Gravity Method

Basic Concept

The acceleration of gravity at a point on Earth’s surface results from the combined effects of Earth’s total mass and the subsurface mass differences that occur below. Thus, the gravity method considers how subsurface density variations effect the acceleration of gravity as it is measured along the earth’s surface using a gravimeter. By determining the change(s) in gravitational acceleration, which is a result of gravitational forces, the gravity method can be used to deduce subsurface material variations (Wightman and others, 2003).

The first of the two surface-measurable components of gravitational force is due to the total earth and is relatively uniform along the surface of the earth. The second component is of much smaller size and varies due to the density differential of local subsurface earth materials. The smaller component produces the gravity anomalies of interest and can be used for geologic or structural mapping, mineral exploration, void detection, and determining bedrock depth.

Theory

Newton’s Law of Gravitation suggests that any two objects exert a force on each other. Furthermore, this gravitational force is directly proportional to the product of the two masses and inversely proportional to the square of the distance between the centers of the masses. However, because of the magnitude of the gravitational force exerted on objects by celestial bodies, the gravitational force can be simply described as a product of mass and acceleration.

Thus, a relatively small mass that exists within the gravitational field of a reference mass (i.e., celestial body) will experience an acceleration due to gravity (g). The magnitude of g depends on the mass of the celestial body and distance between the centers of the masses. With respect to Earth, g is approximately 9.81 meters per second squared (m/s²). Ignoring other forces, free-falling objects experience this rate of change of velocity while within Earth’s gravitational field.

In honor of Galileo’s famous experiment in Pisa, g is described in the unit Gal, which equates to 0.01 to m/s². In actuality, because of the gravitational attraction and centrifugal forces that give Earth its oblate-spheroidal shape, g varies with latitude and ranges from 976 to 983 Gal. Due to the small scale of the gravity anomalies with which environmental investigations are concerned, measurements are typically collected in mGal or microGal (Telford, 1990, Burger, 2006).

Gravity surveys are conducted with a gravimeter, which is a tool that typically determines gravitational acceleration using the displacement (i.e., extension or compression) of a spring. This system has the ability to measure changes in the gravitational acceleration as precise as a hundredth of a mGal. Gravimeter data can be qualitatively analyzed, whereby dense subsurface bodies show a positive anomaly, and cavities/voids produce negative anomalies.

However, the detailed interpretation of a gravity survey is limited by ambiguity and the assumption of homogeneity. A distribution of small masses at a shallow depth can produce the same effect as a large mass at depth. External control of the density contrast or the specific geometry is required to resolve ambiguity questions. This external control may be in the form of geologic plausibility, borehole logs, or laboratory measurements (Whiteman and others, 2003).

Applications

Because the scale of the gravity anomalies within environmental applications is generally small (i.e., 1 to 10 m targets), microgravity surveys are typically employed. Microgravity surveys often require station spacings of 1 to 10 m and great care to preserve measurement precision and analysis. For this precise work, relative elevations for all stations need to be established to +/-1 to 2 cm, and, thus, a real-time kinematic (RTK) GPS survey and high station densities are required.

Data corrections are necessary to account for numerous naturally occurring parameters and are detailed in many references (e.g., Telford, 1990, Burger, 2006). The latitude correction deals with latitude-derived variability in gravity, and the free-air correction accounts for the vertical distance of the measurement above sea level. The Bouguer correction considers the attraction of rock material and the measurement station due to elevation, and topographic correction deals with materials with a higher elevation than that of the station.

Additionally, regional-residual separation is performed. This process accentuates the small-scale gravity anomalies and removes the larger, regional gravitational effects on the data. Modern software assists in these data corrections, interpretations, and filtering to meet the many possible survey objectives. In the near-subsurface, rock densities may not vary considerably. However, the contrast between unconsolidated materials (e.g. alluvium, dune sediments, etc.) and hard rock is considerable. Therefore, gravity surveys are quite effective for bedrock mapping in addition to the following applications:

Mapping voids or karst features
Mapping landfills
General geologic mapping for conceptual site models (CSMs)
Gravity variations of groundwater dewatering from pump tests

Examples/Case Studies

Balia, R., and Littarru, B., 2010, Geophysical experiments for the pre-reclamation assessment of industrial and municipal waste landfills: Journal of Geophysics and Engineering, v. 7, no. 1, p. 64-74, doi:10.1088/1742-2132/7/1/006.

