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].

Airborne gravity measurements over mountainous areas by using a LaCoste & Romberg air‐sea gravity meter

Geophysics ◽  
2002 ◽  
Vol 67 (3) ◽  
pp. 807-816 ◽  
Author(s):  
Jérôme Verdun ◽  
Roger Bayer ◽  
Emile E. Klingelé ◽  
Marc Cocard ◽  
Alain Geiger ◽  
...  
Keyword(s):  
Data Reduction ◽  
Classical Method ◽  
Gravity Data ◽  
Airborne Gravity ◽  
Mountainous Area ◽  
Vertical Acceleration ◽  
Mountainous Areas ◽  
Free Air ◽  
Gravity Measurements ◽  
Free Air Anomaly

This paper introduces a new approach to airborne gravity data reduction well‐suited for surveys flown at high altitude with respect to gravity sources (mountainous areas). Classical technique is reviewed and illustrated in taking advantage of airborne gravity measurements performed over the western French Alps by using a LaCoste & Romberg air‐sea gravity meter. The part of nongravitational vertical accelerations correlated with gravity meter measurements are investigated with the help of coherence spectra. Beam velocity has proved to be strikingly correlated with vertical acceleration of the aircraft. This finding is theoretically argued by solving the equation of the gravimetric system (gravity meter and stabilized platform). The transfer function of the system is derived, and a new formulation of airborne gravity data reduction, which takes care of the sensitive response of spring tension to observable gravity field wavelengths, is given. The resulting gravity signal exhibits a residual noise caused by electronic devices and short‐wavelength Eötvös effects. The use of dedicated exponential filters gives us a way to eliminate these high‐frequency effects. Examples of the resulting free‐air anomaly at 5100‐m altitude along one particular profile are given and compared with free‐air anomaly deduced from the classical method for processing airborne gravity data, and with upward‐continued ground gravity data. The well‐known trade‐off between accuracy and resolution is discussed in the context of a mountainous area.

The calculation of deflexions of the vertical from gravity anomalies

1950 ◽  
Vol 204 (1078) ◽  
pp. 374-395 ◽  
Keyword(s):  
Gravity Data ◽  
Gravity Anomalies ◽  
Sources Of Error ◽  
Free Air ◽  
Gravity Measurements ◽  
Standard Deviations ◽  
The Difference ◽  
Different Sources

The theory of the application of gravity measurements to geodetic calculations is discussed, and the errors involved in calculating deflexions of the vertical are estimated. If the gravity data are given as free air anomalies from Jeffreys’s (1948) formula, so thdt the second and third harmonics of gravity are assumed known, the orders of magnitude of the standard deviations of the different sources of error are the following: Single deflexion: neglect of gravity outside 20° 1" Difference of deflexions: neglect of gravity outside 5° 0"·5 Calculation of effects of gravity from 0º·05 to 5° 0"·1 Calculation of effects of gravity within 0º·05 between 0"·1 and 0"·5 Estimates of the deflexions are made for Greenwich, Herstmonceux, Southampton and Bayeux, and the difference between Greenwich and Southampton is compared with the astronomical and geodetic amplitudes.

scholarly journals Gravity observations on Santorini island (Greece): Historical and recent campaigns

2021 ◽  
Vol 51 (1) ◽  
pp. 1-24
Author(s):  
Melissinos PARASKEVAS ◽  
Demitris PARADISSIS ◽  
Konstantinos RAPTAKIS ◽  
Paraskevi NOMIKOU ◽  
Emilie HOOFT ◽  
...  
Keyword(s):  
Gravity Anomaly ◽  
Bronze Age ◽  
Gravity Data ◽  
Absolute Gravity ◽  
Bouguer Gravity ◽  
Near Surface ◽  
Bouguer Gravity Anomaly ◽  
Minoan Eruption ◽  
Marine Gravity ◽  
Gravity Measurements

Santorini is located in the central part of the Hellenic Volcanic Arc (South Aegean Sea) and is well known for the Late-Bronze-Age “Minoan” eruption that may have been responsible for the decline of the great Minoan civilization on the island of Crete. To use gravity to probe the internal structure of the volcano and to determine whether there are temporal variations in gravity due to near surface changes, we construct two gravity maps. Dionysos Satellite Observatory (DSO) of the National Technical University of Athens (NTUA) carried out terrestrial gravity measurements in December 2012 and in September 2014 at selected locations on Thera, Nea Kameni, Palea Kameni, Therasia, Aspronisi and Christiana islands. Absolute gravity values were calculated using raw gravity data at every station for all datasets. The results were compared with gravity measurements performed in July 1976 by DSO/NTUA and absolute gravity values derived from the Hellenic Military Geographical Service (HMGS) and other sources. Marine gravity data that were collected during the PROTEUS project in November and December 2015 fill between the land gravity datasets. An appropriate Digital Elevation Model (DEM) with topographic and bathymetric data was also produced. Finally, based on the two combined datasets (one for 2012–2014 and one for the 1970s), Free air and complete Bouguer gravity anomaly maps were produced following the appropriate data corrections and reductions. The pattern of complete Bouguer gravity anomaly maps was consistent with seismological results within the caldera. Finally from the comparison of the measurements made at the same place, we found that, within the caldera, the inner process of the volcano is ongoing both before, and after, the unrest period of 2011–2012.

