Horizontal Offsets Research Articles

Slow rupture in Andaman during 2004 Sumatra-Andaman earthquake: a probable consequence of subduction of 90°E ridge

One of the most enigmatic features of the 2004 Sumatra–Andaman earthquake was the slow rupture speed and low slip on the northern part of the rupture under the Andaman region. We propose that the aseismic 90°E Ridge (NER) on the Indian Plate obliquely subducts under the Andaman frontal arc region. Though other possibilities also exist, we hypothesized that this ridge probably acted as a structural barrier influencing rupture characteristics of the earthquake. Here we present several features of the Andaman region that favour NER subduction under the region, which include (i) comparatively shallow bathymetry and trench depth, (ii) low seismicity, (iii) significant variation in the azimuths of coseismic horizontal offsets due to the 2004 Sumatra–Andaman earthquake, (iv) lack of post-seismic afterslip on the coseismic rupture in the Andaman frontal arc region, (v) low P wave with only small decrease in S wave speed from tomographic studies, (vi) gravity anomalies on the Indian Plate indicating continuation of the ridge under the Andaman frontal arc and (vii) lack of back arc volcanoes in the Andaman region.

Coseismic reverse- and oblique-slip surface faulting generated by the 2008 Mw 7.9 Wenchuan earthquake, China

The Mw 7.9 Wenchuan, China, earthquake ruptured two large thrust faults along the Long-menshan thrust belt at the eastern margin of the Tibetan Plateau. This earthquake generated a 240-km-long surface rupture zone along the Beichuan fault and an additional 72-km-long surface rupture zone along the Pengguan fault. Maximum vertical and horizontal offsets of 6.5 m and 4.9 m, respectively, were measured along the Beichuan fault. A maximum vertical offset of 3.5 m was measured along the Pengguan fault. Coseismic surface ruptures, integrated with aftershocks and industry seismic profiles, show that two imbricate structures have ruptured simultaneously, resulting in the largest continental thrust event ever documented. Large oblique thrusting observed during this earthquake indicates that crustal shortening is an important process responsible for the high topography in the region, as everywhere along the edge of Tibetan Plateau.

A reevaluation of the tolerance to vertical misalignment in stereopsis

The stereoscopic system tolerates some vertical misalignment of the images in the eyes. However, the reported tolerance for an isolated line stimulus (approximately 4 degrees) is greater than for a random-dot stereogram (RDS, approximately 45 arcmin). We hypothesized that the greater tolerance can be attributed to monoptic depth signals (E. Hering, 1861; M. Kaye, 1978; L. M. Wilcox, J. M. Harris, & S. P. McKee, 2007). We manipulated the vertical misalignment of a pair of isolated stereoscopic dots to assess the contribution of each depth signal separately. For the monoptic stimuli, where only one half-image was present, equivalent horizontal and vertical offsets were imposed instead of disparity. Judgments of apparent depth were well above chance, though there was no conventional disparity signal. For the stereoscopic stimuli, one element was positioned at the midline where monoptic depth perception falls to chance but conventional disparity remains. Subjects lost the depth percept at a vertical misalignment of between 44 and 88 arcmin, which is much smaller than the limit found when both signals were provided. This tolerance for isolated stimuli is comparable to the reported tolerance for RDS. We conclude that previous reports of the greater tolerance to vertical misalignment for isolated stimuli arose from the use of monoptic depth signals.

Multivariate analysis of Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) Venus Express nightside and limb observations

