In a context of global change and increasing anthropic pressure on the environment, monitoring the Earth system and its evolution has become one of the key missions of geosciences. Geodesy is the geoscience that measures the geometric shape of the Earth, its orientation in space, and gravity field. Time-variable gravity, because of its high accuracy, can be used to build an enhanced picture and understanding of the changing Earth. Ground-based gravimetry can determine the change in gravity related to the Earth rotation fluctuation, to celestial body and Earth attractions, to the mass in the direct vicinity of the instruments, and to vertical displacement of the instrument itself on the ground. In this paper, we review the geophysical questions that can be addressed by ground gravimeters used to monitor time-variable gravity. This is done in relation to the instrumental characteristics, noise sources, and good practices. We also discuss the next challenges to be met by ground gravimetry, the place that terrestrial gravimetry should hold in the Earth observation system, and perspectives and recommendations about the future of ground gravity instrumentation. 
Plain Language Summary: In a context of global change and increased human vulnerability to terrestrial hazard, monitoring the Earth system is one of the key challenges of geoscience. In particular, terrestrial gravimetry, with its precision at the level of one part of a billion, allows the monitoring of many phenomena, from water resource availability to volcanic activity. This paper reviews the technique, its advantages and limitations, how it has been used in the Earth monitoring, and the next challenges to be met by ground gravimetry.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1002/2017RG000566

Geophysics From Terrestrial Time-Variable Gravity Measurements

Gravity inversion and uncertainty assessment of basement relief via Particle Swarm Optimization

A very fast simulated annealing (VFSA) global optimization is used to interpret residual gravity anomaly Since, VFSA optimization yields a large number of best-fitted models in a vast model space; the nature of uncertainty in the interpretation is also examined simultaneously in the present study The results of VFSA optimization reveal that various parameters show a number of equivalent solutions when shape of the target body is not known and shape factor ‘q’ is also optimized together with other model parameters The study reveals that amplitude coefficient k is strongly dependent on shape factor This shows that there is a multi-model type uncertainty between these two model parameters derived from the analysis of cross-plots However, the appraised values of shape factor from various VFSA runs clearly indicate whether the subsurface structure is sphere, horizontal or vertical cylinder type structure Accordingly, the exact shape factor (15 for sphere, 10 for horizontal cylinder and 05 for vertical cylinder) is fixed and optimization process is repeated After fixing the shape factor, analysis of uncertainty and cross-plots shows a well-defined uni-model characteristic The mean model computed after fixing the shape factor gives the utmost consistent results Inversion of noise-free and noisy synthetic data as well as field data demonstrates the efficacy of the approach

Interpretation of residual gravity anomaly caused by simple shaped bodies using very fast simulated annealing global optimization

A global particle swarm optimization (GPSO) technique is developed and applied to the inversion of residual gravity anomalies caused by buried bodies with simple geometry (spheres, horizontal, and vertical cylinders). Inversion parameters, such as density contrast of geometries, radius of body, depth of body, location of anomaly, and shape factor, were optimized. The GPSO algorithm was tested on noise-free synthetic data, synthetic data with 10% Gaussian noise, and five field examples from different parts of the world. The present study shows that the GPSO method is able to determine all the model parameters accurately even when shape factor is allowed to change in the optimization problem. However, the shape was fixed a priori in order to obtain the most consistent appraisal of various model parameters. For synthetic data without noise or with 10% Gaussian noise, estimates of different parameters were very close to the actual model parameters. For the field examples, the inversion results showed excellent agreement with results from previous studies that used other inverse techniques. The computation time for the GPSO procedure is very short (less than 1 s) for a swarm size of less than 50. The advantage of the GPSO method is that it is extremely fast and does not require assumptions about the shape of the source of the residual gravity anomaly.

Application of Global Particle Swarm Optimization for Inversion of Residual Gravity Anomalies Over Geological Bodies with Idealized Geometries