Gravity data collection with a CG5 gravitymeter for densification of the gravity data around the AUT1 IHRF station

2021 ◽  
Author(s):  
Dimitrios A. Natsiopoulos ◽  
Elisavet G. Mamagiannou ◽  
Eleftherios A. Pitenis ◽  
Georgios S. Vergos ◽  
Ilias N. Tziavos ◽  
...  
Keyword(s):  
Gravity Data ◽  
Gravity Anomalies ◽  
Absolute Gravity ◽  
Dual Frequency ◽  
Height Determination ◽  
Free Air ◽  
Gravity Measurements ◽  
Gravity Database ◽  
Free Air Gravity ◽  
Geological Complexity

<p>Within the GeoGravGOCE project, funded by the Hellenic Foundation for Research Innovation, a main goal has been the densification of the available land gravity database around the eastern part of the city of Thessaloniki, Greece, where the core International Height Reference Frame (IHRF) station AUT1 is located in order to improve regional geoid and potential determination. Hence it was deemed necessary to densify the available gravity data within radiuses of 10 km, 20 km, 50 km and 100 km from the AUT1 core IHRF site. In that frame, and given the geological complexity of the region surrounding Thessaloniki and the significant variations of the terrain, gravity campaigns were appropriately designed and gravity measurements were carried out in order to densify the database and cover as much as possible traverses of varying altitude. The measurements have been carried out with the CG5 gravity meter of the GravLab group and dual-frequency GNSS receivers in RTK mode for orthometric height determination. In this  study we provide details of the gravity campaigns, the measurement principle and the finally derived gravity and free-air gravity anomalies. The mean measurement accuracy achieved was at the ~20 μGal level for the gravity measurements and ~3 cm for the orthometric heights. In all cases the final derived gravity value was based on the absolute point established by the GravLab team at the AUTH seismological station premises with the A10 (#027) absolute gravity meter.</p>

Near-surface geophysical investigation of the 2010 Haiti earthquake epicentral area: Léogâne, Haiti

2016 ◽  
Vol 4 (1) ◽  
pp. T49-T61 ◽  
Author(s):  
Eray Kocel ◽  
Robert R. Stewart ◽  
Paul Mann ◽  
Li Chang
Keyword(s):  
Gravity Data ◽  
Ground Surface ◽  
Earthquake Hazard ◽  
Sediment Type ◽  
Haiti Earthquake ◽  
S Wave ◽  
Geophysical Investigation ◽  
Epicentral Area ◽  
Near Surface ◽  
Fan Delta

The [Formula: see text] Léogâne fan delta in southwestern Haiti borders the epicentral region of the devastating magnitude 7.0 Haiti earthquake of 12 January 2010. The flat plain of the Léogâne area experienced some of the worst shaking, destruction of buildings, and loss of life caused by the Haiti earthquake. This intense shaking was attributed by previous workers to either activation of a blind (no surface expression) thrust fault some 4 km beneath the Léogâne fan delta or to strike-slip motion along a shallow, ground-breaking fault that ruptured the uppermost part of the fan delta. Our research team from the University of Houston and the Haiti Bureau of Mines and Energy collected shallow seismic and gravity data in the fan delta where previous studies of earthquake aftershocks, coastal uplift of coral reefs, and radar interferometry all indicated a maximum amount of coseismic uplift. Our objective was to acquire geophysical information on the subsurface stratigraphy, structure, and material properties of the fan. S-wave seismic studies revealed an average velocity of [Formula: see text] for the first 30 m. These velocity values suggest that the near-surface sediments at Léogâne are a seismically hazardous class D sediment type (National Earthquake Hazard Reduction Program). Interpretation of our various seismic data sets has indicated prolonged sedimentary environments of fluvial channeling and channel migration to a depth of approximately 350 m as expected in this fan delta setting. There was no clear evidence on our seismic reflection lines for substantial faulting in the seismically slow, shallow fan delta sediments. Integrated geophysical data analyses indicated south-dipping seismically slow layers on the southern end of the Léogâne fan with a less well defined northward dip with the broad, anticlinal axis aligned with the area of maximum coseismic uplift at the coast. The sudden, coseismic upheaval of the ground surface above the proposed blind thrust combined with extreme shaking of the seismically weak sediments contributed to the destructiveness of the earthquake on the Léogâne fan delta.

