A BASIC program for 2-D spectral analysis of gravity data and source-depth estimation

Tilt angle filter is an interpretation method that is used to determine the source borders locations from potential fields data. Moreover, the tilt angle is applied for estimation of the anomaly source depth, such as contact-depth method and tilt-depth method. In this paper an application of the tilt angle technique obtained from the first vertical and horizontal gradients of the gravity anomaly from semi-infinite vertical cylindrical source is described. The technique is based on the tilt angle and derivatives ratio. In this approach the depth estimates are proportional to the computed tilt angles and their distances from the cross section center of the anomaly cause on the surface. This new method is termed the tilt-distance-depth (TDD). The method is demonstrated using synthetic gravity data, with and without random noise, and real gravity data from Iran. The results are also compared with the solutions from Euler deconvolution technique and inverse modelling using Modelvision software.

Download Full-text

A review on spectral analysis of magnetic data for depth estimation

Geophysics ◽

10.1190/geo2020-0268.1 ◽

2021 ◽

pp. 1-87

Author(s):

Y. Kelemework ◽

M. Fedi ◽

M. Milano

Keyword(s):

Spectral Analysis ◽

Gravity Data ◽

Window Size ◽

Depth Estimation ◽

Critical Parameters ◽

Estimation Methods ◽

Magnetic Data ◽

Moho Depth ◽

Exponential Factor ◽

Temperature Isotherm

Spectral analysis has been used for studying a variety of geological structures and processes, such as estimation of the depth to the crystalline basement or of the Curie temperature isotherm from magnetic anomalies. However, the analysis is not standard, as it refers to different theoretical frameworks, such as statistical ensembles of homogeneous sources and uncorrelated or fractal random distributed sources. In this review, we aim to unify the approaches by reformulating all the common spectral expressions in the form of a product between a depth-dependent exponential factor and a factor, which we call the spectral correction factor, that incorporates all of the a priori assumptions for each method. This kind of organization might be useful for practitioners to quickly select the most appropriate method for a given study area. We also establish a new formula for extending the Spector and Grant method to the centroid depth estimation. Practical constraints on the depth estimation and intrinsic assumptions/limitations of the different approaches are examined by generating synthetic data of homogenous ensemble sources, random and fractal models. We address the statistical uncertainty of depth estimates using ordinary error propagation on the spectral slope. Critical parameters, such as the window size, are also analyzed in terms of the type of method used and of the geological complexity. We find that the window size is smaller for the centroid/modified centroid methods and larger for the spectral peak, de-fractal, and nonlinear parameter depth estimation methods. In any case, the window size can be large in tectonically stable regions and relatively small over volcanically, tectonically, and geothermally active areas. We finally estimate and discuss the depth to magnetic top and bottom in the Adriatic Sea region (eastern Italy) in the context of heat flow, Moho depth, and gravity data of the region.

Download Full-text

Gravity Analysis for Subsurface Characterization and Depth Estimation of Muda River Basin, Kedah, Peninsular Malaysia

Applied Sciences ◽

10.3390/app11146363 ◽

2021 ◽

Vol 11 (14) ◽

pp. 6363

Author(s):

Muhammad Noor Amin Zakariah ◽

Norsyafina Roslan ◽

Norasiah Sulaiman ◽

Sean Cheong Heng Lee ◽

Umar Hamzah ◽

...

Keyword(s):

River Basin ◽

Rock Type ◽

Gravity Data ◽

Power Spectral Analysis ◽

Depth Estimation ◽

Structural Features ◽

Peninsular Malaysia ◽

Gravity Survey ◽

Basement Rock ◽

Geophysical Techniques

Gravity survey is one of the passive geophysical techniques commonly used to delineate geological formations, especially in determining basement rock and the overlying deposit. Geologically, the study area is made up of thick quaternary alluvium deposited on top of the older basement rock. The Muda River basin constitutes, approximately, of more than 300 m of thick quaternary alluvium overlying the unknown basement rock type. Previous studies, including drilling and geo-electrical resistivity surveys, were conducted in the area but none of them managed to conclusively determine the basement rock type and depth precisely. Hence, a regional gravity survey was conducted to determine the thickness of the quaternary sediments prior to assessing the sustainability of the Muda River basin. Gravity readings were made at 347 gravity stations spaced at 3–5 km intervals using Scintrex CG-3 covering an area and a perimeter of 9000 km2 and 730 km, respectively. The gravity data were then conventionally reduced for drift, free air, latitude, Bouguer, and terrain corrections. These data were then consequently analyzed to generate Bouguer, regional and total horizontal derivative (THD) anomaly maps for qualitative and quantitative interpretations. The Bouguer gravity anomaly map shows low gravity values in the north-eastern part of the study area interpreted as representing the Main Range granitic body, while relatively higher gravity values observed in the south-western part are interpreted as representing sedimentary rocks of Semanggol and Mahang formations. Patterns observed in the THD anomaly and Euler deconvolution maps closely resembled the presence of structural features such as fault lineaments dominantly trending along NW-SE and NE-SW like the trends of topographic lineaments in the study area. Based on power spectral analysis of the gravity data, the average depth of shallow body, representing alluvium, and deep body, representing underlying rock formations, are 0.5 km and 1.2 km, respectively. The thickness of Quaternary sediment and the depth of sedimentary formation can be more precisely estimated by other geophysical techniques such as the seismic reflection survey.

Download Full-text

Spectral analysis and Euler deconvolution technique of gravity data to decipher the basement depth in the Dehradun-Badrinath area

Journal of the Geological Society of India ◽

10.1007/s12594-014-0077-3 ◽

2014 ◽

Vol 83 (5) ◽

pp. 501-512 ◽

Cited By ~ 5

Author(s):

G. K. Ghosh ◽

C. L. Singh

Keyword(s):

Spectral Analysis ◽

Gravity Data ◽

Euler Deconvolution ◽

Deconvolution Technique ◽

Basement Depth

Download Full-text

Source depth estimation using multi-station waveform data

Bulletin of the Seismological Society of America ◽

10.1785/bssa0740051623 ◽

1984 ◽

Vol 74 (5) ◽

pp. 1623-1643

Author(s):

Falguni Roy

Keyword(s):

Depth Estimation ◽

The United States ◽

Digital Data ◽

United States Geological Survey ◽

Travel Times ◽

Source Depth ◽

Time Data ◽

Waveform Data ◽

Level Ii ◽

Level I

Abstract A depth estimation procedure has been described which essentially attempts to identify depth phases by analyzing multi-station waveform data (hereafter called level II data) in various ways including deconvolution, prediction error filtering, and spectral analysis of the signals. In the absence of such observable phases, other methods based on S-P, ScS-P, and SKS-P travel times are tried to get an estimate of the source depth. The procedure was applied to waveform data collected from 31 globally distributed stations for the period between 1 and 15 October 1980. The digital data were analyzed at the temporary data center facilities of the National Defense Research Institute, Stockholm, Sweden. During this period, a total number of 162 events in the magnitude range 3.5 to 6.2 were defined by analyzing first arrival time data (hereafter called level I data) alone. For 120 of these events, it was possible to estimate depths using the present procedure. The applicability of the procedure was found to be 100 per cent for the events with mb > 4.8 and 88 per cent for the events with mb > 4. A comparison of level I depths and level II depths (the depths as obtained from level I and level II data, respectively) with that of the United States Geological Survey estimates indicated that it will be necessary to have at least one local station (Δ < 10°) among the level I data to obtain reasonable depth estimates from such data alone. Further, it has been shown that S wave travel times could be successfully utilized for the estimation of source depth.

Download Full-text