Spectral signatures measured by Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on the nightside and at the limb of Venus are analyzed with Independent Component Analysis. A methodology has been set up to minimize instrumental effects and to interpret the results on the basis of studies of the most common situations in the data set. The main spectral components commonly retrieved on the nightside include the bulk signal modulated by atmospheric opacity variations, photometric variations in the long‐wavelength atmospheric windows, a branching parameter describing particle size variations, and O2 emission at 1.26 and 1.58 μm. Faint atmospheric windows are detected at 1.51, 1.55, 1.78, and 1.82 μm for the first time. The polar vortex structure is outlined, with two main circular areas made of many concentric rings with alternating particle sizes. Discrete clouds about 100 km across are observed in low opacity conditions. High‐altitude, warm clouds are tentatively observed from the polar vortex down to 55°S. At the limb, the two signatures of CO2 nonlocal thermodynamic equilibrium emission are directly mapped, and the thermal structure of the cloud layers and upper atmosphere is apparent. Surface emission is detected with a spatial resolution limited by atmosphere blurring, reaching ∼35 km in exceptional conditions. Horizontal offsets indicate that the radiation propagates mostly vertically, consistent with the large optical depth and vertical extent of the cloud layer. Intense scattering is suspected to take place at the bottom of the atmosphere, at least in the southern plains.

Distribution of the right-lateral strike–slip motion from the Main Recent Fault to the Kazerun Fault System (Zagros, Iran): Evidence from present-day GPS velocities

GPS measurements across the Kazerun Fault System in the Zagros mountain belt provide first instantaneous velocities on the different segments. These results are closely consistent with the geological fault slip rates (over 150 ka), implying stable velocities over a longer period. The present-day strike–slip motion is distributed from the Main Recent Fault to the N-trending Kazerun Fault System along a preferential en-echelon fault zone included in a more distributed fan-shape fault pattern. The Hormuz salt decoupling layer cannot be the only cause of a sedimentary spreading because seismicity attests these faults are rooted in the basement. The Dena fault (3.7 mm/yr) transfers the MRF fault slip mainly to the Kazerun (3.6 mm/yr) and slightly to the High Zagros and Sabz Pushan faults (1.5 mm/yr), and the Kazerun fault further to the Kareh Bas fault (3.4 mm/yr). Total geological horizontal offsets associated with GPS slip rates help inferring precise fault slip onset ages. The successive onsets deduced by this approach imply that the right-lateral strike-slip activity of the MRF has propagated in time southeastward to the Dena segment, and then to the Kazerun segment and to the Kareh Bas fault.

Coseismic Slip Distribution of the 2001 Kunlun Mountain Pass West Earthquake Constrained by GPS and Insar Data

AbstractThe rupture process of the 2001 Ms 8.1 Kunlun Mountain Pass West earthquake was quite complex. Studies on its coseismic rupture distribution using different datasets, methods, and models have so far produced rather different results. We use coseismic displacement data from the Global Positioning System (GPS) and Interferometric Synthetic Aperture Radar (InSAR) measurements before and after the earthquake to invert for its rupture distribution. We test multiple model parameters, and provide a more detailed result than previous studies under an optimized smoothing constraint. The modeling process consists of three steps: First, assuming a vertical fault plane we obtain an optimal smoothing constraint for the solution based on balancing a trade‐off between the solution resolution and model misfit. Second, we vary dipping angles of fault segments to search for optimal dipping angles in data fitting. Finally, we adopt the fault smoothing constraint and dipping angles obtained from previous two steps and invert for the coseismic rupture distribution. Comparing with previous studies, our results agree better with the field survey results on horizontal surface offsets. We also find that the coseismic displacement amplitudes are asymmetric across the seismic fault, with the displacements south of the fault about 10%~20% larger in amplitude than that north of the fault. This observation can be explained by the fault dipping 80°~81° to the south. We also demonstrate for the first time that a left‐stepping fault about 50 km long in between the west end of the East Kunlun fault and the east end of the Sun Lake fault absorbed 0.1~0.2 m of normal faulting, and the Kunlun Pass fault absorbed about 0.8 m thrust faulting. The seismic moment release is estimated as 9.3 × 1020 N·m, corresponding to a Mw 8.0 earthquake.

