Gravity exploration
2.5 Implementing the gravity method Whatever the objective, applying the gravity method to
imaging the subsurface follows the common sequence of geophysical practice which is described in Section 1.5.
The implementation sequence involves the planning, data acquisition, data processing, and interpretation phases out- lined in Figure 2.7. This section broadly introduces the topic, with more objective-specific details described in Chapters 5, 6, and 7.
2.5.1 Planning phase
Once it is deemed advisable to apply the gravity method to a particular problem, the planning phase is initiated. In this phase the areal size of the survey, data density, and the accuracy of the measurements and their processing are all of interest. The first step in this phase is to develop a model of the subsurface sources of interest. This means that the general location of the sources and their geome- tries, sizes, depths, and density contrasts with the horizon- tally adjacent rock units must be ascertained or estimated.
Commonly the available subsurface information is inade- quate to define these parameters very accurately or there is a range of sources of interest so that a spectrum of models of sources of interest covering the range of uncertainty in the definitive parameters is defined.
All parameters of the sources are of interest, but depth is a particularly significant factor because of the ubiqui- tous role of the inverse distance function in gravity theory (e.g. Equation (3.26)). The density differential is com- monly difficult to estimate because of a paucity of direct density data, particularly on the formations adjacent to the sources of interest. These country rocks generally are poorly studied, and their potential heterogeneity compli- cates the estimation of their density. The lack of direct data typically leads to estimating the densities from gen- eralized tables based on lithologies and a consideration of the controlling characteristics specified in Section 4.5.
In the planning phase one must to estimate the minimal characteristics of the anomalies anticipated in the study.
This can be based on experience in similar projects, but more often involves the approximation of the anticipated sources of anomalies with simplified geometries and the calculation of their anomalies based on estimated densities of the sources and their adjacent formations. The result is a suite of anomaly dimensions and amplitudes that are likely to be encountered.
In addition, the nature of anticipated noise in the grav- ity measurements and anomalies is estimated, again either from experience or by calculation of effects from antici- pated sources. A common source of noise, that is anoma- lies that have wavelengths approximating or less than the wavelengths of sources of interest, results from errors in measuring and processing the gravity field to the anoma- lies. Occasionally these sources also may produce longer- wavelength anomalies due, for example, to slowly varying calibrations of instrumentation. Other important sources of noise are derived from local geological conditions. These latter effects cannot be eliminated, while the former can be minimized through improved measurement and reduc- tion procedures if they are found to be a significant factor
2.5 Implementing the gravity method 35
Statement of problem Densities
-Measures -Estimates Geophysical information
Modeling Define range of subsurface models
Define precision on position control
Define precision and accuracy of observations
and reductions
Define survey size station distribution
and spacing
Perform simple inversion to define source characteristics
Conduct iterative forward modeling to define subsurface
sources Estimate noise -Geological -Observation
-Reduction
Observe gravity, establish base station(s), specify drift control procedures Establish relative
or absolute location and elevation stations
Calculate theoretical gravity at each station and compare with observed
gravity to determine anomaly
Isolate and/or enhance anomalies of interest
Identify anomalies of interest and their sources
Conduct inversion to determine subsurface
sources
Ancillary geological and
geophysical information Report preparation Estimate
regional anomalies
Estimate gravity anomalies
Interpretationphase Dataprocessingphase Planning phaseField measurement phase Geologic
information -Surface -Subsurface
FIGURE 2.7 Flow chart illustrating the principal steps in conducting the gravity method.
36 The gravity method
in analyzing anomalies. The anticipated noise and source anomaly characteristics are used in selecting data density and accuracy requirements of the survey. On the oppo- site end of the spectrum, anomalies due to broader and deeper features than the sources of interest, resulting in longer wavelengths or regional anomalies, generally are estimated from regional anomaly maps of the study area.
Once models of the anticipated sources are specified and their anomalies determined in relation to the expected noise and regional anomalies, the design of the survey begins. This requires specification of the size and location of the survey, the layout of the observations, data density, and the required accuracy of the observations and reduc- tion of the data. Of particular importance is the selection of data density, which is dictated not only by the char- acteristics of sources but also by the intended use of the data.
