Export 25 results:
Sort by: Author Title Type [ Year  (Desc)]
Maksymowicz, A, Chadwell CD, Ruiz J, Trehu AM, Contreras-Reyes E, Weinrebe W, Diaz-Naveas J, Gibson JC, Lonsdale P, Tryon MD.  2017.  Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake. Scientific Reports. 7   10.1038/srep45918   AbstractWebsite

The M-w 8.8 megathrust earthquake that occurred on 27 February 2010 offshore the Maule region of central Chile triggered a destructive tsunami. Whether the earthquake rupture extended to the shallow part of the plate boundary near the trench remains controversial. The up-dip limit of rupture during large subduction zone earthquakes has important implications for tsunami generation and for the rheological behavior of the sedimentary prism in accretionary margins. However, in general, the slip models derived from tsunami wave modeling and seismological data are poorly constrained by direct seafloor geodetic observations. We difference swath bathymetric data acquired across the trench in 2008, 2011 and 2012 and find similar to 3-5 m of uplift of the seafloor landward of the deformation front, at the eastern edge of the trench. Modeling suggests this is compatible with slip extending seaward, at least, to within similar to 6 km of the deformation front. After the M-w 9.0 Tohoku-oki earthquake, this result for the Maule earthquake represents only the second time that repeated bathymetric data has been used to detect the deformation following megathrust earthquakes, providing methodological guidelines for this relatively inexpensive way of obtaining seafloor geodetic data across subduction zone.

DeSanto, JB, Sandwell DT, Chadwell CD.  2016.  Seafloor geodesy from repeated sidescan sonar surveys. Journal of Geophysical Research-Solid Earth. 121:4800-4813.   10.1002/2016jb013025   AbstractWebsite

Accurate seafloor geodetic methods are critical to the study of marine natural hazards such as megathrust earthquakes, landslides, and volcanoes. We propose digital image correlation of repeated shipboard sidescan sonar surveys as a measurement of seafloor deformation. We test this method using multibeam surveys collected in two locales: 2500m deep lightly sedimented seafloor on the flank of a spreading ridge and 4300m deep heavily sedimented seafloor far from any plate boundary. Correlation of these surveys are able to recover synthetic displacements in the across-track (range) direction accurate to within 1m and in the along-track (azimuth) direction accurate to within 1-10m. We attribute these accuracies to the inherent resolution of sidescan data being better in the range dimension than the azimuth dimension. These measurements are primarily limited by the accuracy of the ship navigation. Dual-frequency GPS units are accurate to approximate to 10cm, but single-frequency GPS units drift on the order of 1m/h and are insufficient for geodetic application.

Burgmann, R, Chadwell D.  2014.  Seafloor geodesy. Annual Review of Earth and Planetary Sciences, Vol 42. 42:509-534.   10.1146/annurev-earth-060313-054953   AbstractWebsite

Seafloor geodetic techniques allow for measurements of crustal deformation over the similar to 70% of Earth's surface that is inaccessible to the standard tools of tectonic geodesy. Precise underwater measurement of position, displacement, strain, and gravity poses technical, logistical, and cost challenges. Nonetheless, acoustic ranging; pressure sensors; underwater strain-, tilt- and gravimeters; and repeat multibeam sonar and seismic measurements are able to capture small-scale or regional deformation with approximately centimeter-level precision. Pioneering seafloor geodetic measurements offshore Japan, Cascadia, and Hawaii have substantially contributed to advances in our understanding of the motion and deformation of oceanic tectonic plates, earthquake cycle deformation in subduction zones, and the deformation of submarine volcanoes. Nontectonic deformation related to down-slope mass movement and underwater extraction of hydrocarbons or other resources represent other important targets. Recent technological advances promise further improvements in precision as well as the development of smaller, more autonomous, and less costly seafloor geodetic systems.

