scholarly journals Gravity modelling of the lower crust in Sardinia (Italy)

1997 ◽  
Vol 40 (5) ◽  
Author(s):  
M. T. Carrozzo ◽  
R. Balia ◽  
M. Loddo ◽  
D. Luzio ◽  
C. Margiotta ◽  
...  
Keyword(s):  
Lower Crust ◽  
Local Density ◽  
A Priori ◽  
Seismic Refraction ◽  
Bouguer Anomaly ◽  
Bouguer Anomalies ◽  
Velocity Models ◽  
Gravity Measurements ◽  
Low Pass ◽  
Inversion Techniques

In this paper an example is given of an application of statistical techniques to the Bouguer anomalies analysis in order to design a simple crustal model using few a priori assumptions. All gravity measurements carried out in Sardinia have been collected and processed. The Bouguer anomalies have been calculated according to local density estimates. 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. The density model obtained is compatible with some velocity models achieved from the interpretation of the seismic refraction profiles carried out within the European Geotraverse project.

An interpretation of gravity measurements on the west coast of Canada

1969 ◽  
Vol 6 (3) ◽  
pp. 463-474 ◽  
Author(s):  
R. A. Stacey ◽  
L. E. Stephens
Keyword(s):  
Gravity Field ◽  
Bouguer Anomaly ◽  
Vancouver Island ◽  
Coast Mountains ◽  
Bouguer Anomalies ◽  
Queen Charlotte Islands ◽  
Gravity Measurements ◽  
Terrain Corrections ◽  
Tectonic Belt ◽  
Local Anomalies

The survey area lies close to the continental margin and includes parts of the Insular Tectonic Belt and the Coast Mountains igneous and metamorphic complex, which are part of the Cordilleran (geological) Region. In an endeavor to clarify the structure of the Insular Tectonic Belt and the Coast Mountains complex, gravity measurements have been made using Worden or LaCoste and Romberg meters at 12–15 km intervals throughout the Queen Charlotte Islands, Vancouver Island, and the coastal areas of the British Columbia mainland. Measurements have been made at the same interval using a LaCoste and Romberg underwater gravity meter wherever the depth of water is less than 300 fathoms (< 540 m) along the fiords of the mainland coast and over the continental shelf. The observed gravity values have been reduced to Bouguer anomalies and terrain corrections have been calculated using either Bible's tables or a computer system based on the attraction of the rectangular prismatic block.The major features of gravity field are: (1) a positive Bouguer anomaly along the western edge of the area, which is associated with the change from continental to oceanic crust, and (2) a negative anomaly along the Coast Mountains, which is attributed to the thickening of the continental crust below these mountains. On the eastern side of the Queen Charlotte Islands, Hecate Strait, Queen Charlotte Sound, and Vancouver Island, the average Bouguer anomaly is approximately zero, with local anomalies superimposed on a fairly flat gravity field. Several of these local anomalies are related to density variations in the surface rocks.

scholarly journals Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental break-up

2014 ◽  
Vol 6 (1) ◽  
pp. 1335-1370 ◽  
Author(s):  
K. Becker ◽  
D. Franke ◽  
R. B. Trumbull ◽  
M. Schnabel ◽  
I. Heyde ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Small Body ◽  
Seismic Refraction ◽  
South Atlantic ◽  
The South ◽  
Cross Sectional ◽  
Velocity Models ◽  
Rifted Margins ◽  
Break Up

Abstract. High-velocity lower crust (HVLC) and seaward dipping reflector sequences (SDRs) are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal four times larger cross sectional areas of HVLC than at the South American margin, a finding that is in sharp contrast to the distribution of the flood basalts in the Paraná-Etendeka Large Igneous Provinces (LIP). Also, the position of the HVLC with respect to the seaward dipping reflector sequences varies consistently along both margins. Close to the Falkland-Agulhas Fracture Zone a small body of HVLC is not accompanied by seaward dipping reflectors. In the central portion of both margins, the HVLC is below the inner seaward dipping reflector wedges while in the northern area, closer to the Rio Grande Rise/Walvis Ridge, large volumes of HVLC extend far seawards of the inner seaward dipping reflectors. This challenges the concept of a simple extrusive/intrusive relationship between seaward dipping reflector sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and break-up process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple shear dominated extension.

scholarly journals Asymmetry of high-velocity lower crust on the South Atlantic rifted margins and implications for the interplay of magmatism and tectonics in continental breakup

Solid Earth ◽  
2014 ◽  
Vol 5 (2) ◽  
pp. 1011-1026 ◽  
Author(s):  
K. Becker ◽  
D. Franke ◽  
R. Trumbull ◽  
M. Schnabel ◽  
I. Heyde ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Small Body ◽  
Seismic Refraction ◽  
South Atlantic ◽  
Large Igneous Province ◽  
Asymmetric Distribution ◽  
The South ◽  
Velocity Models ◽  
Rifted Margins

