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Topic: Bouguer anomalies

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	Gravity anomaly - Wikipedia, the free encyclopedia
		A gravity anomaly is the difference between the observed gravity and its theoretical value, which is calculated at the surface of an global spheroid (ellipsoid of Hayford or WGS84) by rather simple formulas (2 functions of latitude).
		Bouguer anomalies are ideal for geophysics because they show the effects of different rock densities of the subsurface.
		The Bouguer anomalies usually are negative in the mountains because of isostasy: the rock density of their roots is lower, compared with the surrounding earth's mantle.
	en.wikipedia.org /wiki/Gravity_anomaly (428 words)

	Recent abstracts (Site not responding. Last check: 2007-10-13)
		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.
		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.
	deeptow.whoi.edu /abstracts/jgr99.html (414 words)

	Gravity/Magnetic Glossary (Site not responding. Last check: 2007-10-13)
		The gravity anomalies observed in the Bouguer field are caused by lateral density contrasts within the sedimentary section, crust and sub-crust of the earth.
		Anomalies caused by mass inhomogeneities between station elevation and datum and which were measured at the original station elevations remain in the data unless special corrections are made.
		The Bouguer correction requires an estimation of bulk density to calculate and eliminate the gravitational effect of the subsurface mass between point of gravity measurement and a datum.
	www.geop.itu.edu.tr /~onur/seis/dic/gravmag.html (2611 words)

	Gravity and Magnetic data: ISOSTATIC RESIDUAL GRAVITY MAP (Site not responding. Last check: 2007-10-13)
		Most long-wavelength anomalies (longer than approx 250 km) on a Bouguer gravity map are caused by deep-seated density distributions that buoyantly support the topography in a manner consistent with the principle of isostasy (Simpson and others, 1986).
		Bouguer gravity anomalies related to isostasy are prevalent in California because of the extreme topographic relief in the State (Oliver, 1980; Jachens and Griscom, 1985), and they are particularly strong near the coast, where an eastward to northeastward decrease in gravity reflects the transition from thin oceanic crust to thicker continental crust.
		In such areas as California, the Bouguer gravity anomalies associated with isostatic support of topography are so strong that they tend to distort or even mask the lower-amplitude anomalies caused by density distributions in the middle to upper crust, those anomalies most easily correlatable with rocks exposed at the surface (Jachens and Griscom, 1985).
	www.johnmartin.com /earthquakes/eqsafs/safs_905.htm (722 words)

	Physical geodesy
		In this formula, stands for gravity anomalies, differences between true and normal (reference) gravity, and S is the Stokes function, a kernel function derived by Stokes in closed analytical form.
		These anomalies are called free-air anomalies, and are the ones to be used in the above Stokes equation.
		In geophysics, these anomalies are often further reduced by removing from them the attraction of the topography, which for a flat, horizontal plate (Bouguer plate) of thickness H is given by
	www.brainyencyclopedia.com /encyclopedia/p/ph/physical_geodesy.html (1177 words)

	USGS Fact Sheet 078-95 -- Geophysical Data Grids for the Conterminous United States
		The magnetic anomaly and gamma-ray data sets are gridded at a 2- kilometer grid interval; the gravity anomaly and topographic-bathymetric data sets are gridded at a 4-kilometer grid interval.
		Magnetic anomalies are produced by variations in the distribution of iron minerals, usually magnetite, in the rocks of the Earth's crust.
		Topographic and bathymetric data are used in the reduction and interpretation of magnetic anomaly and gravity anomaly data and in the construction of geologic and topographic maps.
	pubs.usgs.gov /fs/fs-0078-95/FS078-95.html (634 words)

	SCO Web: Gravity Anomaly Maps (Site not responding. Last check: 2007-10-13)
		P. Bouguer first observed this anomaly in 1735, noting that the deviation of a plumb-line in gravitational observations was less than a calculated value.
		Bouguer anomalies are calculated for a sea-level datum using density values of 2.67 grams per centimeter cubed for rock density and 1.00 grams per centimeter cubed for water.
		This is one of the most current gravity anomaly maps produced for Wisconsin and includes several figure insets that display isometric data, gravity trend surface, residual gravity, regional tectonic features, gravity and magnetic profiles, and gravity and geologic profiles.
	www.geography.wisc.edu /sco/maps/gravityanomaly.php (726 words)

