Ship Gravity Data Research Articles

Toward 1-mGal accuracy in global marine gravity from CryoSat-2, Envisat, and Jason-1

More than 60% of the Earth's land and shallow marine areas are covered by > 2 km of sediments and sedimentary rocks, with the thickest accumulations on rifted continental margins (Figure 1). Free-air marine gravity anomalies derived from Geosat and ERS-1 satellite altimetry (Fairhead et al., 2001; Sandwell and Smith, 2009; Andersen et al., 2009) outline most of these major basins with remarkable precision. Moreover, gravity and bathymetry data derived from altimetry are used to identify current and paleo-submarine canyons, faults, and local recent uplifts. These geomorphic features provide clues to where to look for large deposits of sediments. While current altimeter data delineate large offshore basins and major structures, they do not resolve some of the smaller geomorphic features and basins (Yale et al., 1998; Fairhead et al., 2001). Improved accuracy and resolution is desirable: to facilitate comparisons between continental margins; as an exploration tool and to permit extrapolation of known structures from well-surveyed areas; to follow fracture zones out of the deep-ocean basin into antecedent continental structures, to define and compare segmentation of margins along strike and identify the position of the continent-ocean boundary; and to study mass anomalies (e.g., sediment type and distribution) and isostatic compensation at continental margins. In this article, we assess the accuracy of a new global marine gravity model based on a wealth of new radar altimetry data and demonstrate that these gravity data are superior in quality to the majority of publicly available academic and government ship gravity data.

Validation of ERS-1 and High-Resolution Satellite Gravity with in-situ Shipborne Gravity over the Indian Offshore Regions: Accuracies and Implications to Subsurface Modeling

Geoid and gravity anomalies derived from satellite altimetry are gradually gaining importance in marine geoscientific investigations. Keeping this in mind, we have validated ERS-1 (168 day repeat) altimeter data and very high-resolution free-air gravity data sets generated from Seasat, Geosat GM, ERS-1 and TOPEX/POSEIDON altimeters data with in-situ shipborne gravity data of both the Bay of Bengal and the Arabian Sea regions for the purpose of determining the consistencies and deviations. The RMS errors between high resolution satellite and ship gravity data vary from 2.7 to 6.0 mGal, while with ERS-1 data base the errors are as high as 16.5 mGal. We also have generated high resolution satellite gravity maps of different regions over the Indian offshore, which eventually have become much more accurate in extracting finer geological structures like 85° E Ridge, Swatch of no ground, Bombay High in comparison with ERS-1satellite-derived gravity maps. Results from the signal processing related studies over two specific profiles in the eastern and western offshore also clearly show the advantage of high resolution satellite gravity compared to the ERS-1 derived gravity with reference to ship gravity data.

Crustal structure and rift flank uplift of the Adare Trough, Antarctica

The Adare Trough, located 100 km northeast of Cape Adare, Antarctica, represents the extinct third arm of a Tertiary spreading ridge between East and West Antarctica. It is characterized by pronounced asymmetric rift flanks elevated up to over 2 km above the trough's basement, accompanied by a large positive mantle Bouguer anomaly. On the basis of recently acquired seismic reflection and ship gravity data, we invert mantle Bouguer anomalies from the Adare Trough and obtain an unexpectedly large oceanic crustal thickness maximum of 9–10.5 km underneath the extinct ridge. A regional positive residual basement depth anomaly between 1 and 2.5 km in amplitude characterizes ocean crust from offshore Victoria Land to the Balleny Islands and north of Iselin Bank. The observations and models indicate that the mid/late Tertiary episode of slow spreading between East and West Antarctica was associated with a mantle thermal anomaly. The increasing crustal thickness toward the extinct ridge indicates that this thermal mantle anomaly may have increased in amplitude through time during the Adare spreading episode. This scenario is supported by a mantle convection model, which indicates the formation and strengthening of a major regional negative upper mantle density anomaly in the southwest Pacific in the last 50 million years. The total amount of post‐26 Ma extension associated with Adare Trough normal faulting was about 7.5 km, in anomalously thick oceanic crust with a lithospheric effective elastic thickness (EET) between 3.5 and 5 km. This corresponds to an age between 3 and 5 million years based on a thermal boundary layer model and supports a scenario in which the Adare Trough formed soon after spreading between East and West Antarctica ceased, confined to relatively weak lithosphere with anomalously thick oceanic crust. There is little evidence for major subsequent structural activity in the Adare trough area from the available seismic data, indicating that this part of the West Antarctic Rift system became largely inactive in the early Miocene, with the exception of minor structural reactivation which is visible in the seismic data as offsets up to end of the early Pliocene.

