CORRELATING SEA‐SURFACE AND AERIAL GRAVITY MEASUREMENTS

Geophysics ◽  
1966 ◽  
Vol 31 (1) ◽  
pp. 264-266 ◽  
Author(s):  
Stephen Thyssen‐Bornemisza
Keyword(s):  
Ground Surface ◽  
Sea Surface ◽  
Surface Gravity ◽  
Sea Floor ◽  
Gravity Measurements ◽  
Vertical Gradients

When sea‐surface gravity observations were supplemented by corresponding values from the airborne meter, average vertical gradients of gravity could be computed. In a borehole these gradients are observable by moving the borehole gravity meter up and down to another level (Thyssen‐Bornemisza, 1963, 1964, 1965a), but measurements taken on two horizontal profiles separated by the constant vertical interval h could furnish only relative gradient values or variations in the profile direction. Of course, gravity profiles on the ground surface or the sea floor can be likewise supplemented by aerial observations.

scholarly journals Porosity of very young oceanic crust from sea floor gravity measurements

1998 ◽  
Vol 25 (11) ◽  
pp. 1959-1962 ◽  
Author(s):  
Matthew J. Pruis ◽  
H. Paul Johnson
Keyword(s):  
Oceanic Crust ◽  
Sea Floor ◽  
Gravity Measurements

Introduction

1967 ◽  
Vol 252 (777) ◽  
pp. 169-171 ◽  
Keyword(s):  
Southern Ocean ◽  
Oceanic Islands ◽  
Sea Surface ◽  
Large Measure ◽  
Sea Floor ◽  
Nature Conservancy ◽  
Antarctic Ecosystem ◽  
History Of ◽  
Antarctic Continent ◽  
The Antarctic

Mr President, ladies and gentlemen: it is my pleasure, in opening this two-day conference on the terrestrial Antarctic ecosystem, to welcome you as contributors of papers and, as I shall hope, participants in the discussions with which we will conclude each of the four sessions of our meeting. This symposium was first suggested and has, in very large measure, been organized by Dr Martin Holdgate whom we regretfully, but nevertheless most warmly congratulate on his recent translation from the post of Senior Biologist of the British Antarctic Survey to that of Deputy Director of the Nature Conservancy. The furtherance of Antarctic biology in recent years owes much to Dr Holdgate’s energetic and imaginative direction, and I am glad to have this opportunity of acknowledging our indebtedness to him for arranging this discussion. The Antarctic continent, half as large again as Australia, and the surrounding Southern Ocean, in area about one-fifth of the world’s sea surface were, by their very remoteness from the maritime nations of the northern hemisphere, late of exploration. But, while it is little more than 75 years since man first set foot on the Antarctic continent, the more accessible waters of the Southern Ocean have an appreciably longer history of exploration, dating from the pioneering voyages of Captain Cook some 200 years ago. Biological investigations in Antarctica were, therefore, for long concerned almost entirely with observations and studies of animals living in the open ocean or on the sea floor rather than with the terrestrial and freshwater floras and faunas of the continental margin and oceanic islands which, either because of difficulties of access or limitations of time imposed by ships’ programmes, were rarely surveyed in detail.

Effects of terrain on borehole gravity data

Geophysics ◽  
1980 ◽  
Vol 45 (2) ◽  
pp. 234-243 ◽  
Author(s):  
J. R Hearst ◽  
J. W. Schmoker ◽  
R. C. Carlson
Keyword(s):  
Gravity Data ◽  
Radial Distance ◽  
Ground Surface ◽  
Absolute Maximum ◽  
Near Surface ◽  
Free Air ◽  
Gravity Measurements ◽  
Terrain Elevation ◽  
Lower Magnitude ◽  
Terrain Features

The effect of terrain on gravity measurements in a borehole and on formation density derived from borehole gravity data is studied as a function of depth in the well, terrain elevation, terrain inclination, and radial distance to the terrain feature. The vertical attraction of gravity [Formula: see text] in a borehole resulting from a terrain element is small at the surface and reaches an absolute maximum at a depth of about one and one‐half times the radial distance to the terrain element, then decreases at greater depths. The effect of terrain on calculated formation density is proportional to the vertical derivative of [Formula: see text] and is maximum at the surface, passes through zero where |[Formula: see text]| is greatest, and reaches a second extremum of opposite sign to the first and of much lower magnitude. Accuracy criteria for borehole‐gravity terrain corrections show that elevation accuracy requirements are most stringent for a combination of nearby terrain features and near‐surface gravity stations. Sensitivity to terrain inclination is also greatest for this combination. The measurement of the free‐air gradient of gravity, commonly made’slightly above the ground surface, is extremely sensitive to topographic irregularities within about 300m of the measurement point. The effect of terrain features 21.9 to 166.7 km from the well [Hammer’s (1939) zone M through Hayford‐Bowie’s (1912) zone O] on calculated formation density is nearly constant with depth. At these distances, the terrain correction will be equivalent to a dc shift of about [Formula: see text] of average elevation above or below the correction datum. The effect of topography beyond 166.7 km is not likely to exceed [Formula: see text].

