Determination of Earth’s Crust Movement at Kwoi, Kaduna State Using Gravity Method

Abstract

The earth’s crust is deforming due to plate tectonic which causes earth tremor. Monitoring of this deformation is essential for the preservation of life and property. Gravity Survey plays an important role in crustal deformation monitoring which means a change in geometric configuration whose monitoring problem is to determine the spatial relationship of a set of object point relative to a number of reference points. The aim of this study is to determine the movement of the Earth’s crust at Kwoi, Kaduna State and its environs, using Gravimetric method in other to monitor the effect of Earth tremor that occurs within the study area and the above aim were achieved through the following objectives: acquisition of GNSS and gravity data of the study area, the reductions of the observed gravity data, determination and analyses of the GNSS and gravity data using Trimble business center and Gravsoft and conducting some test using necessary statistics. The gravity data acquisition was done using LaCoste and Romberg (G664) gravimeter while The GNSS field data acquisition (Observation) was done on a Static mode for minimum of thirty minutes per session using Trimble R8 Model 3 dual frequency GNSS receiver. Trimble Business Centre (TBC) software with its accessories were used for the processing of the coordinate data to obtain the adjusted latitude, longitude and ellipsoidal height of the study area, Gravsoft works with Notepad to process raw gravity data, to correct for errors and to also obtain the gravity value of the study area, Microsoft Excel were used to average the gravity data before using Gravsoft to process. The results of this work shows the adjusted coordinates of all the stations, the gravity value, gravity anomaly, free air anomaly and bouguer anomaly of the stations which were observed in three different epoch. The comparative analyses of the three epochs were done and represented using a graph though there exist a minimal change between the epochs which needs constant monitoring or observation to avoid future ocurance of natural disaster on life and properties. Keyword: Tremor, Gravity, Earth Tremor, Gravsoft, Global Navigation Satellite System, GNSS, Geoid Model, Ellipsoidal earth. 1.0 INTRODUCTION Gravity is a potential field; it is a force that acts at a distance (Mariita 2007). The gravity method is a non-destructive geophysical technique that measures differences in the earth’s gravitational field at specific locations. It has found numerous applications in engineering, environmental and geothermal studies including locating voids, faults, buried stream valleys, water table levels and geothermal heat sources. The success of the gravity method depends on the different earth materials having different bulk densities (mass) that produce variations in the measured gravitational field. These variations can then be interpreted by a variety of analytical and computers methods to determine the depth, geometry and density that causes the gravity field variations. For better definition of the bodies causing the perturbations in the gravity field, the gravity data should be collected with small stations spacing, such as 1 km. For engineering investigations this may be as low as 5 meters or less. In addition, gravity station elevations must be determined to within 0.2 meters. Using the highly precise locations and elevations plus all other quantifiable disturbing effects, the data are processed to remove all these predictable effects. The most commonly used processed data are known as Bouguer gravity anomalies, measured in mGal. The interpretation of Bouguer gravity anomalies ranges from just manually inspecting the grid or profiles for International Journal of Scientific and Research Publications, Volume 11, Issue 4, April 2021 366 ISSN 2250-3153 This publication is licensed under Creative Commons Attribution CC BY. http://dx.doi.org/10.29322/IJSRP.11.04.2021.p11250 www.ijsrp.org variations in the gravitational field to more complex methods that involves separating the gravity anomaly due to an object of interest from some sort of regional gravity field. From this, bodies and structures can be inferred which may be of geothermal interest. Volcanic centres, where geothermal activity is found, are indicators of cooling magma or hot rock beneath these areas as shown by the recent volcanic flows, ashes, volcanic domes and abundant hydrothermal activities in the hot springs. Gravity studies in volcanic areas have effectively demonstrated that this method provides good evidence of shallow subsurface density variations, associated with the structural and magmatic history of a volcano. The earth’s terrain is not at rest but moves slowly due probably to the nature of the earth itself and the underground man-made conditions of the earth’s crust. It is, therefore, very important to be sure of the movements of earth that serve human life. Hence, a lot of studies for determining