Sensation-Targeted Motor Control: Every Spike Counts? Focus on: “Whisker Movements Evoked by Stimulation of Single Motor Neurons in the Facial Nucleus of the Rat”

Traditionally, sensory processing and motor control have been studied separately, reflecting the belief that sensory and motor streams remain independent until linked via cortical “associative” areas. Although this belief no longer dominates neuroscience, the traditional tendency to continue to

Detecting co-seismic displacements in glaciated regions: An example from the great November 2002 Denali earthquake using SPOT horizontal offsets

We use SPOT image pairs to determine horizontal offsets associated with the Mw 7.9 November 2002 Denali earthquake in the vicinity of Slate Creek, AK. Field measurements and aerial photographs are used to further characterize the geometry of the surface rupture. Aerial photographs show that shear localization occurs where the rupture trace is linear, and distributed off-fault deformation is common at fault bends and step-overs, or at geologic contacts between rock, glacial sediments, and ice. The displacement field is generated using a sub-pixel cross correlation technique between SPOT images taken before and after the earthquake. We identify the effects of glacier motion in order to isolate the tectonic displacements associated with the Denali earthquake. The resulting horizontal displacement field shows an along-strike variation in dextral shear, with a maximum of approximately 7.5 m in the east near 144° 52′W, which decreases to about 5 m to the west near 145° 45′W. If the November 2002 earthquake represents the long-term behavior of the Denali fault, it implies a westward decrease in the long-term dextral slip rate. A possible mechanism to accommodate the westward decreasing slip on the Denali fault is to transfer fault slip to adjacent east-trending contractional structures in the western region of the central Alaskan Range.

Rethinking Geometric Design Standards for Bike Paths

The “single file” and “side-by-side” travel characteristics of bicyclists and pedestrians should be considered for the determination of stopping sight distances, horizontal sightline offsets, and minimum lengths of vertical curves for bike paths. This paper describes these unique travel behaviors and classifies them into two cases (Case I and Case II) for geometric design. It presents a set of new design equations and charts that consider these two design cases and introduces design methods to supplement current AASHTO standards and guidelines. The paper then provides a practical procedure for using the new charts and equations for design of bike paths. It is recommended that the new analytical means of evaluating the effect of side-by-side riding on geometric design of bike paths should be included in the revision of the AASHTO Bike Guide for the Development of Bicycle Facilities.

Real-time kinematic tracking of towed AEM birds

In the absence of attitude and altitude sensors directly attached to the bird, helicopter airborne electromagnetic (AEM) data are typically interpreted assuming that the sensor bird maintains a fixed attitude as well as fixed vertical and horizontal offsets relative to the helicopter during survey. Laser altimeters fitted to the bird can be used for measuring bird-height over land, but these altimeters do not necessarily function over seawater, and in this case a fixed vertical offset is subtracted from the helicopter altimetry to estimate the height of the bird above sea level. With current navigation technology, these assumptions could be overcome by incorporating suitable altimetry and navigation sensors into AEM systems. We constructed an airborne testing rig to represent an AEM bird and fitted GPS, inertial navigation, and altimetry sensors to accurately measure bird attitude and height above seawater (as required for bathymetric mapping) during typical and atypical AEM-survey flight conditions. Bird height above sea level was measured with radar and laser altimeters, and was also estimated from the GPS receiver height. Bird attitude was obtained from the inertial navigation unit (INU) data and was compared with attitude data derived from a triangular configuration of three GPS antennas. Each antenna was linked to a pair of GPS receivers to allow comparison between dual-frequency, high-fidelity and single-frequency, low-fidelity measurements.Bird attitude and altimetry measurements were recorded during surveys flown offshore Tickera Bay (Spencer Gulf, SA) and within Broken Bay (Sydney, NSW). These surveys were flown at a maximum altitude of 180 m, with bird roll angles up to about ± 40°. Using dual-frequency GPS receivers, the agreement between heights derived from GPS and laser (radar) altimeter data is typically ~0.3 m (0.6 m). GPS antenna separations computed during flight from measured GPS positions gave agreement to within 0.4% (typically 0.2%) of measured values. The agreement between pitch and roll angles computed from GPS antenna positions and INU measurements was within ~1° and 2° respectively, neglecting the effects of any offsets in the alignment of coordinate axes between the two systems. A comparison of pitch and roll angles obtained from single and dual-frequency GPS receivers showed that the accuracy of pitch and roll angles obtained from single-frequency GPS receivers is generally about ± 2°. The discrepancy in results from the different GPS receivers increases at various points along the flight path. This increase was attributed to a decrease in accuracy in results from the single frequency GPS receivers. Roll and pitch angle profiles show oscillations consistent with harmonic (pendulum) motion of the bird at the end of the tow cable connecting the bird to the helicopter. We recommend the use of inertial navigation interfaced to a single dual-frequency GPS receiver for accurate attitude and position measurements, combined with laser and radar altimetry sensors. The future implications of this study are that we expect to accurately measure attitude and altitude of a towed bird over seawater and that consequently these altimetry and attitude sensors will be implemented on a new helicopter TEM system (SeaTEM) specifically designed for bathymetric mapping.

