Continuous Gravity Measurements Research Articles

A Atmospheric Correction of FG5 Absolute Gravimetry Data using Measured Air Pressure

Changes in atmospheric density affect atmospheric pressure, which is a key factor affecting high-precision gravity measurement. Currently, atmospheric correction of absolute gravity measurements uses the empirical admittance value recommended by the International Association of Geodesy (−0.3 μGal/mbar); however, the actual admittance value changes with atmospheric mass and time. In this study, we determine the effect of using measured admittance values for absolute gravity correction. First, high-precision relative gravimeters (GWR OSG-057, Scintrex CG5) are used for continuous gravity measurements. Then, air pressure measured by the pressure sensor equipped by FG5 absolute gravimeter is used to obtain the atmospheric admittance using the iterative least squares method, which is compared with the theoretical atmospheric admittance. Taking FG5-257 as an example, we use the measured admittance for atmospheric correction of absolute gravity at four different elevations (Lhasa, Nagqu, Gar, and Suining, China). The results are as follows: 1) According to co-location measurements in Lhasa, CG5 and OSG gravimeter measured admittance values exhibit comparable precision (–0.332 ± 0.003 μGal/mbar and –0.332 ± 0.001 μGal/mbar, respectively), though the CG5 has larger standard deviation. 2) After correction using the measured admittance, changes in set standard deviation and measurement precision are approximately 0.01 μGal; however, the effect on the measurement results does not exceed 1 μGal, which is equivalent to the measurement precision of FG5. Therefore, measured admittance values are only recommended for atmospheric correction of high-precision absolute gravity measurements.

Gravity Monitoring of Underground Flash Flood Events to Study Their Impact on Groundwater Recharge and the Distribution of Karst Voids

AbstractFlash flood events are expected to become increasingly common with the global increases in weather extremes. They are a significant natural hazard that affects karst landscapes, which host large resources of drinking water worldwide. The role played by underground flood events in the karst aquifer recharge is complex due to the heterogeneity of the basement which remains poorly understood. We present the analysis of 20 incave flash flood events affecting the Rochefort karst system (Belgium) using continuous gravity measurements at one single station and water level sensors installed in caves. Underground flood events typically produce a peak in the gravity signal, due to an increase in the associated mass change. After the flood, the gravity values drop but remain slightly increased compared to before the flood event. Via forward gravity modeling, we demonstrate that this remaining anomaly can be reasonably explained by the infiltration of local rainfall within the karst system rather than by allogenic recharge of the aquifer. Flash floods are mainly restricted to connected voids. This allows us to utilize them as proxies to investigate the distribution of cavities in the karst system. Forward modeling of the gravitational attraction induced by the mapped caves being flooded yields a gravity signal much smaller than the observed one. We conclude that at least 50% more cavities than those previously mapped are required to match the measured anomalies. This presents opportunities for implementing similar approaches in other diverse porous media, using gravity monitoring of hydrological processes (e.g., infiltration fronts, hydrothermalism, or tide effects in coastal aquifers) as proxies to characterize underground properties.

The Benefits of Using a Network of Superconducting Gravimeters to Monitor and Study Active Volcanoes

AbstractWe present results from a mini‐array of three iGrav superconducting gravimeters (SGs) at Mount Etna. This is the first network of SGs ever installed on an active volcano. Continuous gravity measurements at active volcanoes are mostly accomplished with spring gravimeters that can be operated even under harsh field conditions. Nevertheless, these instruments do not provide reliable continuous measurements over periods longer than a few days due to the instrumental drift and artifacts driven by ambient parameters. SGs are free from these instrumental effects and thus allow to track even small gravity changes (1–2 μGal) over a wide range of time scales (minutes to months). However, SGs need host facilities with main electricity and a large installation surface, implying that they cannot be deployed in close proximity to the active structures of tall volcanoes. At Mount Etna the three iGrav SGs were installed at distances from the summit active craters ranging between 3.5 and 15 km. Despite the relatively unfavorable position of the installation sites, we show that these instruments can detect meaningful (i.e., volcano‐related) changes that would otherwise remain hidden, like, for example, the weak gravity signature (within a few μGal) of gas buildup at intermediate depth in the plumbing system of Etna, during noneruptive intervals. Our results prove that iGrav SGs are powerful tools to monitor and study active volcanoes and can provide unique information on the bulk processes driving volcanic activity.