THE ANOMALOUS FREE‐AIR VERTICAL GRADIENT IN BOREHOLE EXPLORATION

Geophysics ◽  
1965 ◽  
Vol 30 (3) ◽  
pp. 441-443 ◽  
Author(s):  
Stephen Thyssen‐Bornemisza
Keyword(s):  
Observation Interval ◽  
Vertical Gradient ◽  
Gravitational Effects ◽  
Free Air ◽  
Gravity Measurements ◽  
Source Of Error ◽  
Vertical Gradients ◽  
Interpretation Of Results

Hammer (1950) and Smith (1950), in discussions about gravity measurements in vertical shafts and boreholes, have pointed out that gravitational effects originating in zones or layers above or below the gravity‐meter observation interval may easily produce anomalous free‐air vertical gradients. These anomalous gradients cannot well be corrected without the proper density information and, therefore, would represent a possible source of error in the interpretation of results.

Stability of Submarine Foundation Pits Under Wave Loads

2012 ◽  
Author(s):  
Julian Bubel ◽  
Jürgen Grabe
Keyword(s):  
Ground Surface ◽  
Friction Angle ◽  
Shallow Foundation ◽  
Cohesionless Soil ◽  
Wave Loads ◽  
In Situ Tests ◽  
Sea Floor ◽  
The North ◽  
The Stability ◽  
Wave Induced

Shallow foundation structures offer ecological benefits compared to pile foundations as less noise is emitted at sea floor level during construction process. On the other hand, shallow offshore foundations can rarely be placed on top of the sea floor. Weak soils usually need to be excavated to place the foundation structure on more stable ground and thus, anthropogenic submarine pits result. Steep but stable slopes of the pit meet both economic and ecologic aims as they minimise material movement and sediment disturbance. According to Terzaghi [1] the angle β between slope and the horizontal of the ground surface of cohesionless soil is at most equal to the critical state friction angle φc. However, it can be observed that natural submarine slopes of sandy soils are always much more shallow. Artificial (temporary) slopes do not appear and behave as natural submarine slopes, since the latter are already shaped by perpetual loads of waves, tide and mass movements. Physical simulations of different scales were presented at the OMAE 2011 [2] to analyse the stability of artificial submarine slopes of sandy soil in the North Sea. The laboratory tests focused on gravitational forces and impacts from the excavation processes. This paper presents additional numerical simulations of wave-induced bottom pressure on the suggested submarine foundation pits. Furthermore, in-situ tests will be performed in 2012 and 2013. Both dredging process and resulted foundation pits will be considerably surveyed.

scholarly journals Estimation of Bouguer correction density based on underground and surface gravity measurements and precise modelling of topographic effects – two case studies from Slovakia

2018 ◽  
Vol 48 (4) ◽  
pp. 319-336 ◽  
Author(s):  
Pavol Zahorec ◽  
Juraj Papčo
Keyword(s):  
Laser Scanning ◽  
Airborne Lidar ◽  
Surface Gravity ◽  
Karst Area ◽  
Topographic Effects ◽  
Railway Tunnel ◽  
Detailed Surface ◽  
Active Coal ◽  
Gravity Measurements ◽  
Gravimetric Modelling

Abstract We present a simple and straightforward method for estimating the mean density of topographic masses based on underground gravity measurements along with topography modelling. Two examples under different conditions are given, the first coming from a railway tunnel passing through a Mesozoic karst area and the second from an active coal mine situated in a Neogene sedimentary basin. Relative gravity measurements were processed and corrected by topographic effect modelling based on high-precision airborne LiDAR-derived elevation models. In addition, detailed mining tunnel gravimetric modelling based on terrestrial laser scanning data is presented. Resulted mean (bulk) densities are compared with those obtained from detailed surface gravity measurements as well as with available rock-samples density analysis.

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