scholarly journals GEOPHYSICAL STUDIES OF BASIN STRUCTURES ALONG THE EASTERN FRONT OF THE SIERRA NEVADA, CALIFORNIA

Geophysics ◽  
1964 ◽  
Vol 29 (3) ◽  
pp. 337-359 ◽  
Author(s):  
J. H. Healy ◽  
Frank Press
Keyword(s):  
Sierra Nevada ◽  
Gravity Data ◽  
El Paso ◽  
Gravity Anomalies ◽  
Density Contrast ◽  
Seismic Velocities ◽  
Owens Valley ◽  
Eastern Front ◽  
Garlock Fault ◽  
Mohorovičić Discontinuity

A seismic and gravity survey along the eastern front of the Sierra Nevada, California, between southern Owens Valley and the Garlock fault, outlines a series of basins with maximum depths ranging from 5,000 to 9,000 ft. These basins follow the front of the Sierra Nevada in a continuous chain with one interruption of about 10 miles near Little Lake. The gravity anomalies indicate that the basins are bounded by a series of high‐angle faults rather than a single large fault. The seismic velocities in the basin deposits appear to correlate with the stratigraphy of the section exposed in the El Paso Mountains. A comparison of Bouguer anomalies with seismic depths indicates a density contrast of 0.35 g/cc in basins less than 3,000 ft deep, and an average but widely varying density contrast of 0.25 g/cc in basins 4,000 to 8,000 ft deep. A digital‐computer program for automatic computation of basin depths from gravity anomalies was evaluated and found to be useful in this type of analysis. Changes in the depth to the Mohorovičič discontinuity cannot produce regional gradients as large as the regional gradients observed in the area of the survey. Either structure on an intermediate crustal boundary or lateral changes in crustal densities, or a combination of these, is required to explain the gravity data.

Gravity interpretation using Fourier transforms and simple geometrical models with exponential density contrast

Geophysics ◽  
1993 ◽  
Vol 58 (8) ◽  
pp. 1074-1083 ◽  
Author(s):  
D. Bhaskara Rao ◽  
M. J. Prakash ◽  
N. Ramesh Babu
Keyword(s):  
Los Angeles ◽  
Fourier Transforms ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Density Contrast ◽  
Model Parameters ◽  
Exponential Density ◽  
Exponential Density Contrast ◽  
Gravity Interpretation

The decrease of density contrast in sedimentary basins can often be approximated by an exponential function. Theoretical Fourier transforms are derived for symmetric trapezoidal, vertical fault, vertical prism, syncline, and anticline models. This is desirable because there are no equivalent closed form solutions in the space domain for these models combined with an exponential density contrast. These transforms exhibit characteristic minima, maxima, and zero values, and hence graphical methods have been developed for interpretation of model parameters. After applying end corrections to improve the discrete transforms of observed gravity data, the transforms are interpreted for model parameters. This method is first tested on two synthetic models, then applied to gravity anomalies over the San Jacinto graben and Los Angeles basin.

Stable inversion of gravity anomalies of sedimentary basins with nonsmooth basement reliefs and arbitrary density contrast variations

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 754-764 ◽  
Author(s):  
Valéria C. F. Barbosa ◽  
João B. C. Silva ◽  
Walter E. Medeiros
Keyword(s):  
Gravity Anomaly ◽  
Gravity Data ◽  
A Priori ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
Sedimentary Basins ◽  
Upper And Lower Bounds ◽  
Density Contrast ◽  
Good Precision ◽  
Rift Basins