	Gravity anomalies in the Channel Islands -- Day 96 (2): 89 -- Geological Magazine
		On Alderney the anomalies are clearly dependent on the nature of the outcropping rock-types, and permit approximate values for the thickness of two outcropping rock masses to be obtained.
		On Guernsey the anomalies are not closely related to the surface geology, and suggest that the northeastern coastal area is underlain by a body of dense rock, possibly of ultrabasic composition.
		The anomalies on Jersey indicate that the sedimentary rocks of western Jersey are underlain at no great depth by rock of density comparable to that of granite.
	geolmag.geoscienceworld.org /cgi/content/abstract/96/2/89 (176 words)

	734/834: Class 27, Gravity anomalies
		A gravity anomaly is the difference between measured "g" and "g" of some standard earth model (often mislabeled "theoretical gravity").
		When we use this anomaly we must be sure to state what our standard density is. For many years it’s been common to use 2670 kg/m^3, an early estimate for the mean density of continental crust.
		Bouguer anomalies are the common ones used in applied geophysics.
	earth.unh.edu /esci834/docs/class_27.html (761 words)

	The San Andreas Fault System, California - Chapter 9 (Site not responding. Last check: 2007-10-13)
		Anomalies in the Earth's gravity and magnetic fields - for example, local deviations of the measured fields from those predicted on the basis of simplified Earth models - primarily reflect lateral variations in density and magnetization that generally are not included in such simple models.
		These anomalies can be interpreted qualitatively to infer the general spatial distribution of rock types in the subsurface, and quantitatively, through the use of efficient computer-based modeling techniques (Saltus and Blakely, 1983; Chuchel, 1985; Blakely and Simpson, 1986), to determine the geometries and specific locations of concealed rock bodies.
		The magnetic and gravity anomalies northwest of Point Arena and west of the San Andreas fault are such conspicuous features and so obviously truncated by the San Andreas fault that we would expect to find their counterparts somewhere on the opposite side of the fault.
	education.usgs.gov /california/pp1515/chapter9.html (8507 words)

	CHALLENGER DIVISION FOR SEAFLOOR PROCESSES (Site not responding. Last check: 2007-10-13)
		The anomalies were corrected for a density difference between basin sediments and water of 1400 kg m-3 and filtered for wavelength between 10 and 190 km.
		The gravity anomalies show two regional strike directions, which are presumably associated with Tertiary extension and older weakness zones in the basement.
		Within the investigated wavelength residual Bouguer gravity anomaly maps derived from satellite altimetry and ship-track bathymetry are as suitable for investigating regional trends as maps derived from ship-track gravity measurements.
	www.soc.soton.ac.uk /CHD/staff_stu/cbe/bouguer.htm (355 words)

	Pan-American Center for Earth and Environmental Studies
		A good example of the utility of gravity anomalies is their use to delineate the geometry and lateral extent of basins that contain ground water resources.
		Bouguer Correction = BC = 2pgrh, where g is the International gravitational constant, r is the density, and h = (elevation - datum elevation).
		Over most continental areas, Bouguer anomaly values are large negative numbers, thus the usual practice of padding the edges of a grid with zeros prior to applying a Fourier transform and filtering will create large edge effects.
	paces.geo.utep.edu /gravity/getstart.htm (6798 words)

	[No title] (Site not responding. Last check: 2007-10-13)
		The Bouguer anomalies are derived from the 50th-degree and -order spherical harmonic gravity model, GMM-1, and the 8th-degree and -order topography model, M.TOPOGRAPHY.8X8.SPH.HARM.txt.done, referred to a geopotential surface, as described in Smith and Zuber, 'The shape of Mars and the topographic signature of the hemispheric dichotomy', Science, v.
		The Bouguer correction to the gravitational potential is calculated, for spherical harmonic degree l=1,2,...,8 as: CB = 3(rho/rhobar) / (2l + 1) The topographic reduction density rho was 2900 kg/m3, and mean planetary density rhobar was 3340 kg/m3.
		The resulting anomalies were then inverted for the topography of a moho (Mohorovicic Discontinuity) at a depth of 50 km, assuming a laterally constant density structure for the crust and mantle, and a density contrast between crust and mantle of 600 kg/m3.
	wufs.wustl.edu /geodata/viking_mariner9/readme.txt (1284 words)

	Physical geodesy (Site not responding. Last check: 2007-10-13)
		Welcome to the role of international co-operation in physical geodesy.) The geoid, or mathematical mean sea surface, is defined not only on the seas, but also under land; it is the equilibrium water surface that would result, would sea water be allowed to move freely (e.g., through tunnels) under the land.
		In geophysics, these anomalies are often further reduced by removing from them the attraction of the topography, which for a flat, horizontal plate (Bouguer plate) of thickness H is given by :
		In case the terrain is not a flat plate (the usual case!) we use for H the local terrain height value but apply a further correction called the terrain correction (TC).
	www.serebella.com /encyclopedia/article-Physical_geodesy.html (1437 words)

