DEEP GRAVITY INTERPRETATION BY STRIPPING

Geophysics ◽  
1963 ◽  
Vol 28 (3) ◽  
pp. 369-378 ◽  
Author(s):  
Sigmund Hammer
Keyword(s):  
Gravity Data ◽  
New Technique ◽  
Gamma Density ◽  
Gravity Effects ◽  
Definition Of ◽  
Gravity Interpretation ◽  
Overlying Strata

“Old gravity data never die.” Review interpretations of gravity surveys can be made whenever warranted by new geological concepts, development information, or improved techniques. Important additional uses can be gained by extending the interpretation to deeper horizons by calculating and subtracting the gravity effects of overlying strata whose structure becomes known in detail from shallower development. This constitutes the new technique reported in this paper. An essential new factor which makes gravity “stripping” practicable is the advent of the gamma‐gamma density log which determines subsurface density relations in strata penetrated by development drilling. Combined stratigraphic and density information defines the mass anomalies in the upper strata. Subtracting the calculated shallower gravity influences improves the definition of the deeper gravity prospects. Applications of the method are illustrated by selected examples.

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.

A method for inversion of 1D vertical soundings of gravity anomalies

Geophysics ◽  
2018 ◽  
Vol 83 (2) ◽  
pp. G15-G23
Author(s):  
Andrea Vitale ◽  
Domenico Di Massa ◽  
Maurizio Fedi ◽  
Giovanni Florio
Keyword(s):  
Potential Field ◽  
Field Data ◽  
Gravity Data ◽  
Finite Size ◽  
Gravity Anomalies ◽  
Real Data ◽  
Gravity Inversion ◽  
Chain Process ◽  
Potential Field Data ◽  
Gamma Density

We have developed a method to interpret potential fields, which obtains 1D models by inverting vertical soundings of potential field data. The vertical soundings are built through upward continuation of potential field data, measured on either a profile or a surface. The method assumes a forward problem consisting of a volume partitioned in layers, each of them homogeneous and horizontally finite, but with the density changing versus depth. The continuation errors, increasing with the altitude, are automatically handled by determining the coefficients of a third-order polynomial function of the altitude. Due to the finite size of the source volume, we need a priori information about the total horizontal extent of the volume, which is estimated by boundary analysis and optimized by a Markov chain process. For each sounding, a 1D inverse problem is independently solved by a nonnegative least-squares algorithm. Merging of the several inverted models finally yields approximate 2D or 3D models that are, however, shown to generate a good fit to the measured data. The method is applied to synthetic models, producing good results for either perfect or continued data. Even for real data, i.e., the gravity data of a sedimentary basin in Nevada, the results are interesting, and they are consistent with previous interpretation, based on 3D gravity inversion constrained by two gamma-gamma density logs.

Shallow and deep crustal structure of the western Sverdrup Basin, arctic Canada

1984 ◽  
Vol 21 (8) ◽  
pp. 902-919 ◽  
Author(s):  
L. W. Sobczak ◽  
A. Overton
Keyword(s):  
Gravity Data ◽  
Seismic Refraction ◽  
Sediment Accumulation ◽  
Density Control ◽  
Gravity Effects ◽  
Sverdrup Basin ◽  
Deep Crustal Structure ◽  
Crystalline Crust ◽  
Free Air Anomaly ◽  
Drill Holes

