Publications

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2003
Sasagawa, GS, Crawford W, Eiken O, Nooner S, Stenvold T, Zumberge MA.  2003.  A new sea-floor gravimeter. Geophysics. 68:544-553.   10.1190/1.1567223   AbstractWebsite

A new reservoir management application uses precise time-lapse gravity measurements on the sea floor to detect seawater infiltration in offshore natural gas fields during production. Reservoir models for the North Sea Troll field predict gravity changes as large as 0.060 mGal within a 3-5-year period. We have constructed and deployed a new instrument-the ROVDOG (Remotely Operated Vehicle-deployed Deep-Ocean Gravimeter) system-for this application. Because the measurements must be relocated accurately (within 3 cm), we required a gravimeter which could be handled by an ROV and placed atop sea-floor bench marks. We have built an instrument based upon the Scintrex CG-3M land gravimeter. Motorized gimbals level the gravimeter sensor within a watertight pressure case. Precision quartz pressure gauges provide depth information. A shipboard operator remotely controls the instrument and monitors the data. The system error budget considers both instrumental and field measurement uncertainties. The instrument prototype was deployed in the North Sea during June 1998; 75 observations were made at 32 stations. The standard deviation of repeated gravity measurements was 0.026 mGal; the standard deviation of pressure-derived heights, for repeated measurements, was 1.4 cm. A refined instrument was deployed in August 2000 in a three-sensor configuration. Multiple sensors improved the precision by averaging more samples without incurring additional survey time. A total of 159 measurements were made at 68 station. The standard deviation of repeated measurements was 0.019 mGal; the standard deviation of pressure-derived heights was 0.78 cm. A ROVDOG pressure case rated to 4500 m depth has also been constructed. This system was deployed with the Alvin manned submersible in November 2000 to a depth of 2700 m.

Nooner, SL, Sasagawa GS, Blackman DK, Zumberge MA.  2003.  Structure of oceanic core complexes: Constraints from seafloor gravity measurements made at the Atlantis Massif. Geophysical Research Letters. 30   10.1029/2003gl017126   AbstractWebsite

[1] Using the DSV Alvin, the relative seafloor gravimeter ROVDOG was deployed at 18 sites on the Atlantis Massif (located at the ridge-transform intersection of the Mid-Atlantic Ridge and the Atlantis Transform Fault near 30degreesN, 42degreesW). These data along with previously collected shipboard gravity and bathymetry provide constraints on the density structure of this oceanic core complex. A series of quasi 3-D forward models suggests that symmetric east and west-dipping density interfaces bound the core of the massif with dip angles of 16degrees-24degrees in the east and 16degrees-28degrees in the west, creating a wedge with a density of 3150-3250 kg/m(3). The dip angle in the east is steeper than that of the surface slope, suggesting that the detachment fault surface does not coincide with the density boundary. The resulting low-density layer is interpreted as a zone of serpentinization.

2002
Ridgway, JR, Zumberge MA.  2002.  Deep-towed gravity surveys in the southern California Continental Borderland. Geophysics. 67:777-787.   10.1190/1.1484521   AbstractWebsite

We surveyed two sites in the southern California continental borderland with a newly developed instrument, a towed deep ocean gravimeter-a gravity sensor that can be towed a few tens of meters above the sea floor. During its development phase, we used the instrument to survey two regions off the coast of southern California. The first was along two tracks in the San Diego Trough. The second was over a seamount-like feature named Emery Knoll. Results of the trough survey reveal a basin with a geometry consistent with seismic data. We observed no significant density contrast across the San Diego Trough fault in the near-surface sediments. The survey of Emery Knoll shed light on the question of the origin of this structure; modeling the knoll to determine its bulk density suggested a nearly uniform structure surrounded by sedimentary basins with a more massive central intrusive body. Derived densities of 2850 kg/m(3) for the knoll and 3050 kg/m(3) for the central intrusion assume that no deep unmodeled sources exist directly underneath the knoll. The gravity data favor a model of metamorphic basement rock uplifted and containing igneous intrusives.