Abstract. High-velocity lower crust (HVLC) and seaward-dipping reflector (SDR) sequences are typical features of volcanic rifted margins. However, the nature and origin of HVLC is under discussion. Here we provide a comprehensive analysis of deep crustal structures in the southern segment of the South Atlantic and an assessment of HVLC along the margins. Two new seismic refraction lines off South America fill a gap in the data coverage and together with five existing velocity models allow for a detailed investigation of the lower crustal properties on both margins. An important finding is the major asymmetry in volumes of HVLC on the conjugate margins. The seismic refraction lines across the South African margin reveal cross-sectional areas of HVLC 4 times larger than at the South American margin, a finding that is opposite to the asymmetric distribution of the flood basalts in the Paraná–Etendeka Large Igneous Province. Also, the position of the HVLC with respect to the SDR sequences varies consistently along both margins. Close to the Falkland–Agulhas Fracture Zone in the south, a small body of HVLC is not accompanied by SDRs. In the central portion of both margins, the HVLC is below the inner SDR wedges while in the northern area, closer to the Rio Grande Rise-Walvis Ridge, large volumes of HVLC extend far seaward of the inner SDRs. This challenges the concept of a simple extrusive/intrusive relationship between SDR sequences and HVLC, and it provides evidence for formation of the HVLC at different times during the rifting and breakup process. We suggest that the drastically different HVLC volumes are caused by asymmetric rifting in a simple-shear-dominated extension.

Crustal velocity models for the Archean Abitibi greenstone belt from seismic refraction data

1995 ◽  
Vol 32 (2) ◽  
pp. 149-166 ◽  
Author(s):  
Gilles Grandjean ◽  
Hua Wu ◽  
Donald White ◽  
Marianne Mareschal ◽  
Claude Hubert
Keyword(s):  
Lower Crust ◽  
Greenstone Belt ◽  
Velocity Structure ◽  
Complex Structure ◽  
Seismic Refraction ◽  
Seismic Energy ◽  
Crustal Layer ◽  
Middle Crust ◽  
Velocity Models ◽  
Abitibi Greenstone Belt

We present velocity models for two seismic wide–angle-refraction profiles across the Archean Abitibi greenstone belt and the Pontiac Subprovince. The seismic profiles are 210 and 220 km long. Traveltime inversion and amplitude forward modelling were used to obtain two-dimensional velocity structure and interface geometry. The main features of the velocity models include (1) three crustal layers; (2) variable velocities (5.6–6.4 km/s) in the upper crust (~0–12 km), with the higher velocities generally associated with mafic metavolcanics and the lower velocities with metasediments and granitic plutons; (3) a relatively uniform middle crust (~12–30 km) with velocities ranging from 6.4 to 6.6 km/s; (4) a velocity increase of 0.3 km/s across the middle crust–lower crust boundary; (5) a lower crust (~30–40 km) with velocities increasing from 6.9 km/s at the top to 7.3 km/s at the base; (6) an average upper mantle velocity of 8.15 km/s; (7) depth to Moho of about 40 km in the north-central Abitibi belt, decreasing southward to 37 km beneath the Pontiac Subprovince; and (8) observed attenuation of seismic energy propagating through the Casa–Berardi deformation zone, suggesting a complex structure in this fault zone. The velocity model is generally consistent with seismic reflection interpretations that suggest that the shallow supracrustal assemblages form an allochthonous veneer, overlying a mid-crustal imbricate sequence of metaplutonic and metasedimentary rocks. The uniform-velocity structure below 12 km depth indicates that the tectonic zones juxtaposing disparate crustal blocks may have limited depth extent. The 40 km thick crust and 10 km thick high-velocity lower crustal layer exceed the thicknesses observed in other studies of Archean crust.

Application of cross correlation to HREM images of surfaces

1987 ◽  
Vol 45 ◽  
pp. 754-755
Author(s):  
D. E. Luzzi ◽  
L. D. Marks ◽  
M. I. Buckett
Keyword(s):  
Cross Correlation ◽  
A Priori ◽  
Matrix Material ◽  
Correlation Method ◽  
Accurate Knowledge ◽  
Complex Image ◽  
Fourier Filtering ◽  
The Matrix ◽  
Low Pass ◽  
Data Reduction Technique

As the HREM becomes increasingly used for the study of dynamic localized phenomena, the development of techniques to recover the desired information from a real image is important. Often, the important features are not strongly scattering in comparison to the matrix material in addition to being masked by statistical and amorphous noise. The desired information will usually involve the accurate knowledge of the position and intensity of the contrast. In order to decipher the desired information from a complex image, cross-correlation (xcf) techniques can be utilized. Unlike other image processing methods which rely on data massaging (e.g. high/low pass filtering or Fourier filtering), the cross-correlation method is a rigorous data reduction technique with no a priori assumptions.We have examined basic cross-correlation procedures using images of discrete gaussian peaks and have developed an iterative procedure to greatly enhance the capabilities of these techniques when the contrast from the peaks overlap.