An analysis of gravity data along two detailed profiles over the western Sverdrup Basin in the Canadian Arctic supports a seismic refraction model. Drill holes as deep as 5.4 km near the profiles provide excellent density control. Mean densities in the thickest part of the basin exceed those in the thinner parts by an average of 0.13 Mg/m3.Bouguer anomalies corrected for the effect of water, sedimentary, and crystalline layers indicate significant anomalies that vary in width from 20 to 275 km and in amplitude from 3 to 46 mGal (30 to 460 μm s−2). These can all be explained by density structures within the sedimentary column. Sedimentary thickness along the profiles varies from 9 to 17.4 km, crystalline thickness varies from 18 to 33 km, and the total crustal thickness varies from 34 to 42 km. The analysis also shows: (1) negative gravity effects of about 60 to 120 mGal (600 to 1200 μm s−2) due to the mass deficiency of the water and sediments are offset by positive gravity effects of similar magnitude due to crustal thinning; (2) isostatic compensation of water and sediments by a mantle antiroot is evident from a regional free-air anomaly near zero and the apparent inverse variation of sedimentary thickness with the thickness of the crystalline crust; (3) in the thickest part of the basin, undulations at the sedimentary–crystalline boundary are in phase with and smaller in amplitude than undulations at the crust–mantle boundary; conversely, in the thinnest part of the basin, these undulations are out of phase and larger in amplitude.These effects may be explained by stretching of a crystalline crust and a general decrease in crustal rigidity with depth during sediment accumulation and subsequent orogenic events.

Robust polynomial fitting method for regional gravity estimation

Geophysics ◽  
1991 ◽  
Vol 56 (1) ◽  
pp. 80-89 ◽  
Author(s):  
J. F. Beltrão ◽  
J. B. C. Silva ◽  
J. C. Costa
Keyword(s):  
Gravity Data ◽  
Crustal Thickening ◽  
Polynomial Fitting ◽  
Shallow Subsurface ◽  
Residual Field ◽  
Fitting Method ◽  
Definition Of ◽  
Regional Gravity ◽  
Lower Crustal ◽  
Polynomial Surface

Standard polynomial fitting methods are inconsistent in their formulation. The regional field is approximated by a polynomial fitted to the observed field. As a result, in addition to the nonuniqueness in the definition of the regional field, the fitted polynomial is strongly influenced by the residual field (observed field minus regional field). We present a regional‐residual separation method for gravity data which uses a robust procedure to determine the coefficients of a polynomial fitted to the observations. Under the hypothesis that the regional can be modeled correctly by the polynomial surface, the proposed method minimizes the influence of the residual field in the fitted surface. The proposed method was applied to real gravity data from Ceará state, Brazil, and produced information on zones of possible crustal thickening and the occurrence of lower‐crustal granulitic rocks thrust into the shallow subsurface.

2D gravity inversion of a complex interface in the presence of interfering sources

Geophysics ◽  
2007 ◽  
Vol 72 (2) ◽  
pp. I13-I22 ◽  
Author(s):  
Fernando J. Silva Dias ◽  
Valeria C. Barbosa ◽  
João B. Silva
Keyword(s):  
Gravity Data ◽  
Synthetic Data ◽  
Gravity Inversion ◽  
Spectral Content ◽  
Robust Fitting ◽  
Mafic Intrusions ◽  
Fitting Procedure ◽  
Gravity Response ◽  
Geologic Setting ◽  
Gravity Interpretation

We present a new semiautomatic gravity interpretation method for estimating a complex interface between two media containing density heterogeneities (referred to as interfering sources) that give rise to a complex and interfering gravity field. The method combines a robust fitting procedure and the constraint that the interface is very smooth near the interfering sources, whose approximate horizontal coordinates are defined by the user. The proposed method differs from the regional-residual separation techniques by using no spectral content assumption about the anomaly produced by the interface to be estimated, i.e., the interface can produce a gravity response containing both low- and high-wavenumber features. As a result, it may be applied to map the relief of a complex interface in a geologic setting containing either shallow or deep-seated interfering sources. Tests conducted with synthetic data show that the method can be of utility in estimating the basement relief of a sedimentary basin in the presence of salt layers and domes or in the presence of mafic intrusions in the basement or in both basement and the sedimentary section. The method was applied to real gravity data from two geologic settings having different kinds of interfering sources and interfaces to be interpreted: (1) the interface between the upper and lower crusts over the Bavali shear zone of southern India and (2) the anorthosite-tonalite interface over the East Bull Lake gabbro-anorthosite complex outcrop in Ontario, Canada.