scholarly journals Forty-three years of absolute gravity observations of the Fennoscandian postglacial rebound in Finland

2021 ◽  
Vol 95 (2) ◽  
Author(s):  
Mirjam Bilker-Koivula ◽  
Jaakko Mäkinen ◽  
Hannu Ruotsalainen ◽  
Jyri Näränen ◽  
Timo Saari
Keyword(s):  
Gravity Data ◽  
Gravity Change ◽  
Global Navigation Satellite Systems ◽  
Absolute Gravity ◽  
Data Sets ◽  
Postglacial Rebound ◽  
Land Uplift ◽  
Satellite Systems ◽  
Gravity Measurements ◽  
Global Navigation Satellite

AbstractPostglacial rebound in Fennoscandia causes striking trends in gravity measurements of the area. We present time series of absolute gravity data collected between 1976 and 2019 on 12 stations in Finland with different types of instruments. First, we determine the trends at each station and analyse the effect of the instrument types. We estimate, for example, an offset of 6.8 μgal for the JILAg-5 instrument with respect to the FG5-type instruments. Applying the offsets in the trend analysis strengthens the trends being in good agreement with the NKG2016LU_gdot model of gravity change. Trends of seven stations were found robust and were used to analyse the stabilization of the trends in time and to determine the relationship between gravity change rates and land uplift rates as measured with global navigation satellite systems (GNSS) as well as from the NKG2016LU_abs land uplift model. Trends calculated from combined and offset-corrected measurements of JILAg-5- and FG5-type instruments stabilized in 15 to 20 years and at some stations even faster. The trends of FG5-type instrument data alone stabilized generally within 10 years. The ratio between gravity change rates and vertical rates from different data sets yields values between − 0.206 ± 0.017 and − 0.227 ± 0.024 µGal/mm and axis intercept values between 0.248 ± 0.089 and 0.335 ± 0.136 µGal/yr. These values are larger than previous estimates for Fennoscandia.

Breaks in lithology: Interpretation problems when handling 2D structures with a 1D approximation

Geophysics ◽  
2010 ◽  
Vol 75 (4) ◽  
pp. WA179-WA188 ◽  
Author(s):  
Alan Yusen Ley-Cooper ◽  
James Macnae ◽  
Andrea Viezzoli
Keyword(s):  
Radial Distance ◽  
The Other ◽  
Conductive Layer ◽  
Near Surface ◽  
Depth Images ◽  
Modeling Methodology ◽  
Wide System ◽  
Receiver System ◽  
Airborne Electromagnetic ◽  
Detection Capabilities

Most airborne electromagnetic (AEM) data are processed using successive 1D approximations to produce stitched conductivity-depth sections. Because the current induced in the near surface by an AEM system preferentially circulates at some radial distance from a horizontal loop transmitter (sometimes called the footprint), the section plotted directly below a concentric transmitter-receiver system actually arises from currents induced in the vicinity rather than directly underneath. Detection of paleochannels as conduits for groundwater flow is a common geophysical exploration goal, where locally 2D approximations may be valid for an extinct riverbed or filled valley. Separate from effects of salinity, these paleochannels may be conductive if clay filled or resistive if sand filled and incised into a clay host. Because of the wide system footprint, using stitched 1D approximations or inversions may lead to misleading conductivity-depth images or sections. Near abrupt edges of an extensive conductive layer, the lateral falloff in AEM amplitudes tends to produce a drooping tail in a conductivity section, sometimes coupled with alocal peak where the AEM system is maximally coupled to currents constrained to flow near the conductor edge. Once the width of a conductive ribbon model is less than the system footprint, small amplitudes result, and the source is imaged too deeply in the stitched 1D section. On the other hand, a narrow resistive gap in a conductive layer is incorrectly imaged as a drooping region within the layered conductor; below, the image falsely contains a blocklike poor conductor extending to depth. Additionally, edge-effect responses often are imaged as deep conductors with an inverted horseshoe shape. Incorporating lateral constraints in 1D AEM inversion (LCI) software, designed to improve resolution of continuous layers, more accurately recovers the depth to extensive conductors. The LCI, however, as with any AEM modeling methodology based on 1D forward responses, has limitations in detecting and imaging in the presence of strong 3D lateral discontinuities of dimensions smaller than the annulus of resolution. The isotropic, horizontally slowly varying layered-earth assumption devalues and limits AEM’s 3D detection capabilities. The need for smart, fast algorithms that account for 3D varying electrical properties remains.