Direct measurement of evapotranspiration from a forest using a superconducting gravimeter

AbstractEvapotranspiration (ET) controls the flux between the land surface and the atmosphere. Assessing the ET ecosystems remains a key challenge in hydrology. We have found that the ET water mass loss can be directly inferred from continuous gravity measurements: as water evaporates and transpires from terrestrial ecosystems, the mass distribution of water decreases, changing the gravity field. Using continuous superconducting gravity measurements, we were able to identify daily gravity changes at the level of, or smaller than, 10−9 nm s−2 (or 10−10 g) per day. This corresponds to 1.7 mm of water over an area of 50 ha. The strength of this method is its ability to enable a direct, traceable and continuous monitoring of actual ET for years at the mesoscale with a high accuracy.

Observed secular gravity trend at Onsala station with the FG5 gravimeter from Hannover

Abstract Annual absolute gravity measurements with a FG5 instrument were performed in Onsala Space Observatory by the Institute of Geodesy of the Leibniz Universität Hannover from 2003 to 2011 and have been continued with the upgraded meter FG5X in 2014. Lantmäteriet, Gävle, with their FG5 absolute gravimeter have visited Onsala since 2007. Because small systematic errors may be inherent in each absolute gravimeter, their measuring level and a resulting bias (offset) between the instruments must be controlled over time by means of inter-comparison. From 2007 to 2014, 8 direct comparisons took place well distributed over the time span. A complete re-processing of the absolute gravity observations with the Hannover instrument has been conducted to improve the reduction of unwanted gravity effects. A new tidal model is based on continuous time series recorded with the GWR superconducting gravimeter at Onsala since 2009. The loading effect of the Kattegat is described with a varying sea bottom pressure (water and air mass load) and has been validated with the continuous gravity measurements. For the land uplift,which is a result of the still ongoing glacial isostatic adjustment in Fennoscandia, a secular gravity trend of −0.22 μGal/yr was obtained with a standard deviation of 0.17 μGal/yr. That indicates a slight uplift but is still not significantly different from zero.

Volcanic signatures in time gravity variations during the volcanic unrest on El Hierro (Canary Islands)

AbstractGravity changes occurring during the initial stage of the 2011–2012 El Hierro submarine eruption are interpreted in terms of the preeruptive signatures during the episode of unrest. Continuous gravity measurements were made at two sites on the island using the relative spring gravimeter LaCoste and Romberg gPhone‐054. On 15 September 2011, an observed gravity decrease of 45 μGal, associated with the southward migration of seismic epicenters, is consistent with a lateral magma migration that occurred beneath the volcanic edifice, an apparently clear precursor of the eruption that took place 25 days later on 10 October 2011. High‐frequency gravity signals also appeared on 6–11 October 2011, pointing to an occurring interaction between a magmatic intrusion and the ocean floor. These important gravity changes, with amplitudes varying from 10 to −90 μGal, during the first 3 days following the onset of the eruption are consistent with the northward migration of the eruptive focus along an active eruptive fissure. An apparent correlation of gravity variations with body tide vertical strain was also noted, which could indicate that concurrent tidal triggering occurred during the initial stage of the eruption.

Continuous gravity measurements reveal a low-density lava lake at Kīlauea Volcano, Hawai‘i

On 5 March 2011, the lava lake within the summit eruptive vent at Kīlauea Volcano, Hawai‘i, began to drain as magma withdrew to feed a dike intrusion and fissure eruption on the volcanoʼs east rift zone. The draining was monitored by a variety of continuous geological and geophysical measurements, including deformation, thermal and visual imagery, and gravity. Over the first ∼14 hours of the draining, the ground near the eruptive vent subsided by about 0.15 m, gravity dropped by more than 100 μGal, and the lava lake retreated by over 120 m. We used GPS data to correct the gravity signal for the effects of subsurface mass loss and vertical deformation in order to isolate the change in gravity due to draining of the lava lake alone. Using a model of the eruptive vent geometry based on visual observations and the lava level over time determined from thermal camera data, we calculated the best-fit lava density to the observed gravity decrease — to our knowledge, the first geophysical determination of the density of a lava lake anywhere in the world. Our result, 950±300 kgm−3, suggests a lava density less than that of water and indicates that Kīlaueaʼs lava lake is gas-rich, which can explain why rockfalls that impact the lake trigger small explosions. Knowledge of such a fundamental material property as density is also critical to investigations of lava-lake convection and degassing and can inform calculations of pressure change in the subsurface magma plumbing system.