OPTIMIZATION OF INTEGRATED PRODUCTION MODEL CALCULATIONS

2020 ◽  
pp. 67-72
Author(s):  
I.S. Senkin ◽  
L.I.. Akmadieva ◽  
E.V. Belonogov
Keyword(s):  
Input Data ◽  
Integrated Model ◽  
Production Model ◽  
New Technique ◽  
Model Calculations ◽  
Integrated Production ◽  
Commercial Package ◽  
Calculation Results ◽  
A New Technique ◽  
Definition Of

The article discusses a new technique for optimizing the calculations of the integrated model, all stages of the calculations are analyzed. The analysis of the necessary input data for the integrated model is carried out and the definition of the calculation results according to the obtained method is given. Comparison of calculations using the developed method and a commercial package is obtained. Opportunities for applying the obtained solution are proposed.

THE EFFECT OF FINITE DATA LENGTH IN THE SPECTRAL ANALYSIS OF IDEAL GRAVITY ANOMALIES

Geophysics ◽  
1976 ◽  
Vol 41 (1) ◽  
pp. 44-55 ◽  
Author(s):  
Robert D. Regan ◽  
William J. Hinze
Keyword(s):  
Spectral Analysis ◽  
Fourier Transforms ◽  
Gravity Data ◽  
Gravity Anomalies ◽  
Transformation Process ◽  
Definition Of ◽  
Horizontal Cylinders ◽  
Data Windows ◽  
Source Of Error ◽  
Data Length

The interpretation of gravity data which has been transformed from the space domain to the frequency domain is subject to limitations imposed by the transformation process and the nature of the data. One potential source of error in the spectral analysis of gravity anomalies is the incomplete definition of the anomaly due to insufficient data length. Comparison of theoretical Fourier transforms and the transforms of finite‐length segments of gravity anomalies due to two‐dimensional vertical slabs and horizontal cylinders indicates that the interpretation error can be held to less than ten percent if the profile length is at least six times the maximum depth to the source of the anomaly. This result was achieved with a rectangular data window in the transformation process. Greater error resulted from the use of the Bartlett, Parzen, and Tukey data windows.

APPROXIMATION IN GRAVITY INTERPRETATION CALCULATIONS

Geophysics ◽  
1974 ◽  
Vol 39 (2) ◽  
pp. 205-222 ◽  
Author(s):  
Sigmund Hammer
Keyword(s):  
Thin Plate ◽  
Center Of Mass ◽  
Maximum Error ◽  
Horizontal Cylinder ◽  
The Body ◽  
Finite Width ◽  
Practical Applications ◽  
Gravity Effects ◽  
Body Shapes ◽  
Gravity Interpretation

Rigorous calculation of gravity effects of geologic bodies for the interpretation of subsurface structure from exploration gravity surveys is time‐consuming and tedious. Resort to electronic computers is convenient but not always necessary. Simple hand calculations can be instructive and are often adequate and useful. This paper reports the errors involved and ranges of validity of various simplifying approximations for commonly used geometric body shapes: (a) equivalent sphere approximation for an infinitely long horizontal cylinder, (b) circular plate approximation for horizontal slabs of finite width, (c) vertical line element and thin horizontal plate approximations for vertical cylinders of variable radii and heights (dome, salt dome, or plug), and (d) thin‐plate approximations for vertical dikes and horizontal fault blocks. The results are evaluated in terms of dimensionless parametric ratios, usually a characteristic dimension of the body versus depth to center of mass. Errors in the approximate calculations are remarkably small for several of the cases considered and the simplified results are valid for given precision over a rather large range of dimensions. For example, the maximum error in the thin‐plate approximation for an horizontal fault plate is less than one percent for a thickness/depth ratio ranging up to 0.75 which represents a very substantial fault throw. This particular example is more than adequate for most practical applications in geologic interpretations of gravity. On the other hand, some examples are shown to be unsuitable for simple approximation calculations.