Computer Application in Geosciences: Imaging of the Subsurface by Structural Coupled Inversion of Potential Field Data

2014 ◽  
Vol 644-650 ◽  
pp. 2670-2673
Author(s):  
Jun Wang ◽  
Xiao Hong Meng ◽  
Fang Li ◽  
Jun Jie Zhou
Keyword(s):  
Potential Field ◽  
Field Data ◽  
Gravity Data ◽  
Computer Application ◽  
Magnetic Data ◽  
Imaging Method ◽  
Inversion Algorithm ◽  
Constant Property ◽  
Near Surface ◽  
Potential Field Data

With the continuing growth in influence of near surface geophysics, the research of the subsurface structure is of great significance. Geophysical imaging is one of the efficient computer tools that can be applied. This paper utilize the inversion of potential field data to do the subsurface imaging. Here, gravity data and magnetic data are inverted together with structural coupled inversion algorithm. The subspace (model space) is divided into a set of rectangular cells by an orthogonal 2D mesh and assume a constant property (density and magnetic susceptibility) value within each cell. The inversion matrix equation is solved as an unconstrained optimization problem with conjugate gradient method (CG). This imaging method is applied to synthetic data for typical models of gravity and magnetic anomalies and is tested on field data.

Estimating saturation and density changes caused by CO2 injection at Sleipner — Using time-lapse seismic amplitude-variation-with-offset and time-lapse gravity

2017 ◽  
Vol 5 (2) ◽  
pp. T243-T257 ◽  
Author(s):  
Martin Landrø ◽  
Mark Zumberge
Keyword(s):  
Gravity Data ◽  
Time Lapse ◽  
Co2 Injection ◽  
Injection Point ◽  
Amplitude Variation With Offset ◽  
Seismic Method ◽  
Seismic Amplitude ◽  
Gravity Measurements ◽  
Measured Time ◽  
Best Fit

We have developed a calibrated, simple time-lapse seismic method for estimating saturation changes from the [Formula: see text]-storage project at Sleipner offshore Norway. This seismic method works well to map changes when [Formula: see text] is migrating laterally away from the injection point. However, it is challenging to detect changes occurring below [Formula: see text] layers that have already been charged by some [Formula: see text]. Not only is this partly caused by the seismic shadow effects, but also by the fact that the velocity sensitivity for [Formula: see text] change in saturation from 0.3 to 1.0 is significantly less than saturation changes from zero to 0.3. To circumvent the seismic shadow zone problem, we combine the time-lapse seismic method with time-lapse gravity measurements. This is done by a simple forward modeling of gravity changes based on the seismically derived saturation changes, letting these saturation changes be scaled by an arbitrary constant and then by minimizing the least-squares error to obtain the best fit between the scaled saturation changes and the measured time-lapse gravity data. In this way, we are able to exploit the complementary properties of time-lapse seismic and gravity data.

scholarly journals Research Article. A new gravity laboratory in Ny-Ålesund, Svalbard

2017 ◽  
Vol 7 (1) ◽  
Author(s):  
K. Breili ◽  
R. Hougen ◽  
D. I. Lysaker ◽  
O. C. D. Omang ◽  
B. Tangen
Keyword(s):  
Noise Level ◽  
Gravity Data ◽  
Ocean Tide ◽  
Gravity Gradients ◽  
Svalbard Archipelago ◽  
Ocean Tide Loading ◽  
Lower Elevation ◽  
Gravity Measurements ◽  
Space Geodetic Techniques ◽  
Under Construction

AbstractThe Norwegian Mapping Authority (NMA) has recently established a new gravity laboratory in Ny-Ålesund at Svalbard, Norway. The laboratory consists of three independent pillars and is part of the geodetic core station that is presently under construction at Brandal, approximately 1.5 km north of NMA’s old station. In anticipation of future use of the new gravity laboratory, we present benchmark gravity values, gravity gradients, and final coordinates of all new pillars. Test measurements indicate a higher noise level at Brandal compared to the old station. The increased noise level is attributed to higher sensitivity to wind.We have also investigated possible consequences of moving to Brandal when it comes to the gravitational signal of present-day ice mass changes and ocean tide loading. Plausible models representing ice mass changes at the Svalbard archipelago indicate that the gravitational signal at Brandal may differ from that at the old site with a size detectable with modern gravimeters. Users of gravity data from Ny-Ålesund should, therefore, be cautious if future observations from the new observatory are used to extend the existing gravity record. Due to its lower elevation, Brandal is significantly less sensitive to gravitational ocean tide loading. In the future, Brandal will be the prime site for gravimetry in Ny-Ålesund. This ensures gravity measurements collocated with space geodetic techniques like VLBI, SLR, and GNSS.

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