Four Major Strike-slip Faults on the Moon

Summary Four of the most conspicuous lunar lineaments are examined in relation to surrounding topography and are found to be faults with maximum horizontal offsets of between 12 and 32 km. The analyses provide evidence of protracted growth of the faults and of periods of plasticity of the rocks, and the results support the hypothesis of a crustal meridional compression.

Albacora Leste Field - P-50 FPSO Mooring System

This article, written by Technology Editor Dennis Denney, contains highlights of paper OTC 18243, "Albacora Leste Field Development - FPSO P-50 Mooring- System Concept and Installation," by F.E.N. Brandao, C.C.D. Henriques, J.B. de Araujo, O.C.G. Ferreira, and C. dos Santos Amaral, Petrobras, prepared for the 2006 Offshore Technology Conference, Houston, 1–4 May. The mooring system for the P-50 floating production, storage, and offloading (FPSO) vessel is an 18-line, partially compliant spread-mooring system, with 10 lines to the bow and eight lines to the stern. The mooring lines comprise polyester ropes and steel chain in a semitaut configuration. This project was the heaviest synthetic-line mooring system designed for a floating production unit. Introduction Since the late 1990s, free-fall piles, "torpedoes," have been used offshore Brazil. Initially, torpedoes were used to moor flexible risers to the sea bottom, then later to moor semisubmersible drilling rigs. In April 2001, the classification group Bureau Veritas certified a design called T-43 (a 43-tonne torpedo pile). These torpedoes were used to moor a deepwater drilling rig and the floating storage unit Avaré, a former 30,000-DWT tanker, installed in the Santos basin offshore Brazil. The mooring analyses of the P-50 FPSO showed that the required anchor-holding power would be approximately 1000 tonnes and the use of a differentiated compliance anchoring system with polyester ropes would require the use of anchors with high vertical-holding capacity, such as vertical-loaded anchors (VLAs) or suction piles. VLAs had been used to moor semisubmersible floating production units, such as P-27, P-36, and P-40 in the Campos basin. However, for the holding capacity required to moor the P-50 (a 270,000-DWT tanker), three or four anchor-handling vessels (AHVs) with sufficient bollard pull would be required to embed the anchors in the soil. Experience with the P-43 installations showed that the required installation time could be much shorter with torpedo piles than with VLAs. In October 2000, development of a torpedo that could meet P-50 requirements began and resulted in the T-98 design (a 98-tonne torpedo). P-50 Mooring System P-50, a former very large crude carrier converted into an FPSO, operates in 1240 m water depth in the Albacora Leste field, Campos basin, Brazil. The heading of the unit is 208° from true north, and the mooring pattern is shown in Fig. 1. The seabed has a maximum slope of 4°, and the deepest anchor was set at a water depth of 1384 m. Because flexible risers are connected to the FPSO, the mooring-system configuration was designed not to exceed the maximum horizontal offset and rotation limits around the vertical axis in the intact and damaged (one line broken) conditions. On the basis of the riser specifications and operating experience, the horizontal offsets were set not to exceed 10% of the water depth in the intact condition and 15% in the damaged condition. Each mooring line consists of studless chains and polyester ropes, designed for 25-year life.