Results of Gravity Observations Using a Superconducting Gravimeter at the Tibetan Plateau

The tidal and nontidal gravity change characteristics in the Tibetan Plateau region were investigated using the continuous gravity measurements recorded with a superconducting gravimeter (SG) installed in Lhasa from December 8, 2009 to September 30, 2011. The results indicated that the precision of the tidal gravity observations with the SG in Lhasa was very high. The standard deviation of the harmonic analysis for the gravity tides was 0.498 nm s^(-2), and the uncertainties of amplitude factors in the four main tidal waves (i.e., O1, K1, M2 and S2) were better than 0.002%. In addition, the diurnal gravity tide observations clearly revealed a pattern of nearly diurnal resonance. As a result, it is affirmed that the station should act as a local tidal gravity reference in the Tibetan Plateau and its adjacent regions. The load effects of oceanic tides are so weak that the resulting perturbation in the gravimetric factors is less than 0.6%. However, the load effects of the local atmosphere on either the tidal or the nontidal gravity observations are significant, although no seasonal variations have been found. After removing the atmospheric effects, the standard deviation of the harmonic analysis for the gravity tides decreased obviously from 4.160 to 0.498 nm s^(-2). Having removed the load effects of oceanic tides and local atmosphere, it is found that the tidal gravity observations are significantly different from those expected theoretically, which may be related to active tectonic movement and extremely thick crust in the Tibetan Plateau region. In addition, the Earth's free oscillations excited by 2011 Tohoku-Oki Mw 9.0 Earthquake were successfully detected.

Characteristics of tidal gravity changes in Lhasa, Tibet, China

Tidal gravity changes arise from the response of the solid Earth to the tidal forces of the Sun, Moon and planets close to the Earth, and are a comprehensive reflection of the structure and distribution of physical properties of the Earth’s interior. As a result, observations of tidal gravity changes are the basis of studies on other global and/or regional dynamic processes. The characteristics of tidal gravity changes in the region of the Tibetan Plateau were investigated through continuous gravity measurements recorded with a superconducting gravimeter (SG) installed in Lhasa over a year. Through contrast measurements with a spring gravimeter LaCoste-Romberg ET20 at the same site, the gravity observations in Lhasa were scaled to the international tidal gravity reference in Wuhan. Meanwhile, the scale factor of the SG was determined accurately as −777.358 ± 0.136 nm s−2 V−1, which is about 2.2% less than the value provided by the manufacturer. The results indicate that the precision of the tidal gravity observations made with the SG in Lhasa was very high. The standard deviation was 0.459 nm s−2, and the uncertainties of for the four main tidal waves (i.e. O1, K1, M2 and S2) were better than 0.006%. In addition, the observations of the diurnal gravity tides had an obvious pattern of nearly diurnal resonance. As a result, it is affirmed that the Lhasa station can provide a local tidal gravity reference for gravity measurements on the Tibetan Plateau and its surrounding regions. The loading effects of oceanic tides on tidal gravity observations in Lhasa are so weak that the resulting perturbations in the gravimetric factors are less than 0.6%. However, the loading effects of the local atmosphere on either the tidal or nontidal gravity observations are significant, although no seasonal variations were found. After removal of the atmospheric effects, the standard deviation of the SG observations in Lhasa decreased obviously from 2.009 to 0.459 nm s−2. Having removed the loading effects of oceanic tides and local atmosphere, it was found that the tidal gravity observations made with the SG in Lhasa significantly differed by about 1% from those expected theoretically, which may be related to active tectonic movement and the extremely thick crust in the region of the Tibetan Plateau. A more-certain conclusion requires longer accumulation of SG data and further associated theoretical studies.

Experimental detection of the inner core translational triplet

The translational oscillation of the solid inner core is one of the Earth’s fundamental normal modes, which is also called Slichter mode. The normal mode should be split to form a triplet due to the Earth’s rotation and ellipticity. In this study, according to the splitting pattern of Slichter mode, an attempt has been made to detect the possibility of Slichter triplet’s existence by using the product spectra of the long-term continuous gravity measurements from the superconducting gravimeters (SG) at 6 globally-distributed permanent stations in the Global Geodynamics Projects network. The results indicate that the background noise level of the global SG observations is 0.0158 nms−2 and the magnitude threshold of any global harmonic signals, which may be detected by the global SG, is 0.0152 nms−2 in the subtidal frequency band from 0.162 to 0.285 cph in which Slichter triplet may occur. it implies that the signatures, related to the triplet, may be identified in the global SG observations, if they exist. It is found that there is a group of global harmonic signatures with the periods of 5.310, 4.995 and 4.344 h emerging significantly from the background noise in the global SG observations. They are in good agreement with the splitting pattern of Slichter mode. It implies that this group of signatures may be related to the inner core translational oscillations. The associated density contrast across the inner core boundary may be deduced as between the values provided in the Earth models of the PREM and 1066 A.