A new two-and-a-half dimensional (2.5D) regularized inversion scheme has been developed for the interpretation of residual gravity data by a dipping thin-sheet model. This scheme solves for the characteristic inverse parameters (depth to top z, dip angle θ, extension in depth L, strike length 2 Y, and amplitude coefficient A) of a model in the space of logarithms of these parameters (log(z), log(θ), log(L), log(Y), and log(|A|)). The developed method has been successfully verified on synthetic examples without noise. The method is found stable and can estimate the inverse parameters of the buried target with acceptable accuracy when applied to data contaminated with various noise levels. However, some of the inverse parameters encountered some inaccuracy when the method was applied to synthetic data distorted by significant neighboring gravity effects/interferences. The validity of this method for practical applications has been successfully illustrated on two field examples with diverse geologic settings from mineral exploration. The estimated inverse parameters of the real data investigated are found to generally conform well with those yielded from drilling. The method is shown to be highly applicable for mineral prospecting and reconnaissance studies. It is capable of extracting the various characteristic inverse parameters that are of geologic and economic significance, and is of particular value in cases where the residual gravity data set is due to an isolated thin-sheet type buried target. The sensitivity analysis carried out on the Jacobian matrices of the field examples investigated here has shown that the parameter that can be determined with the superior accuracy is θ (as confirmed from drilling information). The parameters z, L, Y, and A can be estimated with acceptable accuracy, especially the parameters z and A. This inverse problem is non-unique. The non-uniqueness analysis and the tabulated inverse results presented here have shown that the parameters most affected by the non-uniqueness are L and Y. It has also been shown that the new scheme developed here is advantageous in terms of computational efficiency, stability and convergence than the existing gravity data inversion schemes that solve for the characteristic inverse parameters of a sheet/dike.

/pdf/2-5d-regularized-inversion-for-the-interpretation-of-28m1ziby1w.pdf

2.5D regularized inversion for the interpretation of residual gravity data by a dipping thin sheet: numerical examples and case studies with an insight on sensitivity and non-uniqueness

This combination of textbook and reference manual provides a comprehensive account of gravity and magnetic methods for exploring the subsurface using surface, marine, airborne and satellite measurements It describes key current topics and techniques, physical properties of rocks and other Earth materials, and digital data analysis methods used to process and interpret anomalies for subsurface information Each chapter starts with an overview and concludes by listing key concepts to consolidate new learning An accompanying website presents problem sets and interactive computer-based exercises, providing hands-on experience of processing, modeling and interpreting data A comprehensive online suite of full-color case histories illustrates the practical utility of modern gravity and magnetic surveys This is an ideal text for advanced undergraduate and graduate courses and reference text for research academics and professional geophysicists It is a valuable resource for all those interested in petroleum, engineering, mineral, environmental, geological and archeological exploration of the lithosphere

Gravity and Magnetic Exploration: Principles, Practices, and Applications

Gravity and Magnetic Exploration: Introduction

Overview The gravity method of geophysical exploration is based on the measurement of variations in the gravity field caused by horizontal variations of density within the subsurface. It is an important technique for many problems that involve subsurface mapping, and it is the principal method in a number of specific types of geological studies. The method, which has its roots in geodetic studies from the seventeenth to the twentieth centuries, has developed rapidly over the past century as a result of significant technological advances: primarily high-accuracy gravity measuring instruments for use on land, sea, and air, and in space, as well as the increasing computational and graphics power of digital computers. These advances are backed up by continuing improvements in data processing, interpretational schemes, practical experience, and surveying methodologies. Traditionally the gravity method has been used primarily in regional characterization of the Earth for determining the architecture of the crust, identifying potentially favorable regions for resource exploration, and developing conceptual exploration models. This is made possible by the millions of gravity observations now available worldwide in both public and commercial data sets. The accuracy now available in gravity observations and the relative ease of measurement have made the method viable for exploration objectives such as assessing subsurface changes over time and combined interpretation with seismic reflection mapping.

Gravity and Magnetic Exploration: The gravity method

Overview Magnetic interpretation, as in gravity interpretation, operates at several levels of complexity. It can range from simple identification and location of anomalous magnetic bodies in the subsurface to three-dimensional modeling leading to complete characterization of anomaly sources. However, there are numerous differences in the interpretation of these two potential fields. For example, magnetic anomalies in most investigations are primarily caused by contrasting crystalline rocks containing variable amounts of the trace mineral magnetite. This limits the range of possible anomaly sources that need to be considered, although as in all potential field interpretations no interpretation is unique. Magnetic interpretation is complicated by the dipolar nature of the magnetic field, resulting in both attractive and repulsive effects from an anomaly source, and by the large range of variables that enter into determining the character of a magnetic anomaly. Anomaly characteristics vary significantly with the location and orientation of the source in the geomagnetic field and may be further complicated by the effects of remanent magnetization and internal demagnetization. Nonetheless, various interpretational techniques have been developed for dealing with these complications and shown to be successful in identifying and characterizing magnetic sources. Magnetic anomalies are particularly sensitive to their depth of origin, so special emphasis is placed on depth determination methods. Although these methods generally involve simplifying assumptions of theoretical formulations, with care errors commonly can be limited to roughly 10%.

Afif H. Saad

Papers

Gravity and Magnetic Exploration: Principles, Practices, and Applications

Gravity and Magnetic Exploration: Introduction

Gravity and Magnetic Exploration: The gravity method

Gravity and Magnetic Exploration: Magnetic anomaly interpretation

Gravity and Magnetic Exploration: The magnetic method