	Current research - Northwest Oregon - Gravity study through the Tualatin Mountains
		The first-order feature of the terrain-corrected, Bouguer anomaly profile is a gradual increase from -28.5 mGal at the west portal to a maximum of -15.4 mGal at about 3.8 km from the west portal (Figure 5).
		Superimposed on this broad gradient are a number of step-like anomalies (vertical arrows on Figures 4 and 6) with magnitudes of 1 to 3 mGal and characteristic widths of 100 to 300 m.
		Another gravity anomaly occurs 1200 m from the west portal at the intersection of the tunnel and the surface trace of the Oatfield fault.
	wrgis.wr.usgs.gov /wgmt/pacnw/tunnel (1150 words)

	THE NATIONAL GEODETIC SURVEY GRAVITY DATA BASE
		When free air or Bouguer anomalies were not contained in the input data records, they were calculated according to the formula given in the definitions.
		When the Bouguer anomaly was not coded, it was calculated except when the type code was X. Since the coded Bouguer anomaly included the terrain correction, the latter was subtracted from the former.
		Bouguer anomaly: The free air gravity anomaly corrected for the Bouguer slab of standard density of 2.67 gr/cc.
	www.ngdc.noaa.gov /seg/cdroms/grav99v1/document/html/ngs.htm (2490 words)

	GDR for ESS : Isostatic Residual Gravity Anomalies 2 km grid of Canada (Site not responding. Last check: 2007-10-13)
		This grid of the isostatic residual gravity anomalies shows variations in the gravity field caused by lateral variations in the density of the Earth's crust and upper mantle that reflect variations in composition and thickness.
		Shorter wavelength anomalies, representing nearer surface density contrasts, are enhanced by removing the calculated effects of the isostatic roots that support topographic loads.
		The effect of these mass deficiencies are not accounted for in the Bouguer reduction and there exists an inverse correlation between broad Bouguer anomaly lows and positive topography.
	gdcinfo.agg.nrcan.gc.ca /gdr/reports/can_grav_2k_iso_e.html (1635 words)

	[No title] (Site not responding. Last check: 2007-10-13)
		Flexural modeling of bending of the southern and southeastern borders of the Amazon lithospheric plate under the western border of the Goias Massif and western Parnaiba basin was constrained by 1070 gravity stations between 5°-14°S and 46°-52.5°W. Topography and aeromagnetic data were also used to estimate the loads of the Araguaia thrust belt.
		Bouguer anomalies over the Amazon craton, to the west of the thrust belt, are higher than _20 mGal.
		= 80 km was estimated by comparing the observed Bouguer anomalies with the gravity anomalies caused by bending of the crust-mantle interface of a broken elastic plate model.
	www.elsevier.com /cdweb/journals/08959811/articles/12/1/S089598119900007.abstract.en (216 words)

	[No title] (Site not responding. Last check: 2007-10-13)
		1.Gravity anomalies in Bouguer reduction, mGal*10 2.Compiled on the basis of Gravimetric Map of the Soviet Union.
		Bouguer reduction, delta sigma = 2.67 g/cm**3, a scale of 1:200 000, edited by N.V.Epshtein, 1975.
		As a rule, in mountaninous regions Bouguer anomalies are computed at mean constant density of 2.67 g/cm**3, this helps to avoid using a particular hypothesis about density distribution in the intermediate level.
	www.ngdc.noaa.gov /seg/cdroms/Spitak_EQ/pct/geophysi/g0001.txt (140 words)

	Structural features
		Bouguer corrected gravity data (correction density: 2300 kg/m3; reference level: 820 m a.
		The gravity field is dominated by a strong regional gradient superimposed by an overall negative anomaly within the Azuara structure.
		The local gravity anomalies may be explained by Mesozoic and/or Palaeozoic rocks, which form a structural inner ring covered by low-density Tertiary sediments.
	www.impact-structures.com /spain/impact/geophysic.html (676 words)

	Slave Province Minerals & Geoscience - Bouguer Gravity & Rock Density (Site not responding. Last check: 2007-10-13)
		A Bouguer gravity anomaly map (free-air anomalies offshore) of the Slave Province is shaded by simulated light inclined at 500, emanating from a source at 1100 declination.
		The gravity data set used to generate the map Bouguer gravity anomalies on land, and Free-air gravity anomalies offshore are available for download from the Natural Resources Canada Geoscience Data Repsitory.
		The Bouguer gravity field is expressed in the unit of milligal, abbreviated to mGal.
	nrd.nrcan.gc.ca /slavecomp/gravity_e.php (798 words)