A seismic-based cross-section of the Grenville Orogen in southern Ontario and western Quebec

2000 ◽  
Vol 37 (2-3) ◽  
pp. 183-192 ◽  
Author(s):  
D J White ◽  
D A Forsyth ◽  
I Asudeh ◽  
S D Carr ◽  
H Wu ◽  
...  
Keyword(s):  
Cross Section ◽  
Seismic Refraction ◽  
Shear Zones ◽  
Tectonic Zone ◽  
Grenville Province ◽  
Crustal Layer ◽  
Seismic Reflection Data ◽  
Velocity Models ◽  
Laurentian Margin ◽  
Structural Boundary

A schematic crustal cross-section is presented for the southwestern Grenville Province based on reprocessed Lithoprobe near-vertical incidence seismic reflection data and compiled seismic refraction - wide-angle velocity models interpreted with geological constraints. The schematic crustal architecture of the southwest Grenville Province from southeast to northwest comprises allochthonous crustal elements (Frontenac-Adirondack Belt and Composite Arc Belt) that were assembled prior to ca. 1160 Ma, and then deformed and transported northwest over reworked rocks of pre-Grenvillian Laurentia and the Laurentian margin primarily between 1120 and 980 Ma. Reworked pre-Grenvillian Laurentia and Laurentian margin rocks are interpreted to extend at least 350 km southeast of the Grenville Front beneath all of the Composite Arc Belt. Three major structural boundary zones (the Grenville Front and adjacent Grenville Front Tectonic Zone, the Central Metasedimentary Belt boundary thrust zone, and the Elzevir-Frontenac boundary zone) have been identified across the region of the cross-section based on their prominent geophysical signatures comprising broad zones of southeast-dipping reflections and shallowing of mid-crustal velocity contours by 12-15 km. The structural boundary zones accommodated southeast over northwest crustal stacking at successively earlier times during orogeny (ca. 1010-980 Ma, 1080-1060 Ma, and 1170-1160 Ma, respectively). These shear zones root within an interpreted gently southeast-dipping regional décollement at a depth of 25-30 km corresponding to the top of a high-velocity lower crustal layer.

Reconnaissance seismic refraction-reflection surveys in northwestern New Mexico

1984 ◽  
Vol 74 (4) ◽  
pp. 1263-1274
Author(s):  
Lawrence H. Jaksha ◽  
David H. Evans
Keyword(s):  
New Mexico ◽  
Wave Velocity ◽  
Lower Crust ◽  
Seismic Waves ◽  
Seismic Refraction ◽  
Velocity Model ◽  
P Wave ◽  
Uppermost Mantle ◽  
P Wave Velocity ◽  
Crustal Thinning

Abstract A velocity model of the crust in northwestern New Mexico has been constructed from an interpretation of direct, refracted, and reflected seismic waves. The model suggests a sedimentary section about 3 km thick with an average P-wave velocity of 3.6 km/sec. The crystalline upper crust is 28 km thick and has a P-wave velocity of 6.1 km/sec. The lower crust below the Conrad discontinuity has an average P-wave velocity of about 7.0 km/sec and a thickness near 17 km. Some evidence suggests that velocity in both the upper and lower crust increases with depth. The P-wave velocity in the uppermost mantle is 7.95 ± 0.15 km/sec. The total crustal thickness near Farmington, New Mexico, is about 48 km (datum = 1.6 km above sea level), and there is evidence for crustal thinning to the southeast.

scholarly journals PRELIMINARY GULLY HAZARD ASSESSMENT USING 2D-ERT AND 2D-SRT GEOPHYSICAL SURVEYS: A CASE STUDY IN SOUTHEASTERN NIGERIA

2021 ◽  
pp. 49-65
Author(s):  
GN Egwuonwu ◽  
EI Okoyeh ◽  
DC Agarana ◽  
EG Nwaka ◽  
OB Nwosu ◽  
...  
Keyword(s):  
Seismic Refraction ◽  
P Wave ◽  
Lateritic Soil ◽  
Risk Level ◽  
Clayey Sand ◽  
Southeastern Nigeria ◽  
Velocity Models ◽  
Refraction Tomography ◽  
Geophysical Surveys ◽  
Oriented Parallel