Potential-field signatures of buried Precambrian basement in the Western Canada Sedimentary Basin

2000 ◽  
Vol 37 (11) ◽  
pp. 1453-1471 ◽  
Author(s):  
M Pilkington ◽  
W F Miles ◽  
G M Ross ◽  
W R Roest
Keyword(s):  
Sedimentary Basin ◽  
Gravity Data ◽  
Magnetic Data ◽  
Western Canada ◽  
Precambrian Basement ◽  
Western Canada Sedimentary Basin ◽  
Magmatic Arc ◽  
Magnetic Source ◽  
Definition Of ◽  
Canada Sedimentary Basin

An internally consistent, levelled compilation of magnetic data is derived for Alberta and northeastern British Columbia. With Bouguer gravity data, this compilation is used to refine the definition of Precambrian basement domains within the Western Canada Sedimentary Basin. Magnetic data are draped at a constant distance above the mapped basement surface to reduce the effects of varying magnetic source depths. Automated interpretation methods that effectively map outlines of magnetic sources are used to characterize the internal structure of the domains and to aid in their delineation. The basement domain map thus derived differs from previous interpretations in the extension of domains further to the southwest, due mainly to the availability of new public-domain magnetic data and the more precise definition of domain boundaries, based on the magnetic source location maps. The Nahanni, Hottah, Chinchaga, Thorsby, Vulcan, and Kiskatinaw domains are weakly magnetic and characterized by magnetic sources that are paramagnetic, comprising low-susceptibility silicate minerals. All other domains are characterized by the presence of ferrimagnetic material, most likely magnetite, which has a sufficiently high susceptibility to produce measurable anomalies. The largest anomalies and magnetizations are found in the Fort Nelson, Fort Simpson, Buffalo Head, Talston, Ksituan, and Matzhiwin domains. Such large magnetizations are usually indicative of intermediate igneous rocks associated with magmatic arc environments. Moderate-amplitude anomalies and (or) magnetizations are characteristic of the Nova, Wabamun, Lacombe, Rimbey, Loverna, and Medicine Hat domains, suggesting the presence of ferrimagnetic basic and granitoid rocks. Within some of the moderately magnetic domains are areas of paramagnetic lithologies that produce no magnetic anomalies. The narrower regions of magnetic lows, such as the Thorsby, Kiskatinaw, and Vulcan domains, are interpreted as resulting from demagnetization effects accompanying collision. Since demagnetization zones are limited in areal extent, the wider, more extensive magnetic lows of the Chinchaga and Hottah domains likely result from a combination of boundary demagnetization and a lower bulk magnetization level of crustal lithologies present.

Columbia River Basalt Plateau—An integrated approach to interpretation of basalt‐covered areas

Geophysics ◽  
1985 ◽  
Vol 50 (12) ◽  
pp. 2709-2719 ◽  
Author(s):  
Corine Prieto ◽  
Carolyn Perkins ◽  
Ernest Berkman
Keyword(s):  
Structural Information ◽  
Gravity Data ◽  
Columbia River ◽  
Integrated Approach ◽  
Rock Properties ◽  
Data Sets ◽  
Rock Composition ◽  
Columbia River Basalt ◽  
Integrated Interpretation ◽  
Gravity Interpretation

An interpretation is presented of a 219 km regional profile which traverses the eastern Columbia River Plateau in Washington State. Aeromagnetic, magnetotelluric (MT), and gravity data were first interpreted separately. All three data sets then were satisfied by a single geologic model. The objective of this case study is to illustrate the individual contributions derived from these three geophysical data sets to a final integrated interpretation. The aeromagnetic interpretation has produced regional structural information and data from which rock compositions can be inferred. The MT interpretation shows that the basalt/sediment interface can be determined, and thus a relative sediment thickness can be inferred. The gravity interpretation is dependent upon an additional method to determine either the basalt or basement horizon. In order for the gravity interpretation to approximate depth to basement or sedimentary thickness, the base of the basalt must be determined from another scientific method. From comparison of the regional structural results of the three geophysical techniques we conclude that aeromagnetic or MT data can be used to determine major structural trends. Reasonable rock compositions are also determined from the combined data sets. The interpreter must be aware of the different rock properties measured by each tool when performing an integrated interpretation; comparisons between the various techniques must be based upon similar assumptions. We recommend that detailed, integrated models be included for a thorough evaluation of any basalt‐covered area. The analysis of rock composition and regional structural information thus derived provides a sound basis for a regional tectonic interpretation and subsequent prospect evaluation.

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