Prawirodirdjo, L, McCaffrey R, Chadwell CD, Bock Y, Subarya C.  2010.  Geodetic observations of an earthquake cycle at the Sumatra subduction zone: Role of interseismic strain segmentation. Journal of Geophysical Research-Solid Earth. 115   10.1029/2008jb006139   AbstractWebsite

We use survey mode and continuous GPS data from 1991 to 2007 to examine fault segmentation in the earthquake cycle at the Sumatra megathrust, site of the 26 December 2004 M(w) 9.1 Sumatra-Andaman, the 28 March 2005 M(w) 8.7 Nias-Simeulue, and the 12 September 2007 M(w) 8.4 Mentawai earthquakes. These data, including new observations from 2006 and 2007, allow us to observe the final few years of one earthquake cycle and the beginning of the next. Our analysis reveals that the megathrust is segmented, a characteristic that may persist through multiple earthquake cycles. The Nias-Simeulue earthquake ruptured approximately the same region that broke in 1861, a 300 km long segment abutting the Sumatra-Andaman rupture zone. Farther southeast, the Mentawai segment of the megathrust (0.5 degrees S-5 degrees S), which produced M > 8 earthquakes in 1797 and 1833, is fully locked in the interseismic period but is flanked by two freely slipping regions, the Batu Islands in the NW and Enggano in the SE. The 12 September 2007 Mentawai earthquake sequence ruptured only the southern one third of the 1833 rupture zone. We model postseismic deformation from the Sumatra-Andaman and Nias-Simeulue earthquakes and find that afterslip was concentrated updip and downdip, respectively, from the main shocks. Comparing the velocity fields before and after 2001, we find the subduction zone underneath the Batu Islands and Enggano, which, prior to the earthquakes, was partially to fully coupled, appears now to be slipping freely. Thus, while the segmentation of the subduction zone is preserved, interseismic coupling on the subduction fault may vary with time.

Blum, JA, Chadwell CD, Driscoll N, Zumberge MA.  2010.  Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data. Geophysical Research Letters. 37   10.1029/2010gl043293   AbstractWebsite

Seafloor slope instability in the Santa Barbara Basin, California, poses risk to the region. Two prominent landslides, the Goleta and Gaviota slides, occupy the northern flank, with a scarp-like crack extending east from the headwall of the Gaviota slide towards the Goleta complex. Downslope creep across the crack might indicate an imminent risk of failure. Sub-bottom CHIRP profiles with <1 m accuracy across the crack exhibit no evidence of internal deformation. Daily seafloor acoustic range measurements spanning the crack detected no significant motion above a 99% confidence level of +/- 7 mm/yr over two years of monitoring. These disparate data over different timescales suggest no active creep and that the crack is likely a relict feature that formed concomitantly with the Gaviota slide. Citation: Blum, J. A., C. D. Chadwell, N. Driscoll, and M. A. Zumberge (2010), Assessing slope stability in the Santa Barbara Basin, California, using seafloor geodesy and CHIRP seismic data, Geophys. Res. Lett., 37, L13308, doi: 10.1029/2010GL043293.

Chadwell, CD, Sweeney AD.  2010.  Acoustic Ray-Trace Equations for Seafloor Geodesy. Marine Geodesy. 33:164-186.   10.1080/01490419.2010.492283   AbstractWebsite

One goal of seafloor geodesy is to measure horizontal deformation of the seafloor with millimeter resolution. A common technique precisely times an acoustic signal propagating between two points to estimate distance and then repeats the measurement over time. The accuracy of the distance estimate depends upon the travel time resolution, sound speed uncertainty, and the degree to which the path computed from propagation equations replicates the actual path traveled by the signal. In this paper, we address the error from ray propagation equations by comparing three approximations to Snell's Law with ellipsoidal geometry.