Augmenting satellite altimeter data with ship and land gravity data in the northwest shelf of Australia

High-resolution maps of the marine gravity field can be produced for oil and gas exploration. Ship and airborne gravity data collected by oil companies in the preliminary stages of a survey may be combined with existing public-domain satellite altimetry data, greatly increasing the resolution potential of the latter, and providing a regional gravity field for emplacement of the exploration data. The combination of altimeter data with land and ship gravity data is an iterative process, designed to seamlessly match the data sets. The method is quick and easy to implement, and avoids laborious crossover adjustments of the ship data.

An improved gravimetric geoid for Japan, JGEOID98, and relationships to marine gravity data

A geoid model for Japan, JGEOID98, is determined on a 3 × 3 arc-minute grid in the non-tidal system by a one-dimensional Fast Fourier Transform method with EGM96 as a foundation. The model is referred to the GRS80 ellipsoid/ITRF89 frame. About 570 000 ship gravity data are adjusted for about 50 000 cross-over errors (COEs) and yield significant improvement: root mean square of internal and external COEs after adjustment are 6.8 and 10.4 mGal (0 and 24% of reduction), respectively. Compared with GPS at benchmarks, JGEOID98 shows improvement over JGEOID93 by 29% in short wavelength, and tilt reduction from 2.1 to 1.4 ppm. The remaining trend can be partly explained by gravity errors: overestimation of gravity induced from EGM96 east off Hokkaido, and underestimation south off Taiwan.

The satellite altimeter data derived mean sea surface GSFC98

The GSFC98 mean sea surface (MSS) was computed on a 2' oceanwide grid for latitudes below 80°. The data used included 3‐years of TOPEX data (Cycles 9 to 119), 1.5‐years of ERS‐1 35‐day repeat cycle (Cycles 1 to 18), 2‐years of Geosat ERM data (Cycles 1 to 42), 2 ERS‐1 168‐day repeat cycles, and 18 months of the Geosat Geodetic Mission data. All non‐TOPEX satellite altimeter data were adjusted to the mean of TOPEX data in 2° × 30° blocks. After the adjustment, the mean sea surface height was gridded into 2' nodes using least squares collocation and an iteration procedure to reduce the ocean variability. To validate the mean sea surfaces, three comparisons were made. The GSFC98 MSS, along with OSU95 [Yi, 1995] and CSR95 [Kim et al., 1995], were compared with 6‐years of TOPEX and 3‐years of ERS‐2 mean tracks which were not used in the MSS computations. Finally, the marine gravity anomalies were computed from the three MSS implied geoid undulations using the inverse Stokes integral. The marine gravity anomalies were compared with ship gravity data in selected areas. The ship gravity comparison is an independent assessment of the quality of the MSSs, especially at intermediate and short wavelengths. Finally, the inter‐comparisons between the mean sea surfaces were made. The root mean square values of the differences were 6.8, 6.8, and 7.2 cm between GSFC98/OSU95, GSFC98/CSR95, and OSU95/CSR95. The differences agree well with the error estimation of GSFC98 MSS.

Comparison of altimeter‐derived and ship gravity anomalies in the vicinity of the Gulf of California

This article compares four sets of altimeter‐derived gravity anomalies in the Gulf of California region (22°N to 32°N, 244° E to 255°E) to anomalies from the EGM96 geopotential model and ship gravity data. Three anomaly sets were calculated using the same basic altimeter data (Ceosat geodetic mission data and the two cycles of ERS‐1, 168‐day repeat track) while one, by Hwang, Kao, and Parsons (HKP 1998), incorporated additional data not used in the other computations. The three sets using the same date were calculated by Knudsen and Andersen (KA 1997), Andersen and Knudsen (AK 1998), and Sandwell and Smith (SS 1997). Using a 1/8° × 1/8° grid of points, the mean difference between the altimeter anomalies and the degree 360 EGM96 geopotential model was computed in deep‐ and shallow‐water areas. The differences found reflect the different reference models used in the anomaly recovery process. Comparisons were also made with NIMA‐evaluated ship data, whose average accuracy was on the order of ±10 mgal. The comparisons were done with both shallow (—100 < d < —600 m) and deep (d < —2,000 m)‐waler data points. Comparisons were always better with the deep‐water points [for the AK set: ±7.0 mgal (deep water) versus ±11.1 mgal (shallow water)]. In all comparisons made, the AK set gave the best agreement with the ship data [e.g., ±9.7 mgal (AK), ±11.4 mgal (HKP), ±12.1 mgal(SS), ±10.9 mgal (KA)). The improved AK, over the KA set, reflects the use of the EGM96 model in the AK prediction while the KA set used the OSU91A model. Contour plots of the differences between the altimeter‐derived anomalies and ship data reveal some large differences near coastal regions, with the smallest differences found with the A K values. The causes of the various differences found can be associated with geophysical corrections to the altimeter data, altimeter data editing, data gridding procedure, and anomaly recovery methods.