Abstract: Two examples of combined application of geophysical techniques for the pre-reclamation study of old waste landfills in Sardinia, Italy, are illustrated. The first one concerned a mine tailings basin and the second one a municipal solid waste landfill; both disposal sites date back to the 1970-80s. The gravity, shallow reflection, resistivity and induced polarization methods were employed in different combinations at the two sites, and in both cases useful information on the landfill’s geometry has been obtained. The gravity method proved effective for locating the boundaries of the landfill and the shallow reflection seismic technique proved effective for the precise imaging of the landfill’s bottom; conversely the electrical techniques, though widely employed for studying waste landfills, provided mainly qualitative and debatable results. The overall effectiveness of the surveys has been highly improved through the combined use of different techniques, whose individual responses, being strongly dependent on their specific basic physical characteristic and the complexity of the situation to be studied, did not show the same effectiveness at the two places.

Debeglia, N., Bitri, A., and Thierry, P., 2006, Karst investigations using microgravity and MASW; Application to Orléans, France: Near Surface Geophysics, v. 4, no.4, p. 215-225, doi:10.3997/1873-0604.2005046.

Abstract: An integrated geophysical approach for detecting and characterizing karst structures in an urban environment was applied experimentally to partially explored karst conduits located in Orléans, France. Microgravity was performed to detect voids, in conjunction with Multi-channel Analysis of Surface Waves (MASW) for the purpose of identifying areas of mechanical weakness. Microgravity detected negative anomalies corresponding to known conduits and succeeded in identifying the probable extensions of this network in unexplored areas. Control boreholes located on these extensions encountered several levels of water-saturated voids, probably belonging to the shallowest part of the karst system, overlying the main conduits. Buried urban networks, accurately located by Ground Penetrating Radar (GPR), were shown to have no significant gravity effect. Simulations using the compact inversion approach to characterize the size and density of environmental disturbances confirmed this conclusion. In this context, the gravity method has been shown to be suitable for detecting near-surface (<25 m deep) karst features. The MASW method, which analyses Rayleigh wave propagation, can determine the mechanical behavior of superficial formations and serve as an indicator for subsurface heterogeneities such as voids or fractures. At the Orléans site, MASW evidenced disturbed zones superimposed on gravity anomalies, characterized by the appearance of several dispersion modes, velocity inversions and the attenuation of seismic markers. One of these features was characterized by low velocities and interpreted as an area of mechanical weakness, confirmed by pressure measurements in the boreholes. Repeated gravity measurements, or time-lapse microgravity, were conducted on the anomalous areas to ascertain gravity reproducibility and detect possible temporal variations due to subsurface mass redistribution that may indicate site instability. A two-year experiment revealed low-amplitude gravity changes that were recorded in the two sensitive zones. However, their meaning is still unclear and these changes need to be validated by further reiterations.

Gehman, C., Harry, D., and Sanford, W., 2006, Measuring Groundwater Storage Change in an Unconfined Alluvial Aquifer Using Temporal Gravity Surveys, in Proceedings, 19^th Annual Symposium on the Application of Geophysics to Engineering and Environmental Problems: Environmental & Engineering Geophysical Society, p. 796-806, doi: 10.4133/1.2923720.

Abstract: Over 140 microGal gravity measurements were collected at a stream augmentation site in northeastern Colorado as part of a study to resolve the temporal and spatial relationship between changes in gravity and water table fluctuations. At this site, mass variations occur when water drawn from an alluvial aquifer is pumped into three upland recharge ponds and returns to the aquifer via infiltration and groundwater flow. Gravity data were collected with a ScintrexnCG-5 Autograv meter during and after aquifer pumping at common station locations. Station spacing was roughly 250 meters over an area ofn3.2 km^2, except at the seven pumping wells where stations were spaced at roughly 25 meters. Comparison of the two gravity data sets shows relative gravity decreases near pumping well locations and relative gravity increases near the recharge ponds during pumping. These changes correspond to cones of depression at the pumping wells and infiltration at the recharge ponds, respectively. For example, a 7.2 meter lowering in water table elevation corresponded to a 42 microGal decrease in gravity at one of the pumping wells. Further detailed study of temporal gravity comparisons may determine additional hydrogeological parameters such as local groundwater flow paths and specific yield.

