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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.

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
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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.

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, 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
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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, 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.

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.