Phillips, KA, Chadwell CD, Hildebrand JA.  2008.  Vertical deformation measurements on the submerged south flank of Kilauea volcano, Hawai'i reveal seafloor motion associated with volcanic collapse. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb005124   AbstractWebsite

A four-year seafloor geodetic study was conducted to measure vertical deformation of the submerged south flank of Kilauea volcano on the active Hilina slump. The Hilina slump is a site of significant deformation, major earthquakes with ground cracking and associated tsunami. A new technique was developed to measure vertical deformation on the seafloor using pressure sensors in campaign-style surveys. The data revealed the midslope bench of the offshore slump is uplifting at 9.0 +/- 2.4 cm/a, but transitions to no significant deformation on the outer bench and slump toe. Elastic half-space inverse modeling of these data show that the observed deformation can be fit by seaward slip of 28.1 +/- 7.3 cm/a extending from the East rift zone 27.0 +/- 0.5 km on a gently island-dipping decollement fault plane at a depth of approximately 7 km. Modeling suggests that the outer bench is not currently deforming. Because the majority of Kilauea is submerged, these offshore measurements are necessary to constrain the seaward extent of fault slip and the motion of the outer bench.

Chadwell, CD, Spiess FN.  2008.  Plate motion at the ridge-transform boundary of the south Cleft segment of the Juan de Fuca Ridge from GPS-Acoustic data. Journal of Geophysical Research-Solid Earth. 113   10.1029/2007jb004936   AbstractWebsite

We measure the present-day plate velocity of the Juan de Fuca Ridge 25 km off-axis to be 63.6 +/- 3.6 mm/a at S67.2 degrees E +/- 7.9 degrees degrees (1-sigma) relative to the Pacific plate (PA). This velocity was derived from GPS-Acoustic (GPSA) measurements in 2000, 2001, 2002, and 2003 that observed the position of a seafloor array (44 degrees 43'N, 130 degrees 03'W, 2900 m depth) with a repeatability of +/- 4-6 mm. Three transient events at the Juan de Fuca Ridge and Blanco Transform account for similar to 10% of this motion in viscoelastic modeling, suggesting that the observed GPSA-PA velocity is due primarily to steady state plate dynamics. Subtracting the modeled transient motion gives a velocity of 57.3 +/- 3.9 mm/a at S72.9 degrees E +/- 12.1 degrees degrees (1-sigma), which is consistent at the 95% confidence level with the velocity calculated from the Wilson (1993) 0-0.78 Ma Euler pole. Therefore this site is interpreted to be in a region of continuous, full-rate plate motion, a robust result of this study which holds with and without correcting for transient motions. These results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading-rate ridge. Previously reported geodetic monitoring across the 1-km-wide axial valley from 1994-1999 and 2000-2003 shows no significant extension (Chadwell et al., 1999; Hildebrand et al., 1999; Chadwick and Stapp, 2002; W. W. Chadwick, personal communication, 2006) and seismic monitoring shows no activity. This suggests the crust between 0.5 and 25 km off-axis accommodates similar to 26 mm of aseismic deformation each year through some combination of near-axis fault motion and elastic strain accumulation.

Gagnon, KL, Chadwell CD.  2007.  Relocation of a seafloor transponder - Sustaining the GPS-Acoustic technique. Earth Planets and Space. 59:327-336. AbstractWebsite

Rigid seafloor arrays of three to four precision acoustic transponders have been repeatedly positioned with the GPS-Acoustic technique to measure horizontal plate motion. In the event that one transponder becomes inactive, a replacement transponder must be precisely located relative to the existing array. Here we present a technique to determine the geodetic azimuth and baseline between the inactive and replacement transponders. We include three examples of relocations between 2002 and 2003 on the Juan de Fuca plate and near the Peru-Chile trench, which add 16-29 mm uncertainty to the GPS-Acoustic estimated position. A simulation of optimal network geometry shows that the relocation's contribution to uncertainty can be as low as 10 mm.