A survey may be conducted simply to detect poten- tially interesting anomalous values above the noise level for further study or to conduct quantitative analysis of the anomalies. The latter requires a much higher station density. Survey design may consider the probability of detecting an anomaly (e.g.SambuelliandStrobba, 2002) and may incorporate the fractal dimensions of the source anomaly and the network of stations (e.g.Dimri, 1998). Design of the survey means selecting the appro- priate instrumentation and procedures for making the observations and determining collateral information, such as elevation and surrounding topographic relief, for the reduction of the observations.
It is also necessary at this stage to determine the type of anomaly(ies) that will be used in the analysis of the data. The type of anomaly determines the requirements for collateral data used in the calculation. In many cases, gravity data are acquired secondarily to other geophysi- cal data which control the layout of the survey and the distribution of the observation sites. Such is the case with gravity observed along with seismic reflection data in both land and marine environments.
The large and relatively dense data sets covering large regional areas that are increasingly available from govern- mental, commercial, or academic sources may meet the data requirements of the survey, eliminating the need for a gravity observation program. Generally, these data sets consist of the principal facts of each observation including position (latitude and longitude), observed gravity values commonly tied to an absolute datum, elevation, and ter- rain correction that permit the reduction of gravity data to either free-air anomaly or Bouguer anomaly based on an assumption of the mass between the station and the eleva- tion datum. In many areas several data sets are available
covering the same region, but these may not be of equal value because of unequal data density. Care must be exer- cised to select a data set that has erroneous values removed either through a comprehensive quality assurance program or filtering of high-wavenumber components and also has the highest-quality terrain corrections.
In many cases the data sets have (in addition to the prin- cipal facts) the anomaly values at the observation sites, eliminating eliminating the need for any additional pro- cessing before posting and analysis of the anomaly values.
Commonly, the anomaly data sets consist of anomaly val- ues gridded from the point values, providing a uniform distribution of values for ease in presentation and fur- ther processing. Care must be taken to evaluate the grid interval compared to the distribution of the original obser- vations so that aliasing concerns (Appendix A.4.3) are minimized. Where gravity data are available, actual field surveying may be unnecessary. However, care must be taken in deciding to use existing data by first evaluating the density and accuracy of the available data for the purposes of the study. Furthermore, the dense and high-accuracy data required in many surveys for shallow-source studies related to engineering, environmental, and some energy and mineral resource studies are unavailable from public data centers.
2.5.2 Data acquisition phase
In this phase, the survey as designed in the planning stage is implemented. It is not uncommon that the plans are incom- plete or preliminary at the start of the data acquisition phase because of a lack of information on the subsurface con- ditions and the anticipated gravity signatures. As a result initial field measurements may take the form of a test to determine the character of the noise and regional anoma- lies and the anomalies of interest in the study. Evaluation of these measurements, which is facilitated by real-time data reduction and analysis, is used to determine the valid- ity of the initial plans and forms the basis for modifications in the procedures.
The data acquisition phase of field measurements involves observation of the gravity data and, where called for, reoccupation of previously measured sites to deter- mine the time variation, or drift, of the instruments as well as of the gravity field. Specialized field-survey procedures, documentation, data recording facilities, and ancillary data and information are required for conducting surveys under- water, underground, and in ships and aircraft (e.g.Torge, 1989). In these surveys, as well as land-surface surveys, the final step in this phase is to record and store the data in standardized formats in digital form.
2.5 Implementing the gravity method 37 The timing of the gravity survey is important where
gravity surveying will be deployed along with other geo- physical techniques. It should be done early in the process because the method generally is one of the least costly of the geophysical methods on a per-unit-area or line- distance basis. Thus, it can provide useful information for targeting the positioning of the other more expensive and time-consuming geophysical procedures.