Zumberge, MA, Elsberg DH, Harrison WD, Husmann E, Morack JL, Pettit EC, Waddington ED.  2002.  Measurement of vertical strain and velocity at Siple Dome, Antarctica, with optical sensors. Journal of Glaciology. 48:217-225.   10.3189/172756502781831421   AbstractWebsite

As part of a larger program to measure and model vertical strain around Siple Dome on the West Antarctic ice sheet, we developed a new sensor to accurately and stably record displacements. The sensors consist of optical fibers, encased in thin-wall stainless-steel tubes, frozen into holes drilled with hot water, and stretched from the surface to various depths (up to 985 m) in the ice sheet. An optical system, connected annually to the fibers, reads out their absolute lengths with a precision of about 2 mm. Two sets of five sensors were installed in the 1997/98 field season: one set is near the Siple Dome core hole (an ice divide), and a second set is on the flank 7 km to the north (the ice thickness at both sites is approximately 1000 m). The optical-fiber length observations taken in four field seasons spanning a 3 year interval reveal vertical strain rates ranging from -229 +/- 4 ppm a(-1) to -7 +/- 9 ppm a(-1). In addition to confirming a non-linear constitutive relationship for deep ice, our analysis of the strain rates indicates the ice sheet is thinning at the flank and is in steady state at the divide.

2001
Gee, JS, Webb SC, Ridgway J, Staudigel H, Zumberge MA.  2001.  A deep tow magnetic survey of Middle Valley, Juan de Fuca Ridge. Geochemistry Geophysics Geosystems. 2   10.1029/2001GC000170   AbstractWebsite

We report here results from a deep tow magnetic survey over Middle Valley, Juan de Fuca Ridge. A series of track lines are combined to generate a high-resolution map of the magnetic field anomaly within a 10 x 12 km region surrounding the Bent Hill massive sulfide (BHMS) deposit. A uniformly magnetized body (5 A/m) with a cross section approximating the body inferred from Ocean Drilling Program (ODP) drilling can account for the observed near-bottom magnetic anomaly amplitude. Assuming this magnetization is entirely induced, the average susceptibility (0.11 SI) corresponds to similar to3.5% magnetite + pyrrhotite by volume, consistent with the abundance of these phases observed in drill core samples. However, this uniform magnetization model significantly underestimates the magnetic anomaly measured a few meters above the seafloor by submersible, indicating that the upper portion of the sulfide mound must have a significantly higher magnetization (similar to 10% magnetite + pyrrhotite) than at deeper levels. On a larger scale, the near-bottom magnetic anomaly data show that basement magnetizations are not uniformly near zero, as had been inferred from analysis of the sea surface anomaly pattern. We interpret this heterogeneity as reflecting primarily differences in the degree of hydrothermal alteration. Our results highlight the potential of magnetic anomaly data for characterizing hydrothermal deposits where extensive drill core sampling is not available.

1998
Zumberge, MA, Wyatt FK.  1998.  Optical fiber interferometers for referencing surface benchmarks to depth. Pure and Applied Geophysics. 152:221-246.   10.1007/s000240050152   AbstractWebsite

We have developed and operated optical fiber interferometers for monitoring displacements within boreholes, as part of a program of crustal deformation measurement. These optical tiber strainmeters-a total of twelve instruments at two sites in southern California-were installed to sense the motion of the end-monuments of much longer baseline strainmeters and tiltmeters, allowing correction for any near-surface ground movement. One of the installations was specifically designed to investigate the distribution of deformation with depth, measuring over several borehole length-intervals from 5 m to 50 m. The displacements recorded over year-long time scales along these length intervals range up to 6 mm and show internal consistency and stability at the 50 mu m level. The use of these interferometers to provide correction signals for kilometer-scale crustal strain measurements has resulted in greatly improved records.

1997
Zumberge, MA, Ridgway JR, Hildebrand JA.  1997.  A towed marine gravity meter for near-bottom surveys. Geophysics. 62:1386-1393.   10.1190/1.1444243   AbstractWebsite

Gravity is measured presently on the sea surface and on the sea floor. Surface gravity suffers from loss of resolution over the deep ocean because the perturbing source masses are far from the observer, Bottom measurements recover this resolution, but suffer from poor coverage because of the time needed for each measurement. We have constructed a gravimetry system that combines the rapid data collection capability of a moving platform with the high resolution gained by locating the observations near the bottom. This gravity sensor is tethered to a ship and towed just above the sea floor. The instrument consists of a LaCoste and Romberg shipboard gravity meter modified to fit inside a pressure case that is mounted on a platform designed for towing stability. We have tested it in a survey in the San Diego Trough, a 1000-m-deep sedimented valley in the Pacific Ocean in the California continental borderlands. Multiple gravity tracklines collected there at a depth of 935 m show a resolution of a few tenths of a mGal. The new instrument will be useful for surveys of features whose lateral extent is equal to or less than the ocean depth.

