Gravity Data Processing Research Articles

Processing and Interpretation of Seismic Reflection data from Ogaden Basin, Ethiopia: ABSTRACT

The Ogaden basin is believed to be an extensional basin created during the early rifting of Gondwana and the development of the western Indian Ocean. Preliminary studies conducted by various oil companies and national geoscientists suggest that the area is prospective for hydrocarbon accumulation. Prior to these studies, however, very little was known of the surface geology in this area and virtually nothing was known of the subsurface. The objectives of the study were to implement state-of-the-art seismic and gravity data processing and interpretation techniques in order to identify structural and/or stratigraphic trap sequences and indicate their significance in the hydrocarbon exploration of the basin. In this study, high resolution multi-channel reflection seismic data, acquired in 1993 from the Ogaden Basin, are used. The data were acquired for Hunt Oil Company using Vibroseis as an energy source. Various data enhancement techniques were applied to these data in order to produce an interpretable final seismic section for the identification of possible hydrocarbon habitats. These included discrimination and filtering of coherent noise from the signal, enhancing, focussing of the identified reflectors and static corrections. Statics solutions from shallow wells and automatic refraction statics were compared, with the best results obtained when automaticmore » refraction solutions were applied. Our study has revealed various reflection horizons that can be potential stratigraphic traps at pinch outs and may be porous strata which are terminated by lateral transitions or interconnected lines of either elastic sediments or carbonates. We discuss these results in the light of gravity modelling designed to estimate maximum sediment thickness along the line.« less

Data collection in geophysical field measurements. Example: gravity data processing

Data collection in geophysical field measurements. Example: gravity data processing

A refined method of recovering potential coefficients from surface gravity data

A new method for computing the potential coefficients of the Earth's external gravity field is presented. The gravimetric boundary-value problem with a free boundary is reduced to the problem with a fixed known telluroid. The main idea of the derivation consists in a continuation of the quantities from the physical surface to the telluroid by means of Taylor's series expansion in such a way that the terms whose magnitudes are comparable with the accuracy of today's gravity measurements are retained. Thus not only linear, but also non-linear terms are taken into account. Explicitly, the terms up to the order of the third power of the Earth's flattening are retained. The non-linear boundary-value problem on the telluroid is solved by an iteration procedure with successive approximations. In each iteration step the solution of the non-linear problem is estimated by the solutions of two linear problems utilizing the fact that the non-linear boundary condition may be split into two parts; the linear spherical approximation of the gravity anomaly whose magnitude is significantly greater than the others and the non-linear ellipsoidal corrections. Finally, in order to solve the problem in terms of spherical harmonics, the transform method composed of the fast Fourier transform and Gauss Legendre quadrature is theoretically outlined. Immediate data processing of gravity data measured on the physical Earth's surface without any continuation of gravity measurements to a reference level surface belongs to the main advantage of the presented method. This implies that no preliminary data handling is needed and that the error data propagation is, consequently, maximally suppressed.

Integrated Processing of GPS and Gravity Data

Using phase observations of the Global Positioning System (GPS) we are able to determine relative geometric three-dimensional coordinates in the 0.1–1 ppm level. Most surveying needs, however, require orthometric heights, or more generally expressed to answer the question where water flows. This can be achieved by determining a high-precision geoid in the area under investigation, a lengthy task which often is beyond the knowledge of a surveyor. On the other hand, geoid precision equivalent to that of the GPS baseline componemts is still a challenge for physical geodesy. With OPERA 2.4 (Operational Adjustment) and OPUS (Opera Utilization System) an interactive software system is developed which handles all adjustment problems in one, two, or three dimensions. Besides the densification of existing horizontal networks by GPS baseline components it can process in one integrated approach gravity and height information from a data base in order to come up with horizontal ellipsoidal coordinates and orthometric heights replacing the traditional time consuming and expensive spirit leveling. OPERA 2.4 is able to work purely geometrically or can also be used solely for high-precision local geoid determination. Various pre- and postprocessing capabilities like graphical output are built in. As a practical example the GPS subsidence network in Maine is processed and results are compared to other adjustment techniques.

Application of three‐dimensional interactive modeling in gravity and magnetics

Three‐dimensional (3-D) interactive modeling permits integrated processing and interpretation of gravity and magnetic data, yielding an improved geologic interpretation. 3-D model bodies are constructed from polyhedra of suitable geometry and physical parameters (density and susceptibility), input on an interactive graphics terminal that is tied to a host computer. The method is especially designed for concurrent processing and interpretation in an interactive mode. The effect on gravity of a homogeneous polyhedron is calculated by transforming a volume integral into a sum of line integrals. Magnetic effects can be modeled by using either Poisson’s theorem or a slight modification of the formulas derived for gravity modeling. The interactive modeling program allows the user to change the geometry as well as the density and/or susceptibility of the elementary polyhedra and to observe results quickly during the course of processing. This capability enables the interpreter to decide immediately if and where a tentative geologic structure must be changed for the modeled effect to fit that of a field survey. He is able to drive the device‐dependent process by clear menu functions without any knowledge of the rather complicated data structure and the interaction between the main program and its many subroutines. In addition, application of this method requires considerably less computing time than conventional methods based on the direct evaluation of volume integrals.

