GRAVITY FIELD SURVEYS
The basis for the science and practice of gravimetric measurements is that the gravitational attraction of a body of non-homogeneous density varies from point to point, in response to the distribution of density within the body. For this reason, measurements of the variation, with location, of the gravitational attraction of the Earth can provide valuable information about its subsurface geology. Gravity surveys allow geoscientists an indirect way to “see” beneath the Earth’s surface by sensing differences in densities of rocks as reflected in gravity values measured at the surface. Gravity exploration can help locate faults, mineral or petroleum resources, and ground-water reservoirs. Gravity surveys are relatively inexpensive and can quickly cover large areas of ground.
Gravity meters measure all effects that make up the Earth’s gravity field. Many of these effects are caused by known sources, such as the Earth’s shape and totation, distance from the Earth’s center, topographic relief, and tidal variation. Gravity caused by these sources can be calculated using realistic Earth models and removed from the measured data, leaving gravity anomalies caused by unknown sources. To the geologist, the most important unknown source is the effect of the irregular underground distribution of rocks having different densities.
In surveys aimed at exploring for mineral, petroleum and groundwater exploration, relative changes in g between two locations are useful rather than absolute values of g. This instrument used is thus a relative gravimeter that can measure g with a precision of 0.01. A relative gravity measurement is also made at the nearest absolute gravity station, one of a network of worldwide gravity base stations. The relative gravity measurements are thereby tied to the absolute gravity network.
In gravity field surveys, different levels of precision may be required for surveys with different objectives, and therefore the choice of instrument and the survey procedures must be commensurate with this. The level of precision required will determine the field procedures and the types of corrections that are pertinent. The precision of data from a specific gravimetric survey depends upon:
The precision of gravity measurements
Determination of the elevation,
The position of the stations, and
Various corrections that have to be applied to the measurements.
In gravimetric surveys, there are a number of sources of errors/factors that need to be understood and considered. Let us examine the factors affecting the ultimate precision of gravity, and how to ensure that the required precision is achieved.
Shock and vibration
Extreme temperature shocks
Change of batteries
Long and Short-term drifts
Selection of station location
Wind induced vibration
Shock and Vibration: All relative gravimeters, particularly the steel-spring instruments, are subject to offsets (tares) due to shock and to changes in drift rate when subjected to severe vibration for long periods of time. Quartz springs are more tolerant to these factors.
It is recommended that great care be taken in the transport and use of the gravimeter. A soft cushion support for the gravimeter may be necessary so as to reduce the effect of shock and vibration in a vehicle or aircraft. For manual transport over a long distance, a padded backpack should be used. Shocks should be avoided when setting it on the tripod or on the ground between measurements. In case the instrument has been unavoidably, shocked, the last station reading must be repeated in order to determine if a tare has occurred, so that the required correction may be applied.
Power Down: All gravimeters have a critical dependence on the stability of temperature of their sensors. In spring gravimeters, the gravity sensor element is generally enclosed in a temperature stabilized enclosure. The temperature at which the element is stabilized is set at about 10° above the maximum ambient temperature in which the gravimeter is expected to be used. Power-down occurs most often in the shipment of a gravimeter from one place to another.
If your gravimeter is allowed to power-down, so that the temperature of its element changes drastically (a few 10’s of degrees K) then it will experience a severe temperature shock. After restoration of power, whereas the sensor may quickly restore to its set temperature, (in a matter of hours), the physical effects of the shock will have a much longer duration. Typically, the gravimeter may display an anomalous drift rate for a day or more after power-up. To ensure the maximum performance from your gravimeter after a powerdown, it should be set up again and its return to its normal long-term drift should be monitored. The survey ought not be started unless it is ensured that the long-term drift has settled to a reasonable level where it is linear enough with time to be corrected by a combination of base ties and by software.
It is best practice to keep the gravimeter powered on at all times, to avoid the loss of time in re-establishing it.
Extreme temperature shocks: When the gravimeter is moved from one temperature regime to another (i.e. from an air-conditioned room at 25°C to the field at 40°C, or -20°C), it may suffer a temperature shock which can induce a transient response. It is best to avoid unnecessary temperature shocks by keeping the gravimeter overnight in an environment that is essentially at the field temperature.
Elastic relaxation: When the length of a spring balance gravimeter is changed (due to a change in the position of the proof mass from its null position), it does not immediately return to its original length on removal of load. This is due to the elastic relaxation effect. In such a situation, the gravimeter returns most of the way very quickly, then more slowly. It is observed that this effect is of the order of 5 to 10 µGal for 20 minutes of clamping – hardly an issue in regional surveys. But if the clamping time is longer and/or the objectives of the survey demand high precision (microgravity surveys), this must be taken into account. The elastic relaxation effect can be minimized by adopting the following field practices:
1. When not being transported or in use, place the gravimeter on its tripod and level it. The proof mass will then be in its null position minimizing the elastic relaxation effect.
2. If your instrument has had an uncertain history, set it up at your station and take a series of readings every few minutes. Ensure that the readings have stabilized before making measurements.