Zumberge, MA.  1997.  Precise optical path length measurement through an optical fiber: Application to seafloor strain monitoring. Ocean Engineering. 24:531-542.   10.1016/s0029-8018(96)00029-7   AbstractWebsite

An optical tiber strainmeter intended for measuring tectonic strains on the seafloor is under development. In this instrument, an optical fiber is stretched between two points fixed to the ocean bottom; relative displacement of these points causes a change in the elongation of the fiber. This associated change in optical path length is monitored by an electronic distance meter. The dominant sources of noise in determining the optical path length of the fiber stem from the dependence of the fiber's index of refraction on both wavelength and temperature. In a 50 day long experiment performed in the shallow ocean, a test fiber was installed along a 210 m long baseline on the bottom. The RMS Variation in length was 5 mm except for two displacements of order 10 cm caused by known effects. (C) 1997 Elsevier Science Ltd.

Canuteson, E, Zumberge M, Hanson J.  1997.  An absolute method of vertical seismometer calibration by reference to a falling mass with application to the measurement of the gain. Bulletin of the Seismological Society of America. 87:484-493. AbstractWebsite

We measure the gain of a vertical seismometer by simultaneously recording the output of the seismometer and repeatedly measuring the displacement between the seismometer and a free-falling mass in a vacuum. The falling object provides an inertial reference frame. By comparing the ground motion measured by the seismometer with the independent record of displacement between the seismometer and inertial space, we obtain the gain. It is an absolute measurement of the gain relative to the local Lorentz reference frame, Bootstrap error estimates show that a high precision in the estimate of the gain can be obtained with a small number of individual drops. The method derived can be extended to multi-parameter searches of the vertical response function. The technique is also shown to reduce noise in absolute gravity measurements due to ground noise, Finally, we discuss the potential for replacing vibration isolation schemes in absolute gravity systems with digital noise reduction.

1996
Canuteson, EL, Zumberge MA.  1996.  Fiber-optic extrinsic Fabry-Perot vibration-isolated interferometer for use in absolute gravity meters. Applied Optics. 35:3500-3505.   10.1364/ao.35.003500   AbstractWebsite

In an absolute gravity meter, a laser interferometer measures the position of a test mass that is falling in a vacuum. The calculated value of gravity is the average acceleration of the mass during a set of drops, Since systematic accelerations of the optical system will bias the measured value of gravity, various interferometar geometries have been implemented in the past to isolate the optical system from ground motion. We have developed and tested a low-finesse fiber-optic extrinsic Fabry-Perot interferometer that is fixed to the mass of a critically damped seismometer in which the effects of systematic ground motion and acoustic vibrations are reduced. (C) 1996 Optical Society of America

1995
Zumberge, MA, Canuteson EL, Parker PR.  1995.  Optical fiber gravity meter. , U.S.A.: The Regents of the University of California Abstract
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Parker, PR, Zumberge MA, Parker RL.  1995.  A New Method for Fringe-Signal Processing in Absolute Gravity Meters. Manuscripta Geodaetica. 20:173-181. AbstractWebsite

In all modern absolute gravity meters, an interferometer illuminated with a stabilized laser tracks the motion of a freely falling retroreflector. The value of gravity is measured by timing the passage of interference fringes. Typically, the sinusoidal fringe signal is converted to a series of pulses, a subset of which are input to a time digitizer. In our new system, the fringe signal is digitized with a fast analog-to-digital converter and fit to an increasing-frequency sine wave. In addition to being smaller and less expensive, the system should eliminate some potential systematic errors that may result from imperfect zero-crossing discrimination and pulse pre-scaling.