	RGS : Saskatchewan's Rationalized Digitized Gravity Archives
		In order to produce a consistent, meaningful data set from the Bouguer anomaly maps, it was necessary to calculate the observed gravity for each station and level it to the observed gravity of the Canadian Gravity Anomaly Data Base (CGDB).
		The Bouguer gravity anomaly maps and corresponding elevation maps were digitized by SEM and NIMA between 1992 and 1995.
		Figure 5 is the first vertical derivative of the Bouguer anomalies with an upward continuation of 800 m.
	gdcinfo.agg.nrcan.gc.ca /gdc/sask_grav/index_e.html (2514 words)

	Geoid determination after the first satellite gravity missions
		Unfortunately height anomalies are often considered as some kind of absolute quasi-geoid values, but they are only valid as differences due to the levelling datum zero point problem.
		Anomalies, disturbances, GPS/levelling and gravity gradients (observed in aircrafts or in GOCE) may be mixed in a rigorous manner in approximation methods.
		It is, especially in the future, important to be able to mix gravity anomalies (from ground data at points with known orthometric heights) and gravity disturbances (at points with known ellipsoidal height, contingently observed in an aircraft).
	www.gfy.ku.dk /~cct/torge02.htm (5523 words)

	Calculating Bouguer Anomalies from Observed Gravity Values (Site not responding. Last check: 2007-10-13)
		Two types of Bouguer anomalies are recognized: (1) simple Bouguer anomalies, in which the various factors have been corrected, with the exception of terrain effects, and (2) complete Bouguer anomalies, for which terrain effects have also been removed.
		Although use of the newer formula versus the older one does produce a discrepancy of several milligals, this discrepancy is systematic for localized surveys, and the changes in Bouguer anomalies (reflecting lateral density variations) are consistent whether one uses the old formula or the new one.
		The anomaly caused by a serpentinite is one due to the lateral change in density from graywacke to serpentinite, and it is thus most appropriate to use the 2.60 value as the reduction density.
	seis.natsci.csulb.edu /grannell/secondgravity.html (1285 words)

	Physical geodesy Article, Physicalgeodesy Information (Site not responding. Last check: 2007-10-13)
		In this formula, Δg stands for gravity anomalies, differences between true andnormal (reference) gravity, and S is the Stokes function, a kernel function derived by Stokes in closed analyticalform.
		These anomalies are called free-air anomalies, and are the onesto be used in the above Stokes equation.
		In geophysics, these anomalies are often further reduced by removing fromthem the attraction of the topography, which for a flat, horizontal plate (Bouguer plate) of thickness H isgiven by
	www.anoca.org /gravity/potential/physical_geodesy.html (1132 words)

	[No title]
		Adaptive filtering of gridded Bouguer gravity anomaly data using gridded topography data as a reference signal removes portions of the Bouguer values that are correlated with topography thus revealing trends that may be associated with deeper density anomalies.
		The unfiltered Bouguer gravity anomaly (Figure 2) reveals that the projection of the TT-NRMT is associated with a relative gravity high in the broad low associated with the Cordillera.
		Overall, there is a strong negative correlation between the topography and the Bouguer anomaly, indicating that topography in the study area is isostatically compensated by density variations in the crust and upper mantle.
	www.litho.ucalgary.ca /publications/newsletter9.1/geiger.html (843 words)

	[No title] (Site not responding. Last check: 2007-10-13)
		In continental regions the Bouguer anomaly is commonly used and gives thicknesses for shields as great as 130 km, where geotherms reach temperatures of 800-1000
		obtained from Bouguer anomalies over shields are often upper bounds, rather than estimates, because most of the short-wavelength topography has been removed by erosion, and what remains is incoherent with both the free air and Bouguer gravity anomalies.
		The present study uses free air anomalies, and topography as a load whose geometry is known.
	www.agu.org /pubs/gap/McKenzieD/McKenzieD.html (305 words)

	[No title] (Site not responding. Last check: 2007-10-13)
		Spectral analysis of the Bouguer anomalies has been carried out along selected profiles in order to estimate the mean depth of the Moho discontinuity and that of an infracrustal discontinuity.
		The use of this technique inferred the presence of a discontinuity at a mean depth of ~ 28 km, interpreted as Moho and the likely presence of an infracrustal discontinuity at a mean depth of ~18 km, interpreted as the upper-lower crust transition.
		In order to roughly reconstruct the shape of these interfaces, 2D inversion techniques were applied to the large wavelength components of the Bouguer anomalies, relative to profiles oriented along the E-W direction, extracted from low-pass filtered Bouguer anomaly maps.
	www.ingv.it /~wwwannali/carrozzo405.htm (230 words)

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