The gravity field variation caused by inner core super rotation

Due to the super rotation of the Earth’s inner core, the tilted figure axis of the inner core would progress with respect to the mantle and thus cause the variation of the Earth’s external gravity field. This paper improves the present model of the gravity field variation caused by the inner core super rotation. Under the assumption that the inner core is a stratifying ellipsoid whose density function is fitted out from PREM and the super rotation rate is 0.27∼0.53°/yr, calculations show that the global temporal variations on the Earth’s surface have a maximum value of about 0.79∼1.54×10−3 μGal and a global average intensity of about 0.45∼0.89×10−3 μGal in the whole year of 2007, which is beyond the accuracy of the present gravimetry and even the super conducting gravimeter data. However, both the gravity variations at Beijing and Wuhan vary like sine variables with maximal variations around 0.33 μGal and 0.29 μGal, respectively, in one cycle. Thus, continuous gravity measurements for one or two decades might be able to detect the differential motion of the inner core.

On the capability of recording gravity stations to detect signals coming from volcanic activity: The case of Vesuvius

The goal of this paper is to describe how continuous gravity measurements can improve the geophysical monitoring of a volcano. Here the experience of 15 yr in continuous gravity on Vesuvius is presented. A wide set of dynamic phenomena (i.e. geodynamics, seismicity, volcanic activity) can produce temporal gravity changes, with a spectrum varying from short (1–10 s) to longer (more than 1 yr) periods. An impending eruption, for instance, is generally associated with the ascent of magma producing changes in the density distribution at depth, and leading to ground deformation and gravity changes observed at surface. The amplitude of such gravity variations is often quite small, on the order of 10 − 9 –10 − 8 g (10–10 2 nm/s 2; 1–10 μGal), where g is the mean value of normal gravity (9.806 199 203 m/s 2), so their detection requires instruments with high sensitivity and stability, providing high quality data. Natural, man-made and instrumental sources are present on the gravity records affecting the Signal to Noise Ratio. Such effects may hide the subtle volcanic signals. The main natural noise is due to ocean–atmosphere dynamics and seismic activity. New approaches to model the instrumental response of mechanical gravity sensors (based on the inter-comparison among superconducting, mechanical and absolute gravimeters) and to investigate the temporal trends of the instrumental sensitivity are proposed. In fact, variations of the calibration factors can be considered the main cause preventing the repeatability of high-precision gravity measurements and inducing phase and amplitude perturbations in recorded gravity signals. A modelling of the background gravity noise level was performed at the Vesuvius station. Moreover, the “far field” effects produced by large earthquakes on the gravity station have been also investigated. Finally, the time dependent behaviour of the tidal gravimetric factors, the non-stationary components of the gravity field detected at Vesuvius and the results of absolute and relative gravity measurements are interpreted in the framework of its present-day dynamics, mainly characterized by the low level of seismicity, small ground deformation, gravity changes and moderate gas emission.

A test on a Neuro-Fuzzy algorithm used to reduce continuous gravity records for the effect of meteorological parameters

Gravity measurements are utilized at active volcanoes to detect mass changes linked to magma transfer processes and thus to recognize forerunners to paroxysmal volcanic events. Continuous gravity measurements are now increasingly performed at sites very close to active craters, where there is the greatest chance to detect meaningful gravity changes. Unfortunately, especially when used against the adverse environmental conditions usually encountered at such places, gravimeters have been proved to be affected by meteorological parameters, mainly by changes in the atmospheric temperature. The pseudo-signal generated by these perturbations is often stronger than the signal generated by actual changes in the gravity field. Thus, the implementation of well-performing algorithms for reducing the gravity signal for the effect of meteorological parameters is vital to obtain sequences useful from the volcano surveillance standpoint. In the present paper, a Neuro-Fuzzy algorithm, which was already proved to accomplish the required task satisfactorily, is tested over a data set from three gravimeters which worked continuously for about 50 days at a site far away from active zones, where changes due to actual fluctuation of the gravity field are expected to be within a few microgal. After accomplishing the reduction of the gravity series, residuals are within about 15 μGal peak-to-peak, thus confirming the capabilities of the Neuro-Fuzzy algorithm under test of performing the required task satisfactorily.