We present a new, stable method for interpreting the basement relief of a sedimentary basin which delineates sharp discontinuities in the basement relief and incorporates any law known a priori for the spatial variation of the density contrast. The subsurface region containing the basin is discretized into a grid of juxtaposed elementary prisms whose density contrasts are the parameters to be estimated. Any vertical line must intersect the basement relief only once, and the mass deficiency must be concentrated near the earth’s surface, subject to the observed gravity anomaly being fitted within the experimental errors. In addition, upper and lower bounds on the density contrast of each prism are introduced a priori (one of the bounds being zero), and the method assigns to each elementary prism a density contrast which is close to either bound. The basement relief is therefore delineated by the contact between the prisms with null and nonnull estimated density contrasts, the latter occupying the upper part of the discretized region. The method is stabilized by introducing constraints favoring solutions having the attributes (shared by most sedimentary basins) of being an isolated compact source with lateral borders dipping either vertically or toward the basin center and having horizontal dimensions much greater than its largest vertical dimension. Arbitrary laws of spatial variations of the density contrast, if known a priori, may be incorporated into the problem by assigning suitable values to the nonnull bound of each prism. The proposed method differs from previous stable methods by using no smoothness constraint on the interface to be estimated. As a result, it may be applied not only to intracratonic sag basins where the basement relief is essentially smooth but also to rift basins whose basements present discontinuities caused by faults. The method’s utility in mapping such basements was demonstrated in tests using synthetic data produced by simulated rift basins. The method mapped with good precision a sequence of step faults which are close to each other and present small vertical slips, a feature particularly difficult to detect from gravity data only. The method was also able to map isolated discontinuities with large vertical throw. The method was applied to the gravity data from Reco⁁ncavo basin, Brazil. The results showed close agreement with known geological structures of the basin. It also demonstrated the method’s ability to map a sequence of alternating terraces and structural lows that could not be detected just by inspecting the gravity anomaly. To demostrate the method’s flexibility in incorporating any a priori knowledge about the density contrast variation, it was applied to the Bouguer anomaly over the San Jacinto Graben, California. Two different exponential laws for the decrease of density contrast with depth were used, leading to estimated maximum depths between 2.2 and 2.4 km.

scholarly journals Seafloor Density Contrast Derived From Gravity and Shipborne Depth Observations: A Case Study in a Local Area of Atlantic Ocean

2021 ◽  
Vol 9 ◽  
Author(s):  
Xiaoyun Wan ◽  
Weipeng Han ◽  
Jiangjun Ran ◽  
Wenjie Ma ◽  
Richard Fiifi Annan ◽  
...  
Keyword(s):  
Atlantic Ocean ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Local Area ◽  
Density Variation ◽  
Mean Difference ◽  
Density Contrast ◽  
Data Sets ◽  
Marine Gravity ◽  
Band Pass

Marine gravity data from altimetry satellites are often used to derive bathymetry; however, the seafloor density contrast must be known. Therefore, if the ocean water depths are known, the density contrast can be derived. This study experimented the total least squares algorithm to derive seafloor density contrast using satellite derived gravity and shipborne depth observations. Numerical tests are conducted in a local area of the Atlantic Ocean, i.e., 34°∼32°W, 3.5°∼4.5°N, and the derived results are compared with CRUST1.0 values. The results show that large differences exist if the gravity and shipborne depth data are used directly, with mean difference exceeding 0.4 g/cm3. However, with a band-pass filtering applied to the gravity and shipborne depths to ensure a high correlation between the two data sets, the differences between the derived results and those of CRUST1.0 are reduced largely and the mean difference is smaller than 0.12 g/cm3. Since the spatial resolution of CRUST1.0 is not high and in many ocean areas the shipborne depths and gravity anomalies are much denser, the method of this study can be an alternative method for providing seafloor density variation information.

3D lithospheric structure and density of the NE Atlantic

2020 ◽  
Author(s):  
Maria Laura Gomez Dacal ◽  
Jan Inge Faleide ◽  
Mansour Abdelmalak ◽  
Magdalena Scheck-Wenderoth ◽  
Denis Anikiev ◽  
...  
Keyword(s):  
Gravity Data ◽  
Thickness Distribution ◽  
Tectonic Setting ◽  
Gravity Anomalies ◽  
Density Variation ◽  
Scale Model ◽  
Western Coast ◽  
Seismic Velocities ◽  
Ne Atlantic ◽  
Seismic Profiles