Two-dimensional Electrical Resistivity Tomography (2DERT) and Seismic Refraction Tomography (2DSRT) were concurrently applied in assessment of a gully site with the view of assessing its stability and risk level. Eight profile lines oriented parallel and perpendicular to the boundary of the gully were surveyed. As a result, apparent resistivity model tomograms in the range of 1-9,000 and p-wave velocity models in the range of 300-700 were obtained from the two techniques respectively. Interpretation of the models obtained show predominance of unconsolidated clay, shale intercalates, clayey sand, sandy clay and weathered lateritic soil at shallow depths. Low amplitude undulating refracting layers, landslide slip subsurface and lose horizons were also delineated at shallow depths. The predominance of weak, clayey and unconsolidated lithology at the gully site suggests evidence of unstable gravitational equilibrium which imply environmental hazard. The plausible deductions made from the two

scholarly journals A study of the approaches used to retrieve aerosol extinction, as applied to limb observations made by OSIRIS and SCIAMACHY

2018 ◽  
Vol 11 (6) ◽  
pp. 3433-3445 ◽  
Author(s):  
Landon A. Rieger ◽  
Elizaveta P. Malinina ◽  
Alexei V. Rozanov ◽  
John P. Burrows ◽  
Adam E. Bourassa ◽  
...  
Keyword(s):  
Particle Size ◽  
Radiative Transfer ◽  
A Priori ◽  
Stratospheric Aerosol ◽  
Aerosol Extinction ◽  
Transfer Modelling ◽  
Inversion Techniques ◽  
And Inversion ◽  
Good Agreement

Abstract. Limb scatter instruments in the UV–vis spectral range have provided long-term global records of stratospheric aerosol extinction important for climate records and modelling. While comparisons with occultation instruments show generally good agreement, the source and magnitude of the biases arising from retrieval assumptions, approximations in the radiative transfer modelling and inversion techniques have not been thoroughly characterized. This paper explores the biases between SCIAMACHY v1.4, OSIRIS v5.07 and SAGE II v7.00 aerosol extinctions through a series of coincident comparisons as well as simulation and retrieval studies to investigate the cause and magnitude of the various systematic differences. The effect of a priori profiles, particle size assumptions, radiative transfer modelling, inversion techniques and the different satellite datasets are explored. It is found that the assumed a priori profile can have a large effect near the normalization point, as well as systematic influence at lower altitudes. The error due to particle size assumptions is relatively small when averaged over a range of scattering angles, but individual errors depend on the particular scattering angle, particle size and measurement vector definition. Differences due to radiative transfer modelling introduce differences between the retrieved products of less than 10 % on average, but can introduce vertical structure. The combination of the different scenario simulations and the application of both algorithms to both datasets enable the origin of some of the systematic features such as high-altitude differences when compared to SAGE II to be explained.

A combination of electrical resistivity, seismic refraction, and gravity measurements for groundwater exploration in Sudan

Geophysics ◽  
1981 ◽  
Vol 46 (9) ◽  
pp. 1304-1313 ◽  
Author(s):  
Ronald A. van Overmeeren
Keyword(s):  
Electrical Resistivity ◽  
Seismic Refraction ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Equivalence Problem ◽  
Groundwater Exploration ◽  
Fresh Groundwater ◽  
Subsequent Test ◽  
Additional Information ◽  
Electrical Sounding

In the savannah belt of central Sudan, near the town of Kosti, a regional geophysical survey has been carried out forming part of a groundwater project. Because of the presence of detectable and significant contrasts in physical properties of the subsoil, integrated use could be made of electrical resistivity, seismic refraction, and gravity methods. In the interpretation of multilayer electrical sounding curves, additional subsurface information such as lithological well descriptions and geophysical well logs is normally a necessity for solving the problems of equivalence. Along a profile in the eastern part of the area studied, where additional subsurface information was scarce, 16 vertical electrical soundings have been made. A preliminary simple mathematical interpretation suggested possibilities for the presence of fresh groundwater in the eastern part of the profile. In order to solve the equivalence problem, seismic refraction work was carried out at some selected places; that yielded additional information on depths to bedrock. These seismic data made possible a unique solution of the electrical sounding curves, from which it could be concluded that all groundwater in the area is saline. Subsequent test drilling confirmed these findings. A regional relative Bouguer anomaly map provided a picture of the general geologic structures and made possible rough estimates of depths to bedrock. In areas where the basement rocks are relatively close to the surface, as is the case with the profile presented, the gravity anomalies cannot be correlated with bedrock relief, because the effect is strongly influenced by lateral density variations within the bedrock itself. This is an example of a case where only an integrated application of several geophysical exploration methods can provide the desired hydrogeologic information in an acceptable balance between reliability and cost.

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