A methodology for reducing the effect of meteorological parameters on a continuously recording gravity meter

Gravity changes in geodynamically active areas are usually monitored by using discrete measurements. The temporal resolution, which is only as good as the repeat rate of the observations (usually ranging between one month and one year), is the main drawback of such measurements. To achieve a better temporal resolution and thus improve the possibilities of the investigation task, continuous gravity measurements have to be performed. However, temperature and pressure fluctuations seriously affect the output of continuously running spring gravity meters. In this paper, a methodology allowing the signal from a recording gravity meter to be compensated for the effect of temperature and pressure is discussed. To model how the meter output depends on the interfering signals, both polynomial (LMS) and nonlinear (neuro-fuzzy) forms of three different model structures have been tested. The results obtained through the nonlinear algorithm are satisfactory and could constitute the background for the implementation of a real-time correction system using a fuzzy micro-controller chip.

Continuous near‐bottom gravity measurements made with a BGM‐3 gravimeter in DSV Alvin on the East Pacific Rise crest near 9°31′N and 9°50′N

A Bell BGM‐3 gravimeter has been used to collect continuous, underway, nearbottom (3‐ to 10‐m altitude) gravity measurements from the deep‐diving submersible DSV Alvin during surveys on the East Pacific Rise (EPR) crest near 9°31′N and 9°50′N. Closely spaced (20‐to 30‐m) gravity measurements were made along transects up to 8 km long in both regions. Repeatability of measurements made at the same location on different dives is ∼0.3 mGal. Along‐track spatial resolution of anomalies is ∼130–160 m, with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible. The average upper crustal density of the ridge crest determined from the relationship between depth and free‐water gravity anomalies varies greatly between 9°31′N and 9°50′N. Average upper crustal densities of 2410 kg/m3 for the 9°50′N area and 2690 kg/m3 for the 9°31′N area were calculated. The different densities are not due to differing geometry of the Layer 2A‐2B boundary or a regional cross‐axis gravity gradient. Differences in porosity of the shallow crustal rocks, or a difference in the proportion of low density extrusives to higher‐density dikes and sills within Layer 2A in these two areas, are the likely causes of the different upper crustal densities. Bouguer gravity anomalies near the EPR axis are primarily small amplitude (0.5–2 mGal), are a few hundred meters across, and appear to be lineated parallel to the axis. Larger‐amplitude Bouguer anomalies of up to 4 mGal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor. These ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough (ASCT) at the same latitude. Probable sources of the 0.5‐ to 2‐mGal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through Layer 2A to the surface. A grid survey of the ridge axis near 9°50′N shows Bouguer anomalies lineated along the axis, suggesting that dike swarms do contribute to the observed Bouguer anomalies. The along‐axis continuity of the gravity anomalies is disrupted at a 75‐m offset of the ASCT, suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system. This initial study confirms the ability to conduct high‐resolution, near‐bottom, continuous gravity measurements from Alvin. It also provides important information on how the shallow crustal structure of a fast spreading mid‐ocean ridge develops and how it varies with the surface morphology.

Determination of the gravitational constant by an experiment at a pumped-storage reservoir.

In August 1988 continuous gravity measurements were made in a pumped-storage reservoir in SW Germany. Daily water-level variations of 5--22 m produced, together with the tides, a quasiperiodic gravity signal which could be measured with a resolution well below 1 \ensuremath{\mu}gal${=10}^{\mathrm{\ensuremath{-}}8}$ m ${\mathrm{s}}^{\mathrm{\ensuremath{-}}2}$. The experiment's goal was a search for deviations from Newton's gravitational law, but it can also be viewed as a measurement of the gravitational constant G for effective mass distances of 40--70 m. The deviation of G from the laboratory value was found to be (0.25\ifmmode\pm\else\textpm\fi{}0.4)% and thus is not significantly different from zero.

Crustal structure across the Tasman Sea

Abstract A crustal section across the Tasman Sea between latitudes 31° S and 35° S has been obtained by interpreting continuous gravity measurements with the aid of previous seismic measurements. A quasi-continental type of crust exists under the Dampier Ridge (crustal thickness 20 km), Lord Howe Rise (26 km), West Norfolk Ridge (21 km), and Norfolk Ridge (21 km). The crust beneath the New Caledonia Basin is of oceanic type (9 km). Short wavelength gravity and magnetic anomalies over Lord Howe Rise, West Norfolk Ridge, and Norfolk Ridge suggest that dense, magnetic, intrusive rocks occur within or beneath a thin (< 1 km) sediment cover. The thickness of the sediments in the centre of the New Caledonia Basin is about 1 km and increases to about 2 km at the foot of the Lord Howe Rise and about 3 km at the foot of the West Norfolk Ridge.