Variable density Bouguer processing of gravity data from Herault, France

The Herault permit for oil is located in the south of France in very hilly terrain between Mount Aigoual (1500 m) and the sea at Sete. In the preliminary stage of exploration, it was decided that new Bouguer maps, including a variable density one, and a surface density map should be produced. France is covered by a national gravity map published by BRGM on a scale of 1:80000. It was decided that the original data should be recovered and new data acquired to infill the blanks in order to get stations on an average grid of 1.2 by 1.2 km. The existing data had been acquired along roads with a station spacing of 800 to 1200 m, but the road lines were 3 to 6 km apart. Very detailed mining gravity data were also available from the area around Lodeve. After appropriate sampling of the latter data set, 2041 old stations were retained and 619 new stations were planned (Fig. 1). In most of the permit, the new stations were located along tracks or footpaths over the hills, whereas the old grid was located mainly in the valleys. With 2660 stations in total, the number of stations per 100 km2 varies between 60 and 70, and the largest remaining blanks in very inaccessible areas are about 3 by 3 km in size. The data from these stations were processed using a generalised Nettleton method. After isostatic corrections, structures show up much better on the variabie density Bouguer anomaly map, facilitating interpretation. The surface density map shows the low density of the granites, and the high density of the Permian and Jurassic dolomites.

Lotus 123 - a graphical spreadsheet program used for geophysical calculations

With the upsurge of personal computing on microcomputers several spreadsheet programs have been introduced to perform simple business calculations (budgetting, calculation of project costs, etc.). They provide commercial managers with flexible computing facilities without the need to write additional programs or have costly staff available for relatively simple computing problems. The most widely known programs are Visicalc, Multiplan and Lotus 123. The program under discussion, Lotus 123, was originally developed for the IBM-PC (MS-DOS) and compatible 16 bit microcomputers. This article describes the application of the program to filing and simple processing of geophysical data. The following two examples, taken from fieldwork done by students from the State University, Utrecht in southern Portugal during June 1985, will be discussed: (1) processing of VLF data with a five-point first-derivative filter; (2) field processing of gravity data (corrections for instrument drift, height and latitude, and calculation of the Bouguer anomaly). The purpose of this article is to show geophysicists how to use this user-friendly program in their profession. As far as training is concerned, the use of a standard program is profitable. Data entry is always done in the same manner no matter whether, for example, gravity data or VLF data are involved. It is even possible that users are already acquainted with Lotus 123 from other applications, such as business calculations. It is my experience that students with varying computer expertise can handle data entry and processing completely within a few afternoons. A quick start-up of data filing and primary processing can almost be guaranteed. It took me only one afternoon to establish the spreadsheets for both examples after the fieldwork had already begun and the first gathered data were on my desk. Two further days were needed to refine the spreadsheet model, while the field data were already being processed.

A compact on-line data processing system for the Tokyo Surface Ship Gravity Meter.

As the first step of developing a compact surface ship gravity meter, a compact gravity data processing system on-Iine with a Tokyo Surface Ship Gravity Meter (T.S.S.G.) was newly designed and tested at sea. This system was made compact by using a single board computer for gravity computations, a handhold personal computer for data recording, and a Loran C receiver for navigation. Free-air anomalies are obtained by this on-line processing. Over-all accuracy of this data processing is estimated to be 2-3 mgal; if the error from the Eotvos correction is not considered, the accuracy is estimated to be 0.1 mgal. The data processing system was successfully tested on board a research vessel and used for the gravity measurements around Japan in the case of the Japan-France Cooperative Project KAIKO.

Review of gravity and magnetic data processing systems

The application of high speed computer processing and computer graphics is reviewed and discussed in relation to manipulating and present geophysical, and geological mapping data.

Real-Time Digital Computer Acquisition and Computation of Gravity Data at Sea

A system for the automatic acquisition and computation of gravity data at sea has been tested and is now operational aboard the Research Vessel CHAIN of the Wood's Hole Oceanographic Institution. This is the first recorded instance of on-line acquisition and processing of gravity data by the use of a digital computing system. Sampling and computation are performed in a one minute computation cycle making knowledge of gravity values, gravity anomalies and dead-reckoning position available in real-time. The system consists of a LaCoste-Romberg Sea Gravity Meter, data conversion units for computer compatibility with the gravity meter, ship's gyrocompass and ship's speed log, an IBM 1710 Mod 1 Control System (the combination of a 1620 Computer and a 1711 Data Converter for analog to digital conversion), and typewriter and paper tape input-output units.

TOOLS FOR TOMORROW'S GEOPHYSICS *

AbstractThe introduction of methods for recording and processing field data in digital form on magnetic tape for subsequent processing on electronic computers will result in major improvements in geophysical prospecting techniques. This offers great advantages over analogue recording such as increased dynamic range, greater filter flexibility, more convenience in processing by correlation techniques, freedom from distortion and noise, better adaptability to automatic plotting and contouring, and greater suitability for inverse convolution procedures. Digitally recorded seismic signals can be reliably transmitted long distances by telephone circuits to central data processing centers. Electronic computers facilitate processing of gravity, magnetic, and other potential field data. Smoothing, residual, correlation, and derivative techniques for handling data in digital form can enhance speed and flexibility of processing.