D'Spain, GL, Terrill E, Chadwell CD, Smith JA, Lynch SD.  2006.  Active control of passive acoustic fields: Passive synthetic aperture/Doppler beamforming with data from an autonomous vehicle. Journal of the Acoustical Society of America. 120:3635-3654.   10.1121/1.2346177   AbstractWebsite

The maneuverability of autonomous underwater vehicles (AUVs) equipped with hull-mounted arrays provides the opportunity to actively modify received acoustic fields to optimize extraction of information. This paper uses ocean acoustic data collected by an AUV-mounted two-dimensional hydrophone array, with overall dimension one-tenth wavelength at 200-500 Hz, to demonstrate aspects of this control through vehicle motion. Source localization is performed using Doppler shifts measured at a set of receiver velocities by both single elements and a physical array. Results show that a source in the presence of a 10-dB higher-level interferer having exactly the same frequency content (as measured by a stationary receiver) is properly localized and that white-noise-constrained adaptive beamforming applied to the physical aperture data in combination with Doppler bearnforming provides greater spatial resolution than physical-aperture-alone bearnforming and significantly lower sidelobes than single element Doppler beamforming. A new broadband beamformer that adjusts for variations in vehicle velocity on a sample by sample basis is demonstrated with data collected during a high-acceleration maneuver. The importance of including the cost of energy expenditure in determining optimal vehicle motion is demonstrated through simulation, further illustrating how the vehicle characteristics are an integral part of the signal/array processing structure. (c) 2006 Acoustical Society of America.

Sweeney, AD, Chadwell CD, Hildebrand JA.  2006.  Calibration of a seawater sound velocimeter. IEEE Journal of Oceanic Engineering. 31:454-461.   10.1109/joe.2004.836582   AbstractWebsite

We calibrated a sound velocimeter to a precision of +/- 0.034 m/s using Del Grosso's sound-speed equation for seawater at temperatures of 2, 7.2, 11.7, and 18 degrees C in a tank of seawater of salinity 33.95 at one atmosphere. The sound velocimeter measures the time-of-flight of a 4-MHz acoustic pulse over a 20-cm path by adjusting the carrier frequency within a 70-kHz band until the pulse and its echo are inphase. We used the adjustable carrier frequency to determine the internal timing characteristics of the sound velocimeter to nanosecond precision. Similarly, sound-speed measurements at four different temperatures determined the acoustic pathlength to micrometer precision. The velocimeter was deployed in the ocean from the surface to 4500 dbar alongside conductivity, temperature, and pressure sensors (CTD). We demonstrated agreement of +/- 0.05 m/s (three parts in 10(5))-with CTD-derived sound speed using Del Grosso's seawater equation from 500 to 4500 dbar after removing a bias and a trend.

Osada, Y, Fujimoto H, Kanazawa T, Nakao S, Sakai S, Miura S, Hildebrand J, Chadwell CD.  2006.  Development of a GPS/acoustic seafloor positioning system for 6,000 m water depth and its trial experiments at sea. Journal of the geodetic Society of Japan. 52:171-182. Abstract
Gagnon, K, Chadwell CD, Norabuena E.  2005.  Measuring the onset of locking in the Peru-Chile trench with GPS and acoustic measurements. Nature. 434:205-208.   10.1038/nature03412   AbstractWebsite

The subduction zone off the west coast of South America marks the convergence of the oceanic Nazca plate and the continental South America plate. Nazca - South America convergence over the past 23 million years has created the 6-km-deep Peru - Chile trench, 150 km offshore. High pressure between the plates creates a locked zone, leading to deformation of the overriding plate. The surface area of this locked zone is thought to control the magnitude of co-seismic release and is limited by pressure, temperature, sediment type and fluid content(1). Here we present seafloor deformation data from the submerged South America plate obtained from a combination of Global Positioning System (GPS) receivers and acoustic transponders. We estimate that the measured horizontal surface motion perpendicular to the trench is consistent with a model having no slip along the thrust fault between 2 and 40 km depth. A tsunami in 1996, 200 km north of our site, was interpreted as being the result of an anomalously shallow interplate earthquake(2). Seismic coupling at shallow depths, such as we observe, may explain why co-seismic events in the Peruvian subduction zone create large tsunamis.