Zumberge, M.  1995.  Gravity Meter. McGraw Hill Encyclopedia of Science & Technology. Abstract
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1994
Stevenson, JM, Hildebrand JA, Zumberge MA, Fox CG.  1994.  An Ocean-Bottom Gravity Study of the Southern Juan-De-Fuca Ridge. Journal of Geophysical Research-Solid Earth. 99:4875-4888.   10.1029/93jb02076   AbstractWebsite

We use seafloor and sea surface gravity data to model density structure along the southern Juan de Fuca Ridge. We determine the average density of the shallow oceanic crust at the ridge using seafloor gravity measurements and then use these data in conjunction with sea surface gravity observations to construct density structure models. Of 63 seafloor gravity measurements obtained, 42 observations were distributed over the 3-km left-lateral overlapping rift zone (ORZ), located at latitude 45-degrees-03'N, and separating the Juan de Fuca ridge into the Cleft and Vance segments. A 21-measurement seafloor gravity transect was made perpendicular to the ridge strike at latitude 44-degrees-52'N, a region of linear ridge geometry on the Cleft segment. These seafloor gravity measurements, which are sensitive to shallow crustal density variations, were augmented by roughly 800 km of sea surface gravity measurements. Using a bathymetry-Bouguer anomaly analysis of the seafloor gravity data, we determined the average density of the shallow (roughly upper 2 km) oceanic crust at these two locations to be 2630 kg m-3 +/- 50 kg m-3. Within the uncertainties of the density determinations, there is no difference between the average shallow oceanic crustal density at the linear Cleft segment and at the Cleft-Vance ORZ. Using the seafloor measurements, we modeled the study area's underlying density to provide constraints on its fine-scale structure. Localized porosity of up to 17% within the upper 500 m of oceanic crust (layer 2A) provide a possible explanation for the observed gravity anomalies at the Cleft segment. Two-dimensional density models of the crust underlying the linear Cleft segment (44-degrees-52'N) show that no deep source is required to explain the topographical asymmetry observed between the east and west sides of the ridge axis. For the Cleft-Vance ORZ, three-dimensional modeling suggests low-density material between the rifts, in agreement with thickened layer 2A from seismic observations. In contrast, layer 2A variations do not explain the low density inferred at the northern end of the Cleft segment since seismic observations suggest a thin layer 2A. The magmatic activity recently observed in this region may have a distinct, deeper source.

1991
Zumberge, MA, Hildebrand JA, Stevenson JM, Parker RL, Chave AD, Ander ME, Spiess FN.  1991.  Submarine Measurement of the Newtonian Gravitational Constant. Physical Review Letters. 67:3051-3054.   10.1103/PhysRevLett.67.3051   AbstractWebsite

We have measured the Newtonian gravitational constant using the ocean as an attracting mass and a research submersible as a platform for gravity measurements. Gravitational acceleration was measured along four continuous profiles to depths of 5000 m with a resolution of 0.1 mGal. These data, combined with satellite altimetry, sea surface and seafloor gravity measurements, and seafloor bathymetry, yield an estimate of G = (6.677 +/- 0.013) x 10(-11) m3 s-2 kg-1; the fractional uncertainty is 2 parts in 1000. Within this accuracy, the submarine value for G is consistent with laboratory determinations.

Sasagawa, G, Zumberge MA.  1991.  Absolute Gravity Measurements in California, 1984-1989. Journal of Geophysical Research-Solid Earth and Planets. 96:2501-2513.   10.1029/90jb02283   AbstractWebsite

Repeated absolute gravity measurements have been made at 12 sites in California between 1984.3 and 1989.7. As determined in laboratory tests, the instrument used has an estimated accuracy of 10-mu-Gal (approximately 10(-8) g). The repeatability of the measurements is consistent with this accuracy assessment. No gravity changes above the limits set by instrumental uncertainty and environmental noise are observed in California during this period; the field observations provide upper limits on the rates of secular gravity changes which could be attributed to crustal deformation with a resolution corresponding to vertical displacement rates of 1-2 cm/yr.

1990
Zumberge, MA, Ander ME, Lautzenhiser TV, Parker RL, Aiken CLV, Gorman MR, Nieto MM, Cooper APR, Ferguson JF, Fisher E, Greer J, Hammer P, Hansen BL, McMechan GA, Sasagawa GS, Sidles C, Stevenson JM, Wirtz J.  1990.  The Greenland Gravitational Constant Experiment. Journal of Geophysical Research-Solid Earth and Planets. 95:15483-15501.   10.1029/JB095iB10p15483   AbstractWebsite