Millennial Recurrence of Large Earthquakes on the Haiyuan Fault near Songshan, Gansu Province, China

The Haiyuan fault is a major active left-lateral fault along the northeast edge of the Tibet-Qinghai Plateau. Studying this fault is important in understanding current deformation of the plateau and the mechanics of continental deformation in general. Previous studies have mostly focused on the slip rate of the fault. Paleo- seismic investigations on the fault are sparse, and have been targeted mostly at the stretch of the fault that ruptured in the 1920 M 8.6 earthquake in Ningxia Province. To investigate the millennial seismic history of the western Haiyuan fault, we opened two trenches in a small pull-apart basin near Songshan, in Gansu Province. The excavation exposes sedimentary layers of alternating colors: dark brown silty to clayey deposit and light yellowish brown layers of coarser-grained sandy deposit. The main fault zone is readily recognizable by the disruption and tilting of the layers. Six paleoseismic events are identified and named SS1 through SS6, from youngest to oldest. Charcoal is abundant, yet generally tiny in the shallowest parts of the trench exposures. Thirteen samples were dated to constrain the ages of paleoseismic events. All six events have occurred during the past 3500-3900 years. The horizontal offsets associated with these events are poorly known. However, events SS3 to SS6 appear to be large ones, judging from comparison of vertical separations and widths of fault zones. The youngest event SS1 instead seems to be a minor one, probably the 1990 Mw 5.8 earthquake. Thus, four large events in 3500-3900 years would imply a re- currence interval of about 1000 years. Three events SS2 to SS4 prior to 1990 occurred sometime during 1440-1640 A.D., shortly after 890-1000 A.D. and 0-410 A.D., re- spectively. We tentatively associate them with the 1514 A.D., 1092 A.D., and 143 or 374 A.D. historical earthquakes. Taking 10 2 m of slip for large events (SS3 and SS4), comparable to the 1920 M 8 Haiyuan earthquake, their occurrence times would be consistent with the long-term 12 4 mm/yr estimate of Lasserre et al. (1999). However, a more realistic evaluation of slip rate and its possible change with time requires a more rigorous determination of coseismic slip amounts of past earth- quakes.

Evaluating digital elevation models for glaciologic applications: An example from Nevado Coropuna, Peruvian Andes

This paper evaluates the suitability of readily available elevation data derived from recent sensors – the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and the Shuttle Radar Topography Mission (SRTM) – for glaciological applications. The study area is Nevado Coropuna (6426 m), situated in Cordillera Ampato of Southern Peru. The glaciated area was 82.6 km 2 in 1962, based on aerial photography. We estimate the glacier area to be ca. 60.8 km 2 in 2000, based on analysis of the ASTER L1B scene. We used two 1:50,000 topographic maps constructed from 1955 aerial photography to create a digital elevation model with 30 m resolution, which we used as a reference dataset. Of the various interpolation techniques examined, the TOPOGRID algorithm was found to be superior to other techniques, and yielded a DEM with a vertical accuracy of ± 14.7 m. The 1955 DEM was compared to the SRTM DEM (2000) and ASTER DEM (2001) on a cell-by-cell basis. Steps included: validating the DEM's against field GPS survey points on rock areas; visualization techniques such as shaded relief and contour maps; quantifying errors (bias) in each DEM; correlating vertical differences between various DEM's with topographic characteristics (elevation, slope and aspect) and subtracting DEM elevations on a cell-by-cell basis. The RMS error of the SRTM DEM with respect to GPS points on non-glaciated areas was 23 m. The ASTER DEM had a RMS error of 61 m with respect to GPS points and displayed 200–300 m horizontal offsets and elevation ‘spikes’ on the glaciated area when compared to the DEM from topographic data. Cell-by-cell comparison of SRTM and ASTER-derived elevations with topographic data showed ablation at the toes of the glaciers (− 25 m to − 75 m surface lowering) and an apparent thickening at the summits. The mean altitude difference on glaciated area (SRTM minus topographic DEM) was − 5 m, pointing towards a lowering of the glacier surface during the period 1955–2000. Spurious values on the glacier surface in the ASTER DEM affected the analysis and thus prevented us from quantifying the glacier changes based on the ASTER data.