Prediction of bathymetry in French Polynesia constrained by shipboard data

The south central Pacific Ocean is well covered with satellite‐derived gravity data (ERS‐1, GEOSAT, SEASAT and Topex/Poseidon) but sparsely covered with shipboard depth soundings and gravity measurements. We present an improved method of bathymetric prediction from gravity using both shipboard and satellite data. We employ a condensation model with two layers for the crust to attempt bathymetric prediction using a 2‐D spectra approach. Parameters such as flexural wavelength and oustal density are the major constraints for the model They are determined on a limited area by minimizing the difference between observed bathymetry ship profiles and the model predictions. Then, in regions where there are no ship data, these parameters are used to predict the seafloor topography from ship gravity data combined with satellite grid points. The prediction has an horizontal resolution limit of 8 km and the depth error is usually less than 300 m. This method is better constrained than the ones using satellite altimetry data only. Results on the Western Tuamotu archipelago and on Marquesas Islands, French Polynesia, are presented and discussed.

Gravity anomalies derived from Seasat, Geosat, ERS-1 and TOPEX/POSEIDON altimetry and ship gravity: a case study over the Reykjanes Ridge

SUMMARY We have employed least-squares collocation to derive a gravity field over the Reykjanes Ridge using Seasat, Geosat/ERM, ERS-1 and TOPEX/POSEIDON altimeter data combined with ship gravity. To avoid a crossover adjustment to correct for orbital errors we used mean geoid gradients, which were obtained by averaging gradients over 60, 10 and 36 repeat cycles for Geosat/ERM, ERS-1 and TOPEX/POSEIDON, respectively. The average standard deviations for the Geosat/ERM, ERS-1 and TOPEX/POSEIDON mean gradients are 1.12, 3.25 and 2.01 rad respectively. The standard deviations of the ship gravity, which were assigned based on weightings derived from an analysis of crossing differences, range from 6.72 to 20.17 mgal. The necessary covariances for the least-squares collocation computations were derived using the law of covariance propagation. Before merging with the altimeter data, the ship gravity for each leg was adjusted by removing a quadratic polynomial in time in order to match a satellite-only gravity field in a least-squares sense. The result of the adjustment suggests that most of the ship gravity data collected in the 1960s and the 1970s have average offsets of about 14 mgal, which is close to the offset of the Potsdam Datum. The rms difference between the satellite-only gravity, derived using least-squares collocation, and the adjusted ship gravity is 7.10 mgal, smaller than the rms difference of 12.47 mgal between the satellite-only gravity derived by a Fourier transform method and the adjusted ship gravity. The rms difference between the combined gravity field, derived from both altimetry and ship gravity, and the adjusted ship gravity is 2.65 mgal, suggesting that the former has successfully absorbed the high-frequency component of the gravity signal provided by the latter. The combined gravity field thus features a regionally uniform medium resolution from altimetry and a locally high resolution from ship gravity. The average accuracy estimate given by least-squares collocation for the combined gravity is 5.76 mgal. The combined gravity field reveals detailed tectonic structures over the Reykjanes Ridge related to the interaction of the Iceland hot spot and sea-floor spreading at the ridge.

Oceanwide gravity anomalies from GEOS‐3, Seasat and Geosat altimeter data

Three kinds of satellite altimeter data have been combined, along with 5'×5' bathymetric data, to calculate a 0°.125 ocean wide gridded set of 2.3×106 free‐air gravity anomalies. The procedure used was least squares collocation that yields the predicted anomaly and standard deviation. The value of including the bathymetric data was shown in a test around the Dowd Seamount where the root mean square (rms) difference between ship gravity measurements decreased from ±40 mgal to ±20 mgal when the bathymetry was included. Comparisons between the predicted anomalies and ship gravity data is described in three cases. In the Banda Sea the rms differences were ±20 mgal for two lines. In the South Atlantic rms differences over lines of 2000 km in length were ±7 mgal. For cruise data in the Antarctica region the discrepancies were ±12 mgal. Comparisons of anomalies derived from the Geosat geodetic mission data by Marks and McAdoo [1992] with ship data gave differences of ±6 mgal showing the value of the much denser Geosat geodetic mission altimeter data.