranes. We found that in the absence of fault slip, variation in overburden pressure caused a stress state that preferentially favored intrusions at Crater Flat. However, when we allowed central Yucca Mountain faults to slip in the model, we found that magmatic clustering was not favored at Crater Flat or in the central Yucca Mountain block. Instead, we calculated that the stress field was most encouraging to intrusions near fault terminations, consistent with the location of the most recent volcanism at Yucca Mountain, the Lathrop Wells cone. We found this linked fault and magmatic system to be mutually reinforcing in the model in that Lathrop Wells feeder dike inflation favored renewed fault slip.

Repeatability study of helicopter-borne electromagnetic data

Helicopter-borne electromagnetic (EM) responses depend very much upon the altitude and plan-view flight path, especially when the resistivity of the terrain’s materials varies laterally and/or vertically. Spatially consistent flight paths are required for repeatability analysis of the EM data. Caution should be used in examining the repeatability of the EM data because poor repeatability could result from spatially inconsistent flight paths. However, the apparent resistivity converted from the EM responses is virtually independent of the sensor altitude and directly reflects variations in the resistivity. Therefore, more meaningful repeatability analyses are achieved if the apparent resistivity is used instead of the EM response itself. We have analyzed 32 flights over a control line by using the EM amplitude, the phase, and the apparent resistivity. Our results show that the crosscorrelation for all 496 paired combinations of flights is better for the apparent resistivity than for the EM amplitude or pha...

Terrain Mapping By Reflectorless Laser Rangefinding Systems For Inner Zone Gravity Terrain Corrections

Reflectorless laser rangefmding technology has the ability to generate reflections from many types of materials and angles of incidence without a comer reflector . These laser “guns” have built in inclinometers and compasses which when combined with the laser for distance measurements result in three dimensional mapping, a hand held total station. Although the angle and distance resolution is much poorer than that for total stations, for short (hundred meters) distances the mapping is more than adequate for terrain mapping for gravity terrain corrections. The nearby terrain model is usually estimated visually perhaps with an inclinometer and an optical rangefmder at best, but not digitally and with poor repeatability. The inaccuracy of present inner terrain correction methods can be detrimental to many surveys, and especially for microGal surveys. The combination of digital gravity meters, Global Positioning System locations and the laser gun terrain models result in a totally digital, quantitative gravity surveying system with greater ultimate accuracy. Field procedures and custom software have been developed and combined with a Laser Atlanta Optics Advantage rangefmder for gravity terrain corrections. Examples of surveys in the Dallas, Texas area, in canyons near Sodaville, Nevada, and at an operating mine in northern Nevada are presented. The terrain corrections from laser mapping when compared to corrections from 1:24,000 forty meter USGS and high resolution photogrammetric DTM models, show significant differences. The results demonstrate that the reflectorless laser rangefmder with inclinometer and compass can provide digital, quantitative terrain models for better terrain correction accuracy more efficiently and economically than previously possible and that real time gravity surveying is possible.