2.5.3 Data processing phase
This phase in conducting gravity surveys consists of two basic steps: data reduction, and isolation and enhancement, leading to data that are suitable for interpretation. The field measurements, which commonly consist of the turns on a screw or some variation thereof that are required to bring the mass within a gravimeter to a null position, are converted to gravity units with the calibration unique to the instrument. These observations are then subjected to a broad range of adjustments for the time variation of gravity and to relate them to gravity datums. The resulting values are compared with the theoretical or modeled gravity to produce the gravity anomaly, the difference between the observed gravity and the modeled value at a site.
The process of calculating the gravity anomaly is referred to as reduction, but this should not be construed to consist of reduction of the observations to a datum.
This step involves determining the theoretical gravity at the site, taking into account planetary as well as local effects. A variety of types of anomalies have been devel- oped for specific interpretational purposes. Each takes into account certain effects which may include predicted geo- logical effects. The purpose of the reduction procedure is to convert the field measurements into interpretable form by eliminating all predictable gravity effects leaving only the effects of unknown subsurface mass variations.
After the observed data have been reduced to anomaly form, they are subjected to a process that isolates or enhances the anomalies of interest in the particular sur- vey. The reduced anomaly data consist of a spectrum of anomalies derived from sources covering a range of depths, volumes, geometries, and density contrasts. The cumula- tive effect of the multiplicity of anomalies commonly leads to distortion of the anomalies of interest and difficulties in identifying them.
Isolation and enhancement procedures effectively serve as filters to minimize the effect of short-wavelength noise and long-wavelength regional anomalies upon the anoma- lies of interest in the survey. This filtering commonly is conducted by mathematical digital methods, but in the case of simple extraneous effects the process can be achieved
by graphical procedures. Isolation techniques attempt to delineate the interesting anomalies with a minimum of distortion so as to maintain the integrity of the anomalies for interpretational purposes, whereas enhancement pro- cedures seek to accentuate the anomalies of interest by emphasizing particular attributes of the anomaly such as amplitudes, gradients, and strike directions.
The procedures used in data reduction generally are standardized, with the interpreter only making limited decisions about the parameters involved and the types of factors to be considered. As a result, anomalies cal- culated by different experienced analysts generally are essentially equivalent. The results of the isolation and enhancement process are somewhat different. Experienced analysts often apply specialized filters to anomaly data to isolate or enhance the anomalies of interest, possibly pro- ducing quite different gravity maps or profiles for interpre- tation. The interpreter’s view of the geology of the region and the anomaly sources becomes an important factor in selecting procedures and defining the range of anomalies to isolate or enhance.
2.5.4 Interpretation phase
The interpretational phase of the gravity method consists of making inferences about the subsurface from anomaly values derived from the data processing phase. The pro- cess may be quite involved, using intensive mathemati- cal computations, or may simply consist of a qualitative appraisal of the anomalies. The actual process employed is determined by the complexity of the anomaly pattern, the objective of the survey, the experience of the interpreter, and the available resources.
Interpretation of gravity data usually involves either iterative forward modeling of geologically reasonable sub- surface sources until an approximate match is obtained to the observed anomalies, or inversion wherein the attributes of the sources are determined to some prescribed quan- titative level of error directly from the anomalies. Both procedures can take on various degrees of complexity, but the results of even the most sophisticated methods are not unique. All gravity interpretations are subject to ambiguity, the degree of which is determined by the control that can be exerted on the interpretation from the results of other geophysical methods and related geological information.
The initial interpretation of gravity anomaly data is a physical model or a range of possible models of a specified density contrast with the surrounding Earth materials. In the final stage, this physical model or models must be transcribed into a geological model that will answer the
38 The gravity method
objective of the survey. As a result a strong element of geological expertise is required in gravity interpretation.
2.5.5 Reporting phase
The process is completed with a report on the various phases used in the application. Of particular importance is a clear specification of the assumptions made in the var- ious phases, the error budget, and the optimum interpre- tation of the gravity measurements and the robustness of this interpretation in view of the limitations of the gravity data and the constraints imposed by collateral information.
Computer graphics are very effective and routinely used in promoting gravity data analyses and interpretations.