An Airy-type geophysical experiment was conducted in a 2-km-deep hole in the Greenland ice cap at depths between 213 m and 1673 m to test for possible violations of Newton's inverse square law. The experiment was done at Dye 3, the location of a Distant Early Warning Line radar dome and the site of the deepest of the Greenland Ice-Sheet Program (GISP) drill holes. Gravity measurements were made at eight depths in 183-m intervals with a LaCoste&Romberg borehole gravity meter. Prior to the experiment the borehole gravity meter was calibrated with an absolute gravity meter, and the wireline depth-rinding system used in the borehole logging was calibrated in a vertical mine-shaft against a laser geodimeter. The density of the ice in the region was calculated from measurements taken from ice cores obtained from earlier drilling observations. Ice penetrating radar was employed in order to correct the gravity data for the topography of the ice-rock interface. Surface gravity observations were made to assess the extent to which density variations in the sub-ice rock could affect the vertical gravity gradient. The locations of the gravity observation points were determined with a combination of GPS recording, first-order leveling, and EDM surveying. An anomalous variation in gravity totaling 3.87 mGal (3.87×10−5 m/s2) in a depth interval of 1460 m was observed. This may be attributed either to a breakdown of Newtonian gravity or to unexpected density variations in the rock below the ice.

Ander, ME, Kerr W, Aiken CLV, Glover CC, Zumberge MA.  1990.  An Absolute Wireline Calibration to Support a Test of Newtons Inverse Square Law. Geophysics. 55:920-923.   10.1190/1.1442907   AbstractWebsite

As part of a Greenland ice cap experiment to measure possible scale length violations of Newton’s inverse square law over geophysical scales of 200 to 1500 m, it was necessary to locate the depth of a gravity meter attached to a wireline down a borehole to about 1 part in 10 000. In order to do this, the wireline and cable length measuring system had to be calibrated both before and after the Greenland expedition. The measuring system used a combination of a mechanical wheel measuring device and a magnetic mark counter. The calibration was conducted in a 1200 m vertical mine shaft at the Consolidated Silver Mine in Osborn, Idaho. Distances in the mine shaft were first calibrated to a precision of about 0.005 m using a geodetic laser system model 4L Geodimeter operating at 30 MHz. To calibrate the wireline, it was run up the mine shaft five times before and five times after the Greenland experiment. The calibration before the experiment was good to about 4 parts in 10 000 and the calibration after was accurate to about 1 part in 10 000. A total inelastic stretch of only 0.102 m occurred during the Greenland operation.

Hildebrand, JA, Stevenson JM, Hammer PTC, Zumberge MA, Parker RL, Fox CG, Meis PJ.  1990.  A Sea-Floor and Sea-Surface Gravity Survey of Axial Volcano. Journal of Geophysical Research-Solid Earth and Planets. 95:12751-12763.   10.1029/JB095iB08p12751   AbstractWebsite

Seafloor and sea surface gravity measurements are used to model the internal density structure of Axial Volcano. Seafloor measurements made at 53 sites within and adjacent to the Axial Volcano summit caldera provide constraints on the fine-scale density structure. Shipboard gravity measurements made along 540 km of track line above Axial Volcano and adjacent portions of the Juan de Fuca ridge provide constraints on the density over a broader region and on the isostatic compensation. The seafloor gravity anomalies give an average density of 2.7 g cm−3 for the uppermost portion of Axial Volcano, The sea surface gravity anomalies yield a local compensation parameter of 23%, significantly less than expected for a volcanic edifice built on zero age lithosphere. Three-dimensional ideal body models of the seafloor gravity measurements suggest that low-density material, with a density contrast of at least 0.15 g cm−3, may be located underneath the summit caldera. The data are consistent with low-density material at shallow depths near the southern portion of the caldera, dipping downward to the north. The correlation of shallow low-density material and surface expressions of recent volcanic activity (fresh lavas and high-temperature hydrothermal venting) suggests a zone of highly porous crust. Seminorm minimization modeling of the surface gravity measurements also suggest a low-density region under the central portion of Axial Volcano. The presence of low-density material beneath Axial caldera suggests a partially molten magma chamber at depth.