Gagnon, K, Chadwell D, Spiess FN.  2005.  Evolving method to measure seafloor plate tectonic motions. Sea Technology. 46:49-52. AbstractWebsite
Kussat, NH, Chadwell CD, Zimmerman R.  2005.  Absolute positioning of an autonomous underwater vehicle using GPS and acoustic measurements. Ieee Journal of Oceanic Engineering. 30:153-164.   10.1109/joe.2004.835249   AbstractWebsite

Kinematic global positioning system (GPS) positioning and underwater acoustic ranging can combine to locate an autonomous underwater vehicle (AUV) with an accuracy of +/- 30 cm (2-sigma) in the global International Terrestrial Reference Frame 2000 (ITRF2000). An array of three precision transponders, separated by approximately 700 m, was established on the seafloor in 300-m-deep waters off San Diego. Each transponder's horizontal position was determined with an accuracy of +/- 8 cm (2-sigma) by measuring two-way travel times with microsecond resolution between transponders and a shipboard transducer, positioned to +/- 10 cm (2-sigma) in ITRF2000 coordinates with GPS, as the ship circled each seafloor unit. Travel times measured from AUV to ship and from AUV to transponders to ship were differenced and combined with AUV depth from a pressure gauge to estimate ITRF2000 positions of the AUV to +/- 1 m (2-sigma). Simulations show that +/- 30 cm (2-sigma) absolute positioning of the AUV can be realized by replacing the time-difference approach with directly measured two-way travel times between AUV and seafloor transponders. Submeter absolute positioning of underwater vehicle!; in water depths up to several thousand meters is practical. The limiting factor is knowledge of near-surface sound speed which degrades the precision to which transponders can be located in the ITRF2000 frame.

Zimmerman, R, D'Spain GL, Chadwell CD.  2005.  Decreasing the radiated acoustic and vibration noise of a mid-size AUV. Ieee Journal of Oceanic Engineering. 30:179-187.   10.1109/joe.2004.836996   AbstractWebsite

An Odyssey IIb autonomous underwater vehicle (AUV) made by Bluefin Robotics, Inc., was acquired by the Marine Physical Laboratory Scripps Institution of Oceanography, to conduct research in underwater acoustics as well as provide a platform for other scientific studies. The original Odyssey IIb tail cone was replaced with a ducted fan, vectored thrust system installed on vehicles currently sold by Bluefin. In initial sea tests with the new thrust system, the acoustic self noise levels of the vehicle while underway were 20 to 50 dB higher than typical ocean background noise levels, preventing the vehicle's use as a receiver of low level sounds. Controlled tests were performed to characterize the radiated and vibration noise of the AUV propulsion and actuators. Once this baseline was established, changes were made, mostly to the tail cone propulsion, to decrease the vehicle's self noise. The resulting self noise levels of the AUV from 10 Hz up to 10 kHz measured while underway by a hydrophone mounted on the AUV's inner shroud now are at or below typical shallow water background noise levels except in three bands; below 250 Hz, around 500 Hz, and from 0.9 to 2.0 kHz. The goal of this paper is to describe these changes and their effects in lowering vehicle noise levels.

Sweeney, AD, Chadwell DC, Hildebrand JA, Spiess FN.  2005.  Centimeter-Level Positioning of Seafloor Acoustic Transponders from a Deeply-Towed Interrogator. Marine Geodesy. 28:39-70.: Taylor & Francis   10.1080/01490410590884502   AbstractWebsite

An array of three seafloor transponders was acoustically surveyed to centimeter precision with a deeply-towed interrogator. Measurements of two-way acoustic travel time and hydrostatic pressure made as the interrogator was towed above the array were combined in a least-squares adjustment to estimate the interrogator and transponder positions in two surveys spanning two years. No transponder displacements were expected at this site in the interior of the Juan de Fuca Plate (48?11? N, 127?12? W) due to the lack of active faults. This was confirmed to a precision of ±2 cm by least-squares adjustment. Marginally detectable blunders in the observations were shown to affect the transponder position estimates by no more than 3 mm, demonstrating the geometric strength of the data set. The accumulation of many hundreds of observations resulted in a significant computational burden on the least-squares inversion procedure. The sparseness of the normal matrix was exploited to reduce by a factor of 1000 the number of calculations. The acoustic survey results suggested that the near-bottom sound speed fields during the two surveys were in better agreement than inferred from yearly single-profile conductivity, temperature, and pressure (CTD) measurements.