Performance of a buried object scanning sonar against targets buried in sand at the SAX-04 (Sediment Acoustics Experiment 2004) site

The performance of synthetic aperture processing for imaging buried targets in the band of 3 to 20 kHz is quantified by measuring the SNR of 3-D acoustic images of targets buried in sand at the SAX-04 site. A buried object scanning sonar consisting of an omnidirectional projector and a 40-channel hydrophone line array is towed within 10 m of the seabed at various horizontal offsets to buried targets to measure the SNR of target echoes as a function of grazing angle. Detections of the targets occurring at grazing angles smaller than the critical angle suggest that buried targets can be detected using the evanescent boundary wave thereby improving the range of the sonar for detecting shallow-buried targets. Acoustic images of buried targets also show that backscattering from sand ripples, sediment variability, and sediment layer interfaces limit the performance of the sonar for detecting and imaging buried objects.

Geomorphology and structure of the Jid right-lateral strike-slip fault in the Mongolian Altay mountains

We describe the late Cenozoic structure and evolution of the Jid right-lateral strike-slip fault in the northern Mongolian Altay mountain chain. Parts of the Jid fault have a vertical component of motion, introduced by bending along the fault. Some locations show additional structural complexity at the Earth's surface, with parallel strands having strike-slip, normal and reverse faulting, probably indicating a spatial separation of dip-slip and strike-slip components of motion in cross-section. The Jid fault shows indications of recent movements at scales ranging from drainage re-organisation at the kilometre scale, to ground deformations likely to result from movements during the last earthquake, which occurred ∼870–980 years ago based on OSL dating of deposits exposed in trenching. From the scale of the ruptures the earthquake would have had a magnitude of ∼7.5 with an estimated right-lateral slip of ∼5 m. Numerous horizontal offsets of about 10–15 m, and vertical scarps of ∼5 m in the youngest alluvial deposits show the likely cumulative movement over the last ∼15,000 years, and suggest a maximum slip-rate of roughly 1 mm/yr. The minimum cumulative right-lateral slip on the Jid fault is 1 km, which would accumulate in ∼1 Ma at present-day rates of slip. The relatively small vertical component of motion on the Jid fault is not sufficient to generate topography in the adjacent Kharkhiraa and Türgen Uul mountains, with relief of over 2000 m. The high mountains may therefore result from localised uplift on thrust faults rather than by oblique slip on the Jid fault.

Surfactant effects on buoyancy-driven viscous interactions of deformable drops

Buoyancy-driven interactions of two deformable drops with a bulk-insoluble, nonionic surfactant are studied with a curvatureless boundary-integral algorithm. At moderate drop-to-medium viscosity ratios with a linear equation of state, the surfactant causes breakup of the larger drop to become dominant over breakup of the smaller drop at O ( 0.1 ) values of the dimensionless gas constant β for a surface Péclet number P e s of 100. For P e s ≤ 10 , the surfactant has little effect on breakup for β ≤ 0.2 . For P e s = 1000 , critical horizontal offsets for breakup of the larger drop exceed those for breakup of the smaller drop at β = O ( 0.01 ) . The influence of surfactant on the deformation and breakup of the smaller drop increases when it becomes closer in size to that of the larger drop. Surfactant effects are less significant when the drop-to-medium viscosity ratio is greater than unity. For the case of bubbles, at P e s = 100 , the surfactant is swept to the trailing edges early in the trajectories and leads to cusp formation at β = O ( 0.01 ) .