<p>The NE Atlantic is a tectonically complex region, also interesting in terms of georesources and therefore large areas are well covered by geophysical and geological data. In this work, we present a 3D lithospheric-scale structural and density model of the region including the eastern-most area of Greenland, the western coast of Norway, Iceland and Svalbard. It covers an area of 2000 km in longitude by 2500 km in latitude with a depth of 300 km and a resolution of 10 km. The model was developed by integrating different kinds of data and regional or global previous models, mainly of seismic origin, and constrained by gravity observations.</p><p>The developed model includes the topography, bathymetry and ice thickness obtained from global compilations models. The thickness distribution of sediments was incorporated based on detailed mapping of most areas covered by the model. The structure of the crystalline crust, differentiating between the oceanic and continental areas, is based on seismic information and previous regional models, cross-checked by additional seismic profiles available in the region. The model also includes high velocity/density lower crustal bodies defined by a previous compilation at the Norwegian and Greenland margins and by the analysis of deep seismic profiles in the case of the Iceland area.</p><p>We assigned constant densities to each layer following seismic velocities and literature-suggested values for every lithology. Due to the active tectonic setting of the area and its consequent elevated temperature and thus low density, the portion of mantle included in the model is the only layer with variable density. To obtain the mentioned density variation, we evaluated different seismic tomographic data for the area and converted them into temperatures. To mitigate the poor reliability of the tomographic models at shallow depths and also taking into account that the effect of the temperature in the uppermost mantle is especially important near mid oceanic ridges, we evaluated the thermal effect of this area by running a thermal model. Therefore, we calculated 3D distribution of temperatures for the whole portion of the mantle included in the model to obtain the reduction in density that these temperatures would cause considering the thermal expansivity of mantle rocks. </p><p>The gravity response of the model was calculated and compared to the gravity observations using the 3D interactive software IGMAS+. The developed model includes the latest data and information of the area and, at the same time, reasonably fits the measured gravity anomalies. Comparison of the first-pass 3D gravity model to the observed gravity data detected some residual anomalies that require further differentiation of crustal densities. The new 3D lithosphere-scale model allows us to analyze the structural configuration of the area and interpret its tectonic implications. It also forms the base for thermal and mechanical models to obtain the 3D distribution of physical variables and predict the rheological and dynamic behavior of the wider NE Atlantic region.</p>

scholarly journals IDENTIFICATION OF BURIED ARCHEOLOGICAL OBJECTS WITH RADIAL DERIVATIVES OF MICRO GRAVITY DATA

2019 ◽  
Vol 2 (2) ◽  
pp. 84
Author(s):  
Muhammad Zuhdi ◽  
Bakti Sukrisna ◽  
Syamsuddin Syamsuddin
Keyword(s):  
Gravity Data ◽  
Synthetic Data ◽  
Gravity Anomalies ◽  
Density Contrast ◽  
Horizontal Distance ◽  
Radial Derivative ◽  
Horizontal Derivative ◽  
Almost All ◽  
Derivatives Of ◽  
Micro Gravity

The development of recent gravimetric technology allows us to measure gravity anomalies with accuracy of micro Gal. Micro gravity is able to detect very small gravity anomalies such as anomaly due to buried archeological objects below the earth surface. Radial Derivatives of gravity data is used to sharpen anomaly due to lateral changes of density contrast. Horizontal derivatives carried out by previous researchers have some weaknesses, i.e. the loss of derivative values in certain directions and inconsistence values at the source boundary of the same anomaly edge. To solve the horizontal derivative problem, a radial derivative is made. Radial derivative is derivative of gravity anomaly over horizontal distance in the radial direction from a certain point which is considered as the center of anomaly. There are two kind of radial derivative i.e. First Radial Derivative (FRD) and Second Radial Derivative (SRD). Blade Pattern is another way to enrich the ability of SRD to detect boundary of anomaly source. Synthetic gravity data of buried archeological object was made by counting the response of forward modelling. All of programs and calculation of the models in this research is performed based on Matlab® program. The results of the tests on the synthetic data of the model show that the radial derivative is able to detect the boundaries in buried temples due to density contrast. The advantage of radial derivatives which is a horizontal derivative in the direction of radial compared to ordinary horizontal derivatives is the ability to detect vertical boundaries of various anomaly due to horizontal layers and capable of showing density contrast in almost all directions.