Chadwell, CD.  2003.  Shipboard towers for Global Positioning System antennas. Ocean Engineering. 30:1467-1487.   10.1016/s0029-8018(02)00141-5   AbstractWebsite

Two 12.2 m-high towers for mounting Global Positioning System (GPS) receiver antennas were designed and constructed to provide millimeter-level stability while maintaining portability and accessibility to satellites and deck spaces. A combination of guys and a 3-m horizontal strut provide roll and pitch stability of 2-3 rum observed from 0.1 seconds to 12 days using a combination of GPS and optical/laser devices. The shipboard antenna mounts connect sub-aerial GPS positioning to underwater acoustic ranging that determine the centimeter-level location of seafloor transponders. Observed annually, these seafloor geodetic positions measure seafloor crustal motion for geophysical studies. (C) 2003 Elsevier Science Ltd. All rights reserved.

Osada, Y, Fujimoto H, Miura S, Sweeney A, Kanazawa T, Nakao S, Sakai SI, Hildebrand JA, Chadwell CD.  2003.  Estimation and correction for the effect of sound velocity variation on GPS/Acoustic seafloor positioning: An experiment off Hawaii Island. Earth Planets and Space. 55:E17-E20. AbstractWebsite

A GPS/Acoustic experiment on the southeastern slope of Hawaii Island presented precise seafloor positioning in the condition of large water depth (2.5-4.5 km) and large velocity variations. We estimated sound velocity variations from acoustic ranging, and found that temperature variation can well explain the velocity variation. The effect of daily variation in the sound velocity amounted to +/- 0.7 m on acoustic ranging of 4-7 km with a fixed velocity structure. CTD data observed about every 3 hours could decrease the range residuals to +/- 0.4 m. These large residuals were fairly well canceled in the positioning of the array center of three acoustic transponders. The estimated precision of the array center positioning was about 3 cm in latitude and longitude.

Chadwell, CD, Bock Y.  2001.  Direct estimation of absolute precipitable water in oceanic regions by GPS tracking of a coastal buoy. Geophysical Research Letters. 28:3701-3704.   10.1029/2001gl013280   AbstractWebsite

A buoy-based CPS receiver and meteorological sensor are used to estimate directly the absolute precipitable water (PW) overlying a coastal ocean site 8 km from shore. During an 11-day experiment, one-second CPS data collected at the buoy and at a shore station are combined with 30-second data from four distant CPS stations to estimate the buoy position, zenith wet delay, phase biases, and receiver and satellite clocks using double-differenced phase processing with ambiguity resolution. GPS-derived PW at the buoy compared to radiosonde measurements (20) and to half-hourly GPS-PW values (384) from the nearby shore station show an rms agreement of +/-1.5 mm and +/-1.8 mm, respectively. Hourly means (170) of the GPS-measured vertical motion of the buoy show a +/- 24 mm rms agreement with a NOAA tide gauge, equivalent to about 4 mm of PW. GPS-derived PW from buoys may have the potential to improve weather forecasting, calibration of satellite-based sensors, and climate studies.

Chadwell, CD, Hildebrand JA, Spiess FN, Morton JL, Normark WR, Reiss CA.  1999.  No spreading across the southern Juan de Fuca Ridge axial cleft during 1994-1996. Geophysical Research Letters. 26:2525-2528.   10.1029/1999gl900570   AbstractWebsite

Direct-path acoustic measurements between seafloor transponders observed no significant extension (-10 +/- 14 mm/yr) from August 1994 to September 1996 at the southern Juan de Fuca Ridge (44 degrees 40' N and 130 degrees 20' W). The acoustic path for the measurement is a 691-m baseline straddling the axial cleft, which bounds the Pacific and Juan de Fuca plates. Given an expected full-spreading rate of 56 mm/yr, these data suggest that extension across this plate boundary occurs episodically within the narrow (similar to 1 km) region of the axial valley floor, and that active deformation is occurring between the axial cleft and the plate interior. A cleft-parallel 714-m baseline located 300 m to the west of the cleft on the Pacific plate monitored system performance and, as expected, observed no motion (+5 +/- 7 mm/yr) between the 1994 and 1996 surveys.