and analyzing the earth movement are often implemented. During these studies, the used measurement techniques and systems, which could be geodetic or non-geodetic, are determined considering the movement of the earth, its environmental conditions, and the expected accuracy from the measurements. A few causes of such movements are volcanic eruption which moves downwards on a sloping rock formation, ultimately causing a landslide; change of ground water level, which may result in compression (compaction or consolidation) of the interceded layers of clay and silt the aquifer system; drainage and oxidation of organization soils; dissolution and collapse of susceptible rocks; tidal phenomena; tectonic phenomena; etc. Some serious effects of crustal deformation include loss of level free board and subsequent reduction in flood protection, change in gradient along water conveyance canals, and collapse of engineering structures (Richardus, 1977). Monitoring and analyzing deformations of engineering structures (such as dams, bridges, viaducts, high-rise buildings, etc.) constitutes a special task for geodesists. There are several techniques for measuring the deformations. Geodetic techniques, through a network of points interconnected by angle and/or distance measurements, usually supply a sufficient redundancy of observations for the statistical evaluation of their quality and for detection of errors. They give global information on the behaviour of the deformable structure, while the non-geodetic techniques give localized and locally disturbed information without any check unless compared with some other independent measurements. It is against this background that the determination of earth movement within Kwoi, Kaduna state using gravity data became necessary to give more clear view about the geodynamics of the study area and take into account, the relation between gravity changes and seismic activity. The occurrence of the natural disaster (earth tremor) in some part of the country(Kwoi, Ibadan, Abuja and Ijebu-ode) which is the quivering or vibration movement on ground due to plates tectonics possess a demand for gravity survey for effective monitoring and controlling of crustal movement within the study area and also for the protection of life and property. 2.0 LITERATURE REVIEW 2.1 THEORETICAL FRAMEWORK 2.1.1 Brief Overview of Gravity Gravity is most accurately described by the general theory of relativity (proposed by Albert Einstein in 1915) which describes gravity not as a force, but as a consequence of the curvature of space-time caused by the uneven distribution of mass. The most extreme example of this curvature of space-time is a black hole, from which nothing—not even light—can escape once past the black hole s event horizon. However, for most applications, gravity is well approximated by Newton s law of universal gravitation, which describes gravity as a force which causes any two bodies to be attracted to each other, with the force proportional to the product of their masses and inversely proportional to the square of the distance between them. Gravity surveying uses gravity meters to measure very small variations in the earth’s gravity field. Observations are corrected for instrument drift, earth tide, latitude, station height and the effect of surrounding terrain. Other lesser impacts on gravity observations include atmospheric pressure, ocean loading, polar motion, the height of the water table and the amount of soil moisture. As appropriate to the objectives, precision and accuracy of a survey, all of these effects may be removed using rigorous observational procedures, standardized models and computations. Once the various corrections have been applied to gravity observations, the reduced data is essentially a reflection of variations in the gravity field directly related to changes in rock density and structural architecture. 2.1.2 Theory of Gravity International Journal of Scientific and Research Publications, Volume 11, Issue 4, April 2021 367 ISSN 2250-3153 This publication is licensed under Creative Commons Attribution CC BY. http://dx.doi.org/10.29322/IJSRP.11.04.2021.p11250 www.ijsrp.org The theory of gravity surveying is directly dependent on Newton’s law of gravity: According to Newton, gravitational attraction was caused by an unexplained magnetic force that acted across space and caused mutual attraction between two bodies. Einstein rejected the ideas of Newton. According to the interpretation of the gravitational field which prevailed in the 20th century, gravity was a ‘physical phenomenon’ in that it was generated by ‘masses of matter’. What was supposed to have happened was that matter ‘curved and warped space-time’ and when physical masses of this matter were ‘falling’ or were in orbit, they were actually following a straight line in this curved space time which created stretched holes in the ‘fabric of space-time’. Thus we must infer that somehow space must be composed of a ‘fabric’ of some sort, although no one has eve