3. Wait approximately the same period of time (say five minutes) after unclamping the gravimeter at each of the stations before making your measurements.
Leveling: The value of gravity at a station is that which is determined when the measurement system is aligned along the plumbline, i.e. the direction of the gravity vector at the station. When this is so, the force exerted by the proof mass on the spring is a maximum, and, likewise, the force (mechanical or electrostatic) required to bring the proof mass back to its equilibrium level.
If the measurement system is not precisely vertical i.e. along the plumbline, but deviates by an angle ɵ then the measured value of gravity g1 will be reduced, from the true value g, in accordance with the equation:
g1 = gcosɵ
There may be conditions at a station which make it difficult to maintain your gravimeter in a sufficiently level condition during the measurement. Such conditions may include soft ground such as swamp, wet clay, sand, snow and ice. In addition, roots of large trees may underlie the station location and move due to wind on the trees. Additionally, under these same conditions, movement of the operator during the measurement may change the level of the gravimeter.
Gravity measurements on ice, on lakes and sea margins, often suffer from this problem.
Obviously, such adverse locations should be avoided, as much as possible. Where they cannot be avoided the operator must remain absolutely still during the measurement.
Change of Battery: When a gravimeter supply battery is changed in the field, some change in the observed gravity level may result. This relates to the large difference in supply voltage, at least initially, between the two batteries. Such changes, although small, can be significant for microgravity surveys. In such situations, it is advisable to repeat the station last measured. Observe a series of readings in succession at that station, until they have effectively stabilized. This will happen when the battery voltage has essentially stabilized under load.
Long and Short-Term Drifts: All relative, spring-balance gravimeters display long term drift (over periods of days or longer) related to the relaxation of spring tensions and the aging of critical components, mechanical or electronic. Generally, long term drifts are largest when the gravimeter is new, and progressively diminish with age. Therefore when the instrument is new, the long term drift correction should be checked weekly and thereafter monthly, and suitable corrections applied. Short-term drifts can be compensated by a procedure of base station readings — at least at the beginning and the end of each day, and even more frequently in the case of inadvertent rough handling of the instrument during the survey.
A number of sources of possible errors in gravimeter readings are external to the instrument itself. Several steps may be required to be taken to minimize the effects of these.
Seismic Noise: Some level of seismic noise or ground motion caused by natural or man-made factors is always present at any gravity station. Man-made causes of seismic noise include traffic and industrial noise, particularly in urban and industrial areas. Natural sources include earthquakes, microseisms and local earth resonances. Earthquakes can give rise to accelerations of many g’s, causing serious errors in gravimeter readings. A major earthquake occuring on the other side of the earth can produce ground motions causing accelerations exceeding 100 µGals for periods lasting tens of minutes or longer. Other natural sources may include wind-induced vibrations and tree root movements.
Whatever their source, all of these noises are detrimental to the accuracy of gravity measurements. Their presence is readily seen in the increase in the scatter (standard deviation) of repeated measurements at the station.
To compensate for ambient seismic noise, the measurement should be digitally stacked for a sufficient period of time to achieve the desired level of accuracy, as indicated by the convergence of successive mean values.
Selection of Station Location: Measurement of gravity over infirm ground such as swamps, deep snow, sand, ice, proximity to tree roots result in deterioration of data quality. Whenever possible, such locations are to be avoided.
Although terrain effects due to topographic irregularities can be compensated for, such corrections are mere approximations based on the mean density of topographic features. It is best practice to minimize the need for applying such corrections by selecting station locations on relatively flat areas, at least one metre away from any topographic features exceeding 10 cm in height or depth. The edges of cliffs or steep banks should also be avoided.
Wind Induced Vibration: Vibrations caused due to strong winds during measurements will give rise to undesirable and unnecessary noise. this can be eliminated by simply using a windbreak (a device used to shield the instrument from strong winds). A simple windbreak can be fabricated from a suitably sized sheet of tough canvas or plastic fastened to wooden or aluminum poles that can be pitched in the upwind direction of the gravimeter. The instrument can also be shielded by using a large umbrella, or simply standing upwind of it.
Station Elevation: The elevation of each gravimeter station must be determined with a precision. The level of precision is determined by the objectives of the particular survey. A one meter error in elevation will produce an error of 0.2 mGals. Whereas this magnitude of error may be acceptable in most regional surveys aimed at unraveling the geology and structure of an area, this may be too high when looking for mineral deposits which may produce an anomaly of this magnitude.
With the advent of sophisticated positioning systems, it is possible to determkine the elevation to within ± 10 cm accuracy, thus improving the resultant accuracy of the corrected gravity values to about ± 0.03 mGals.
For microgravity surveys, even this accuracy is not adequate when one wishes to achieve gravity data which is reliable to ± 5 μGals. This would require that the relative station elevation be determined to within ±0 2 cm accuracy. Precise optical leveling is required to achieve this kind of accuracy.
Atmospheric Pressure: Changes in atmospheric pressure result in changes in gravity. Atmospheric pressure is thus significant in microgravity surveys. When considered necessary, effects of atmospheric pressure may be corrected by considering readings from static or moving barometers.
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