5.1 Concepts-This guide summarizes the equipment, field procedures, and interpretation methods used for the determination of subsurface conditions due to density variations using the gravity method. Gravity measurements can be used to map major geologic features over hundreds of square miles and to detect shallow smaller features in soil or rock. In some areas, the gravity method can detect subsurface cavities.
5.1.1 Another benefit of the gravity method is that measurements can be made in many culturally developed areas, where other geophysical methods may not work. For example, gravity measurements can be made inside buildings; in urban areas; and in areas of cultural, electrical, and electromagnetic noise.
5.1.2 Measurement of subsurface conditions by the gravity method requires a gravimeter (Fig. 1) and a means of determining location and very accurate relative elevations of gravity stations.
5.1.2.1 The unit of measurement used in the gravity method is the Gal (in honor of Galileo), based on the gravitational force at the Earth's surface. The average gravity at the Earth's surface is approximately 980 Gal. The unit commonly used in regional gravity surveys is the mGal (10−3 Gal). Typical gravity surveys for environmental and engineering applications require measurements with an accuracy of a few μGals (10−6 Gals), they are often referred to as microgravity surveys.
5.1.2.2 A detailed gravity survey typically uses closely spaced measurement stations (a few meters to approximately 100 meters) and is carried out with a gravimeter capable of reading to a few μGals. Detailed surveys are used to assess local geologic or structural conditions.
5.1.2.3 A gravity survey consists of making gravity measurements at stations along a profile line or grid. Measurements are taken periodically at a base station (a stable noise-free reference location) to correct for instrument drift.
5.1.3 Gravity data contain anomalies that are made up of deep regional and shallow local effects. It is the shallow local effects that are of interest in microgravity work. Numerous corrections are applied to the raw field data. These corrections include latitude, free air elevation, Bouguer correction (mass effect), Earth tides, and terrain. After the subtraction of regional trends, the remainder or residual Bouguer gravity anomaly data may be presented as a profile line (Fig. 2) or on a contour map. The residual gravity anomaly map may be used for both qualitative and quantitative interpretations. Additional details of the gravity method are given in Telford et al (5); Butler (6); Nettleton (7); and Hinze (8).
5.2.2 Table 1 shows that densities of sedimentary rocks are generally lower than those of igneous and metamorphic rocks. Densities roughly increase with increasing geologic age because older rocks are usually less porous and have been subject to greater compaction. The densities of soils and rocks are controlled, to a very large extent, by the primary and secondary porosity of the unconsolidated materials or rock.
5.2.3 A sufficient density contrast between the background conditions and the feature being mapped must exist for the feature to be detected. Some significant geologic or hydrogeologic boundaries may have no field-measurable density contrast across them, and consequently cannot be detected with this technique.
5.2.4 While the gravity method measures variations in density in earth materials, it is the interpreter who, based on knowledge of the local conditions or other data, or both, must interpret the gravity data and arrive at a geologically reasonable solution.
5.3 Equipment:
5.3.1 Geophysical equipment used for surface gravity measurement includes a gravimeter, a means of obtaining position and a means of very accurately determining relative changes in elevation. Gravimeters are designed to measure extremely small differences in the gravitational field and as a result are very delicate instruments. The gravimeter is susceptible to mechanical shock during transport and handling.
5.3.2 Gravimeter-The gravimeter must be selected to have the range, stability, sensitivity, and accuracy to make the intended measurements. Many gravimeters record digital data. These instruments have the capability to average a sequence of readings, to reject noisy data, and to display the sequence of gravity measurements at a particular station. Electronically controlled gravimeters can correct in real time for minor tilt errors, for the temperature of the instrument, and for long-term drift and earth tides. These gravimeters communicate with computers, printers, and modems for data transfer. Kaufmann (10) describes instruments suitable for microgravity surveys. A comprehensive review of gravimeters can be found in Chapin (11).
5.3.3 Positioning-Position control for microgravity surveys should have a relative accuracy of 1 m or better. The possible gravity error for horizontal north-south (latitude) position is about 1 μGal/m at mid-latitudes. Positioning can be obtained by tape measure and compass, conventional land survey techniques, or a differential global positioning system (DGPS).