“Two-way Stretch” of Sialic Crust and Plate Tectonics in the South-west Pacific

ANALYSES of seismic and continuous gravity measurements over the northern Tasman Sea1 have confirmed the existence of “quasi-continental” crust (20–26 km thick) beneath the parallel ridges—Norfolk Ridge, Lord Howe Rise and Dampier Ridge—that extend north-westward from New Zealand (Fig. 1). The intervening major trough, New Caledonia Basin, is shown to have a floor of true oceanic thickness (9 km).

Continental Rise off Eastern North America

During mid-1967 two cruises of the Woods Hole Oceanographic Institution's R/V Chain provided nearly continuous seismic, geomagnetic, and gravity measurements along 8,000 km of ship track. These measurements supplement earlier ones from various sources to provide a comprehensive picture of the composition and geologic history of the continental margin off eastern North America, an area that is much larger than all of the United States east of the Mississippi River. The geomagnetic profiles portray a systematic pattern of positive and negative anomalies that are in accord with the concept of sea-floor spreading, whereby North America separated from Europe and Africa at the beginning of the Permian Period, and drifted westward from the site of rifting (the Mid-Atlantic Ridge) at average rates of 0.8-1.4 cm/year. During all this time the continent has been coupled firmly with the adjacent sea floor, as though both continent and sea floor were on the same conveyor belt. Gravity information suggests that a relict structure of the original rift is preserved in the same general area as the geomagnetic slope anomaly, beneath the seaward part of the continental shelf, the continental slope, or the upper continental rise. It has the form of a complex linear ridge of crystalline rocks that rises above the zone of sharpest landward slope of the Mohorovicic discontinuity. Seismic refraction measurements support the presence of such a ridge, bordered on both sides by linear trenches. The continuous seismic reflection profiles measured during the cruises reveal shallow acoustic basement in the form of a ridge complex that is shallower, but in the same general area, and probably is related to the deep ridge. The ridge and associated trenches served as dams and b sin sinks to trap land-derived sediments during the Mesozoic Era, so that only pelagic silts and clays could reach and be deposited on the irregular oceanic basement seaward of the barrier. During Late Cretaceous to middle Eocene time one or more thick deposits of probably chemical origin formed blankets of deep-sea chert throughout broad abyssal plains, which produce the acoustic reflector known as Horizon A. About middle Eocene time the land-derived sediments filled the trap west of the ridge and prograded eastward over the ridge top and built the present continental rise atop the Mesozoic abyssal plain. Continuous seismic reflection profiles show that the rise is a huge prism of generally seaward-dipping, interbedded pelagic sediments and turbidites that contain many masses of sediment displaced from higher on the continental rise and from the continental slope. Such slides continue to occur, a large one having occurred in 1929. The volume of the Cenozoic continental rise in the study region is nearly 3 million km3, about half the volume of all sediments deposited on basement during Mesozoic time. The interbedding of sandy turbidites with organic-rich silts and clays displaced from the continental slope may constitute a thick sequence of oil reservoir and oil source beds, but no exploratory drilling into them has been done.

Oregon State University sea gravity investigations

In 1963 Oregon State University obtained over 4,000 miles of continuous gravity measurements at sea from the Oregon coast westward to beyond the seaward extension of the San Andreas fault and from Point Mendocino, California, to Astoria Canyon, Oregon. Measurements were made on the U. S. Coast Guard cutter Yocona, a 213‐foot, 1900‐ton ship. The measurements include 7 lines from the coast westward for a distance of 100–300 miles, 15 profiles across the San Andreas fault extension, and 1 profile across the Mendocino escarpment. Free air anomalies for these measurements have been calculated, and a free air anomaly map has been contoured.The reliability and accuracy of LaCoste‐Romberg gravity meter S‐9 was checked against a gravity range which had been previously obtained from an underwater gravity meter off Newport, Oregon. Surface‐ship gravity readings were found to be accurate within 6 mgal, even at Browne corrections up to 450 mgal, where the errors included navigational effects. Comparable errors were obtained at intersections of profiles in the larger gravity program off Oregon. Results of the reliability determinations of meter S‐9 are being prepared for publication, as based on measurements over the Newport gravity range, on measurements past an offshore petroleum platform in the Gulf of Mexico, and from intersections of numerous gravity profiles in the Gulf of Mexico and off the coast of Oregon.