Munk, W, Revelle R, Worcester P, Zumberge M.  1990.  Strategy for future measurements of very-low frequency sea-level change. National Research Council Report, Geophysics Study Committee. :221-227., Washington, D. C.: National Research Council Abstract
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1989
Parker, RL, Zumberge MA.  1989.  An Analysis of Geophysical Experiments to Test Newton Law of Gravity. Nature. 342:29-32.   10.1038/342029a0   AbstractWebsite

Signals reported as evidence for a non-newtonian 'fifth' force at a North Carolina television tower and elsewhere can be explained in a conventional way by postulating small density variations underground. The assumptions employed in earlier analyses which pointed to a failure of the inverse square law are examined and found to be difficult to justify.

Sasagawa, GS, Zumberge MA.  1989.  5-Year Frequency Stability of a Zeeman Stabilized Laser. Applied Optics. 28:824-825.   10.1364/AO.28.000824   AbstractWebsite

A five-year record of the lockpoint frequency of a Zeeman stabilized laser shows an observed drift rate of 0.3 ± 0.5 MHz/yr following an initial drift of 5.7 ± 2.2 MHz/yr in the first eighteen months of intermittent operation. A second Zeeman laser drifted at a rate of -0.8 ± 1.0 MHz/yr over the last 2.5 yr; the frequency drift was -0.2 ± 0.6 MHz/yr over the last 3.3yr. Empirical temperature correctionsto laser frequency measurements produce a slight variance reduction in the data but no effective bias in the drift estimates.

Fisher, E, McMechan GA, Gorman MR, Cooper APR, Aiken CLV, Ander ME, Zumberge MA.  1989.  Determination of Bedrock Topography beneath the Greenland Ice-Sheet by 3-Dimensional Imaging of Radar Sounding Data. Journal of Geophysical Research-Solid Earth and Planets. 94:2874-2882.   10.1029/JB094iB03p02874   AbstractWebsite

In the summer of 1987, approximately 42,600 radar reflections were obtained along 124 radial lines, 5 km long, centered at Dye 3 in southern Greenland. Processing of these data using a three-dimensional kinematic migration algorithm produces a high-resolution image of the rock surface topography beneath the ice sheet. Estimated uncertainties in the position of the rock surface increase where sampling is less dense, such as toward the edges of the survey, but are less than 5 m over most of the survey area. The main structure revealed is a northwest-southeast trending valley in the bedrock that crosses the westward regional dip of the rock surface. Ice thickness increases from approximately 1800 m in the east to approximately 2100 m in the west.

Sasagawa, GS, Zumberge MA, Stevenson JM, Lautzenhiser T, Wirtz J, Ander ME.  1989.  The 1987 Southeastern Alaska Gravity Calibration Range - Absolute and Relative Gravity Measurements. Journal of Geophysical Research-Solid Earth and Planets. 94:7661-7666.   10.1029/JB094iB06p07661   AbstractWebsite

In June 1987 a gravimeter calibration range was set up in southeastern Alaska and the Yukon territory, as part of a geophysical determination of the Newtonian gravitational constant. Absolute gravity measurements were made between the range endpoints using the Institute of Geophysics and Planetary Physics absolute gravity meter. The calibration range spans 171.841±0.014 mGal, with a midpoint g value of 9.81746500 ms−2. Relative gravity meters, including a LaCoste and Romberg borehole gravity meter, were read along this range. A scale factor correction (SFC) for borehole meter 14 was found to be (8.1 ± 1.5) × 10−4, and for meter G-349 the correction was (−3.3 ± 1.7) × 10−4. The SFC for meter D-85 has an upper bound of ±1.0 × 10−4.

Ander, ME, Zumberge MA, Lautzenhiser T, Parker RL, Aiken CLV, Gorman MR, Nieto MM, Cooper APR, Ferguson JF, Fisher E, McMechan GA, Sasagawa G, Stevenson JM, Backus G, Chave AD, Greer J, Hammer P, Hansen BL, Hildebrand JA, Kelty JR, Sidles C, Wirtz J.  1989.  Test of Newtons Inverse-Square Law in the Greenland Ice Cap. Physical Review Letters. 62:985-988.   10.1103/PhysRevLett.62.985   AbstractWebsite

An Airy-type geophysical experiment was conducted in a 2-km-deep hole in the Greenland ice cap at depths between 213 and 1673 m to test for possible violations of Newton’s inverse-square law. An anomalous gravity gradient was observed. We cannot unambiguously attribute it to a breakdown of Newtonian gravity because we have shown that it might be due to unexpected geological features in the rock below the ice.