5.3.4 Elevations-Accurate relative elevation measurements are critical for a microgravity survey. A nominal gravity error of 1 μGal can result from an elevation change of 3 mm. Therefore, elevation control for a microgravity survey requires a relative elevation accuracy of about 3 mm. Elevations are generally determined relative to an arbitrary reference on site but can also be tied to an elevation benchmark. Elevations are obtained by careful optical leveling or by automatic digital levels.
5.4 Limitations and Interferences:
5.4.1 General Limitations Inherent to Geophysical Methods:
5.4.1.1 A fundamental limitation of all geophysical methods is that a given set of data cannot be associated with a unique set of subsurface conditions. In most situations, surface geophysical measurements alone cannot resolve all ambiguities, and some additional information, such as borehole data, is required. Because of this inherent limitation in the geophysical methods, a gravity survey alone can never be considered a complete assessment of subsurface conditions. Properly integrated with other geologic information, gravity surveying is a highly effective, accurate, and cost-effective method of obtaining subsurface information.
5.4.1.2 In addition, all surface geophysical methods are inherently limited by decreasing resolution with depth.
5.4.2 Limitations Specific to the Gravity Method:
5.4.2.1 A sufficient density contrast between the background conditions and the feature being mapped must exist for the feature to be detected. Some significant geologic or hydrogeologic boundaries may have no field-measurable density contrast across them, and consequently cannot be detected with this technique. An interpretation of gravity data alone does not yield a unique correlation between possible geologic models and a single set of field data. This ambiguity can only be resolved through the use of sufficient supporting geologic data and by an experienced interpreter.
5.4.2.2 Interferences Caused by Ambient, Geologic, and Cultural Conditions:
(1) The gravity method is sensitive to noise (vibrations) from a variety of natural ambient and cultural sources. Spatial variations in density caused by geologic factors may also produce unwanted noise.
(2) Ambient Sources of Noise-Ambient sources of noise include earthquakes, microseisms, tides, winds, rain, and extreme temperatures.
(a) Earthquakes-Local earthquakes seldom are a problem during gravity observations. They occur and are gone before they are any inconvenience. Distant earthquakes however, can lead to gravity changes of 100 μGals or more with periods of tens of minutes or more. These effects can delay gravity observations for several hours or even days.
(b) Microseisms-Microseisms are defined as feeble earth tremors due to natural causes such as wind, water, or waves (Sheriff (1)). They are believed to be related to wave action on shorelines and to the passage of rapidly moving pressure fronts whose effects are seen as sinusoidal variations in the gravity data. Their amplitude can readily exceed several tens of μGals.
(c) Earth Tides-Solar and lunar tides affect the force of gravity at the Earth's surface by as much as 300 μGals with a rate of change as large as 1 μGal/min. These solid earth tides are predictable and can be corrected for as a part of gravity data correction procedures.
(d) Wind and Rain-Wind and heavy rain can cause movement of the gravimeter. The gravimeter should be shielded from the wind and rain.
(e) Extreme Temperatures-Extreme temperature changes over short periods of time can cause instrument drift. In order to minimize this effect, the gravimeter should be insulated from extreme heating or cooling. Slow gradual changes in temperature are normally accommodated by repeat base station measurements and drift corrections made as a normal part of the gravity survey.
(f) Geologic Sources of Noise-Geologic sources of noise may include unknown variations in the natural spatial distribution of soil and rock and their densities.
(g) Topography-Hills, mountains, and valleys affect gravity measurements. Depending on the objectives of the survey, topographic corrections may be needed (Hinze (8)).
(h) Cultural Sources of Noise-Cultural sources of noise include vibration from vehicles, heavy equipment, trains, and even persons walking near the gravimeter.
5.4.3 Summary-During the course of designing and carrying out a gravity survey, the sources of ambient, geologic, and cultural noise must be considered and time of occurrence and location noted. The exact form of the interference is not always predictable because it depends upon the type and magnitude of noise and distance from the source of noise.
5.5 Alternate Methods-In some cases, the factors previously discussed may prevent the effective use of the gravity method, and other geophysical (Guide D6429) or non-geophysical methods may be required to investigate subsurface conditions.
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