Seismicity, Stress State, and Style of Faulting of the Ridgecrest-Coso Region from the 1930s to 2019: Seismotectonics of an Evolving Plate Boundary Segment

2020 ◽  
Author(s):  
Egill Hauksson ◽  
Lucile M. Jones
Keyword(s):  
Stress State ◽  
Sierra Nevada ◽  
Plate Boundary ◽  
Frictional Coefficient ◽  
Strike Slip ◽  
Owens Valley ◽  
Boundary Segment ◽  
The North ◽  
Decadal Scale ◽  
Garlock Fault

ABSTRACT Decadal scale variations in the seismicity rate in the Ridgecrest-Coso region, part of the Eastern California Shear Zone, included seismic quiescence from the 1930s to the early 1980s, followed by increased seismicity until the 2019 Mw 6.4 and 7.1 Ridgecrest sequence. This sequence exhibited complex rupture on almost orthogonal faults and triggered aftershocks over an area of ∼90  km long by ∼5–10  km wide, which is a fraction of the area of the previously seismically active Indian Wells Valley and Coso range region. During the last 40 yr, the seismicity has been predominantly the result of strike-slip motion, extending north from the Garlock fault, along the Little Lake and Airport Lake fault zones, and approaching the southernmost Owens Valley fault to the north. The Coso range forms an extensional stepover between these two strike-slip fault systems. This evolution of a plate boundary zone is driven by the northwestward motion of the Sierra Nevada, and crustal extension along the southwestern edge of the Basin and Range Province. Stress inversion of focal mechanisms shows that the postseismic stress state consists of almost horizontal σ1 and vertical σ2. The σ1 is spatially rotated across the Coso range stepover with σ1-trending ∼N17° E to the north, whereas, along the Mw 7.1 mainshock rupture, the trend is ∼N6° E. The friction angles as measured between fault strikes and the σ1 trends correspond to a frictional coefficient of 0.75, suggesting average fault strength. In comparison, the mature Garlock fault has a smaller frictional coefficient of 0.28, similar to weak faults like the San Andreas fault. Thus, it appears that the heterogeneously oriented and spatially distributed but strong Ridgecrest-Coso faults accommodate seismicity at seemingly random places and times within the region and are in the process of self-organizing to form a major throughgoing plate-boundary segment.

scholarly journals Nature of the crust in the northern Gulf of California and Salton Trough

Geosphere ◽  
2019 ◽  
Vol 15 (5) ◽  
pp. 1598-1616 ◽  
Author(s):  
Jolante W. van Wijk ◽  
Samuel P. Heyman ◽  
Gary J. Axen ◽  
Patricia Persaud
Keyword(s):  
Conceptual Model ◽  
Oceanic Crust ◽  
Gulf Of California ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Magnetic Anomalies ◽  
Seismic Velocities ◽  
Potential Field Data ◽  
Serpentinized Peridotite ◽  
Salton Trough

Abstract In the southern Gulf of California, the generation of new oceanic crust has resulted in linear magnetic anomalies and seafloor bathymetry that are characteristic of active seafloor-spreading systems. In the northern Gulf of California and the onshore (southeastern California, USA) Salton Trough region, a thick sedimentary package overlies the crystalline crust, masking its nature, and linear magnetic anomalies are absent. We use potential-field data and a geotherm analysis to constrain the composition of the crust (oceanic or continental) and develop a conceptual model for rifting. Gravity anomalies in the northern Gulf of California and Salton Trough are best fit with crustal densities that correspond to continental crust, and the fit is not as good if densities representative of mafic rocks, i.e., oceanic crust or mafic underplating, are assumed. Because extensive mafic underplated bodies would produce gravity anomalies that are not in agreement with observed gravity data, we propose, following earlier work, that the anomalies might be due to serpentinized peridotite bodies such as found at magma-poor rifted margins. The density and seismic velocities of such serpentinized peridotite bodies are in agreement with observed gravity and seismic velocities. Our conceptual model for the Salton Trough and northern Gulf of California shows that net crustal thinning here is limited because new crust is formed rapidly from sediment deposition. As a result, continental breakup may be delayed.