Chadwell, CD.  1999.  Reliability analysis for design of stake networks to measure glacier surface velocity. Journal of Glaciology. 45:154-164. AbstractWebsite

Measurement of glacier surface velocity provides some constraint on glacier flow models used to date ice cores recovered near the flow divide of remote high-altitude ice caps. The surface velocity is inferred from the change in position of a network of stakes estimated from the least-squares adjustment of geodetic observations-terrestrial and/or spaced-based-collected approximately year apart. The lack of outliers in and the random distribution of the post-fit observation residuals are regarded as evidence that the observations contain no blunders. However, if the network lacks sufficient geometric redundancy the estimated stake positions san shift to fit erroneous observations. To determine the maximum size of these potential undetected shifts, given the covariance of the observations and the approximate network geometry expressions are developed to analyze a network for redundancy number and marginally detectable blunders (internal reliability), and the position shifts from marginally detectable blunders (external reliability). Two stake networks, one on the col of Huascaran (9 degrees 07' S, 77 degrees 37' W; 6050 m a.s.l.) in the north-central Andes of Peru and one on the Guliya ice cap (35 degrees 17' N, 81 degrees 29' E; 6200 ma.s.l.) on the Qinghai-Tibetan Plateau in China, are examined for precision and internal and external reliability.

Spiess, FN, Chadwell CD, Hildebrand JA, Young LE, Purcell GH, Dragert H.  1998.  Precise GPS/Acoustic positioning of seafloor reference points for tectonic studies. Physics of the Earth and Planetary Interiors. 108:101-112.   10.1016/s0031-9201(98)00089-2   AbstractWebsite

Global networks for crustal strain measurement provide important constraints for studies of tectonic plate motion and deformation. To date, crustal strain measurements have been possible only in terrestrial settings: on continental plates and island sites within oceanic plates. We report the development of technology for horizontal crustal motion determination at seafloor sites, allowing oceanic plates to be monitored where islands are not available. Seafloor crustal monitoring is an important component of global strain measurement because about 70% of the Earth's surface is covered by water, and this region contains most of the tectonic plate boundaries and zones of crustal deformation. Using the Global Positioning System (GPS) satellites and underwater acoustics, we have established a geodetic reference site on the Juan de Fuca plate at 2.6 km depth, approximately 150 km off the northwest coast of North America. We measure the baselines between this site and two terrestrial GPS stations on Vancouver Island, British Columbia. The Juan de Fuca plate site is an appropriate setting to develop seafloor observation methods, since it is a well studied area, easily accessible from west coast Canadian and United States ports. Determination of seafloor motion at this site addresses questions related to convergence between the Juan de Fuca and North American plates across the Cascadia Subduction Zone. At the Juan de Fuca seafloor geodetic reference site, we installed precision acoustic transponders on the seafloor, and measured ranges to them from a sound source at a surface platform (ship or buoy), The platform is equipped with a set of three GPS antennas allowing determination of the sound source position at times of signal transmission and reception. Merging the satellite and acoustic data allows determination of the transponder network location in global reference frame coordinates. Data processing to date suggests repeatabilities of +/-0.8 cm north and +/- 3.9 cm east in the seafloor transponder network position relative to reference points on Vancouver Island. (C) 1998 Elsevier Science B.V. All rights reserved.

Chadwell, D, Spiess F, Hildebrand J, Young L, Purcell, George J, Dragert H.  1998.  Deep-sea geodesy; monitoring the ocean floor. GPS World. 9:44-50,52-55., Eugene, OR, United States (USA): Aster Pub. Corp., Eugene, OR AbstractWebsite
Chadwell, CD, Spiess FN, Hildebrand JA, Young LE, Purcell, George J, Dragert H, Segawa J, Fujimoto H, Okubo S.  1997.  Sea floor strain measurement using GPS and acoustics. International Association of Geodesy Symposia. 117:682-689., New York, NY, International (III): Springer-Verlag, New York, NY AbstractWebsite