Slip analysis of the Kokoxili earthquake using terrain-change detection and regional earthquake data

The Kokoxili earthquake occurred on 14 November 2001 along the Kunlun fault. To analyze the type of deformation along several segments of the fault, this study employs Fourier transform terrain-change detection to detect horizontal pixel offsets between time-separated Advanced Spaceborne Thermal Emission and Refl ection Radiometer (ASTER) satellite images. The results allow us to model the slip direction and amount of slip due to the earthquake. Three segments of the fault were analyzed, each with different strain kinematics. Our analysis detected left-lateral slip, a displacement of 311 m, and areas of transpression and transtension in the proximal area of the Kunlun fault. The image geodetic results are consistent with published fi eld measurements and stress analysis using regional earthquake data. The combination of the fi eld measurements, stress and strain analysis, and geodetic results displays patterns of deformation along the fault that are infl uenced by the fault’s geometry. This study demonstrates for the fi rst time the utility of ASTER optical satellite imagery for acquiring quantitative measurements of surface deformation.

Limitations of Two Approximate Methods for Determining the AEM Bird Position in a Conductive Environment

The full potential of airborne electromagnetic (AEM) applications requiring sub-metre depth accuracy cannot be achieved if variable geometry between the receiver and transmitter is neglected. Methods involving photography, laser ranging, and aerodynamic computations have too many limitations to be of general use. We utilise the primary-field technique for estimating the dynamic bird position, assuming a free-space approximation. This procedure is expected to be accurate at high altitude or over a very resistive ground. In these circumstances, averaged bird offsets, relative to the transmitter, can be compared to the nominal receiver bird offset. When using AEM for bathymetric mapping of shallow seawater, the primary field is distorted by conductive seawater, and an inductive-limit approximation is used to correct the primary field. We compare the bird offsets obtained from both uncorrected (free-space) and corrected (inductive-limit) primary fields from survey data recorded over seawater, in order to understand the limitations of these two methods.Primary field values at the receiver increased when flying from a resistive ground to a conductive ground. The increment was greater for the vertical component than the inline component and was more pronounced at 25 Hz than at 12.5 Hz. Our analysis shows that for 25 Hz survey data over seawater, the inductive-limit approximation is an improvement on the free-space approximation, but still does not give the same range of receiver offsets as those obtained from high-altitude primary field data. Averaged bird offsets at high altitude were found to differ from the expected nominal receiver offsets. At 25 Hz, there is only agreement within the upper limits of experimental error. At 12.5 Hz, only the horizontal offsets are in agreement within experimental error. The accuracy of bird offsets at survey altitude is estimated by comparing them with those obtained at high altitude. Differences greater than several metres were observed. If errors greater than several metres in the bird position cannot be tolerated, then the values derived from the primary field method should not be used and alternative methods must be investigated.

A VEP Measure of the Binocular Fusion of Horizontal and Vertical Disparities

Because of the lateral separation of the orbits, the retinal images differ in the two eyes. These differences are reconciled into a single image through sensory and motor fusional mechanisms. This study demonstrates electrophysiologically the effects that normal horizontal and vertical fusional processes have on the processing of monocular position signals. VEPs were recorded in 16 healthy adults in response to a vernier onset-offset target presented to one eye. The vernier offsets appeared and disappeared at 2 Hz and were introduced into bar targets that were oriented either vertically (horizontal offsets) or horizontally (vertical offsets). The magnitude of the offsets was varied over the range of 0.5 to 10 arc min. VEP amplitude was measured as a function of the size of the dynamic offset under monocular viewing conditions and in the presence of two different static targets presented to the other eye. One of the static targets matched the dynamic test, except that it had no vernier offsets. The other static target, the static pedestal, matched the dynamic test, but contained a set of static vernier offsets in locations corresponding to the locations of the dynamic offsets presented to the other eye. VEP amplitude was a monotonically increasing function of vernier offset size under monocular viewing conditions. The addition of the static target without offsets in the other eye resulted in an increased amplitude VEP response. The addition of the static target with vernier offsets resulted in a decrease in VEP amplitude for both horizontal and vertical disparities. The normal process of fusion results in a single visual direction. To obtain a single visual direction, the visual system must synthesize a binocular visual direction that differs from the monocular components. One of the conditions (the static pedestal with offsets) produces binocular visual direction shifts that degrade the appearance of vernier onset-offset, and reduce VEP amplitude for both horizontal and vertical disparities. This characteristic evoked response marker is a promising tool for measuring binocular fusion objectively in patients with strabismus.