Gravity Studies of Earthquake-Related Structures Near Massena, New York

1979 ◽  
Vol 50 (1) ◽  
pp. 13-24
Author(s):  
Reinhard K. Frohlich ◽  
Robert L. Albert ◽  
Frank A. Revetta
Keyword(s):  
New York ◽  
Sea Level ◽  
Gravity Data ◽  
Bouguer Anomaly ◽  
Gravity Anomalies ◽  
River Valley ◽  
Density Contrast ◽  
Positive Anomaly ◽  
Eastern United States ◽  
St Lawrence River

Abstract The causes of the seismicity of the St. Lawrence River Valley are not well understood. As is the case for the entire east coast of North America, epicentral zones often occur in regions where no correlation exists between seismicity and mapped geologic structures. Several explanations have been proposed for such a phenomenon: a) earthquakes occur along unmapped surface faults; b) earthquakes occur along subsurface faults showing no surface expression; or c) the earthquakes are not related to existing faults. Conventional analytical techniques, such as upward and downward continuation, were applied to gravity data from the St. Lawrence River Valley in an attempt to delineate possible seismic–related structures. The analysis of the gravity data indicates that the anomalies trend in a north-northeast direction similar to the structural trends of the Precambrian rocks. The major feature of the Simple Bouguer anomaly map is an extensive positive gravity anomaly centered at Massena, New York. Profiles across the Bouguer gravity anomalies and the up-and downward continued gravity anomalies were reproduced with a two–dimensional modeling technique. Among the various non-unique anomaly-producing structures tested we prefer a model suggesting that the positive anomaly near Massena is derived from two bodies with different density contrasts. The first is a wedge (8 km deep by 35 km wide) located 6 km below sea level with a density contrast of +0.11 gm/cm3 and the second is a smaller body (2 km deep by 6 km wide) located 3.3 km below sea level with a density contrast of +0.2 gm/cm3. The large wedge may represent a sequence of interlayered metasediments and metavolcanics related to the Grenville sequence. The smaller body may represent a mafic intrusive. Several authors have suggested that high gradients of gravity (toward positive) produced by mafic intrusives are associated with earthquakes in the eastern United States. The possible existence of a mafic intrusive near Massena, New York, and its proximity to epicentral zones suggest a similar association for earthquakes in the study area.

scholarly journals Joint Gravity and Seismic Reflection Methods to Characterize the Deep Aquifers in Arid Ain El Beidha Plain (Central Tunisia, North Africa)

Water ◽  
2021 ◽  
Vol 13 (9) ◽  
pp. 1310
Author(s):  
Hajer Azaiez ◽  
Hakim Gabtni ◽  
Mourad Bédir
Keyword(s):  
Seismic Reflection ◽  
Large Scale ◽  
Seismic Analysis ◽  
Gravity Data ◽  
Geophysical Methods ◽  
Gravity Anomalies ◽  
Integrated Approach ◽  
Central Tunisia ◽  
Deep Aquifers ◽  
Advanced Analysis

Electric resistivity sounding and tomography, as well as electromagnetic sounding, are the classical methods frequently used for hydrogeological studies. In this work, we propose the development and implementation of an original integrated approach using the unconventional hydro–geophysical methods of gravity and seismic reflection for the fast, large–scale characterization of hydrogeological potential using the Ain El Beidha plain (central Tunisia) as an analogue. Extending the values of vintage petroleum seismic reflection profiles and gravity data, in conjunction with available geological and hydrogeological information, we performed an advanced analysis to characterize the geometry of deep tertiary (Oligocene and Eocene) aquifers in this arid area. Residual and tilt angle gravity maps revealed that most gravity anomalies have a short wavelength. The study area was mainly composed of three major areas: the Oued Ben Zitoun and Ain El Beidha basins, which are both related to negative gravity trends corresponding to low–density subsiding depocenters. These basins are separated by an important NE–SW trend called “El Gonna–J. El Mguataa–Kroumet Zemla” gravity high. Evaluation of the superposition of detected lineaments and Euler deconvolution solutions’ maps showed several NE–SW and N–S relay system faults. The 3D density inversion model using a lateral and vertical cutting plane suggested the presence of two different tectonic styles (thin VS thick). Results from the gravity analysis were in concordance with the seismic analysis. The deep Oligocene and Eocene seismic horizons were calibrated to the hydraulic wells and surrounding outcrops. Oligocene and Eocene geological reservoirs appear very fractured and compartmented. The faulting network also plays an important role in enhancing groundwater recharge process of the Oligocene and Eocene aquifers. Finally, generated isochron maps provided an excellent opportunity to develop future comprehensive exploration surveys over smaller and more favorable areas’ sub–basins.

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