Nabighian, M.N., Ander, M.E., Grauch, V.J.S., Hansen, R.O., LaFehr, T.R., Li, Y., Pearson, W.C., Peirce, J.W., Phillips, J.D., and Ruder, M.E., 2005, Historical development of the gravity method in exploration: Geophysics, v. 70, no.6, p. 1ND-Z113, doi:10.1190/1.2133785.

Abstract: The gravity method was the first geophysical technique to be used in oil and gas exploration. Despite being eclipsed by seismology, it has continued to be an important and sometimes crucial constraint in a number of exploration areas. In oil exploration the gravity method is particularly applicable in salt provinces, overthrust and foothills belts, underexplored basins, and targets of interest that underlie high-velocity zones. The gravity method is used frequently in mining applications to map subsurface geology and to directly calculate ore reserves for some massive sulfide orebodies. There is also a modest increase in the use of gravity techniques in specialized investigations for shallow targets. Gravimeters have undergone continuous improvement during the past 25 years, particularly in their ability to function in a dynamic environment. This and the advent of global positioning systems (GPS) have led to a marked improvement in the quality of marine gravity and have transformed airborne gravity from a regional technique to a prospect-level exploration tool that is particularly applicable in remote areas or transition zones that are otherwise inaccessible. Recently, moving-platform gravity gradiometers have become available and promise to play an important role in future exploration. Data reduction, filtering, and visualization, together with low-cost, powerful personal computers and color graphics, have transformed the interpretation of gravity data. The state of the art is illustrated with three case histories: 3D modeling of gravity data to map aquifers in the Albuquerque Basin, the use of marine gravity gradiometry combined with 3D seismic data to map salt keels in the Gulf of Mexico, and the use of airborne gravity gradiometry in exploration for kimberlites in Canada.

Wolaver, B.D., Sharp Jr., J.M., and Rodriguez, J.M., 2006, Gravity Geophysical Analysis of Spring Locations in a Karstic Desert Basin, Cuatro Cienegas Basin, Coahuila, Mexico: Gulf Coast of Geological Societies Transactions, v. 56, p. 885-897.

Abstract: This research uses land gravity geophysical surveys to infer subsurface geologic controls on springs in the Cuatro Cienegas Basin of Coahuila, Mexico. Cuatro Cienegas Basin is a National Biosphere Reserve that contains groundwater dependent ecosystems with high species endemism (over 70 local species) in an arid climate. Groundwater discharge from dozens of springs supplies irrigated agriculture and municipal water requirements and links the basin to the Rio Grande. Most Rio Grande flow originates from tributaries in Mexico during droughts in the Rocky Mountains. Effective water resources management depends on sustainable Mexican and Texan transboundary water resource development. Previous studies in the Cuatro Cienegas Basin investigated biologic resources and reconnaissance level hydrogeology but did not explain hydrogeologic controls on spring locations. Springs occur in lines on either side of the Sierra San Marcos carbonate anticline with both hot and cold springs discharging in close proximity. Hydrogeologic cross sections enable the use of classical hydrogeologic models to understand controls on groundwater discharge in regional flow systems like the Cuatro Cienegas Basin. This study uses geophysics to infer subsurface geology beneath Cuatro Cienegas Basin springs to test the hypothesis that spring locations are controlled by subsurface geology. Our initial gravity survey results conducted in January 2006 suggest that groundwater flows along normal faults in some locations and that permeability differences between valley-fill alluvium, alluvial fans, and underlying carbonates is another controlling factor.

References

Burger, H.R., Sheehan, A.F., and Jones, C.H., 2006, Introduction to applied geophysics: Exploring the shallow subsurface: New York, W.W. Norton, 550 p.

Milsom, J., and Eriksen, A., 2013, Field Geophysics (Fourth Edition): Chichester, U.K., Wiley-Blackwell, 304 p.

Telford, W.M., Geldart, L.P., and Sherriff, R.E., 1990, Applied Geophysics (Second Edition): Cambridge, U.K., Cambridge University Press, 793 p.

Wightman, W.E., Jalinoos, F., Sirles, P., and Hanna, K., 2003, Application of Geophysical Methods to Highway Related Problems: Lakewood, CO, Federal Highway Administration, Office of Bridge Technology, 742 p.