The following is a guide to help the operator avoid some of the more common problems encountered in conducting a gravity survey. However, the observant and thoughtful operator is always able to further improve his technique, regardless of the months or years of experience he or she has accumulated. For shipping precautions, please refer to that section of the manual.
Prompt reduction of the gravity data allows the operator to know how good or poor his technique is while he still can recall the details of the field work. Thus he can associate a bump to the carrying case or a rough road with irregular and high drift rates. He soon can learn what care in handling the meter must be provided to attain results of a given quality.
One of the fastest methods of learning the quality of the field work and the performance of the meter is to obtain the gravity tide at the time of each base station reading and calculate the tidecorrected drift the meter has just undergone during the time since the previous base reading. The tide correction may be obtained in the field from precalculated tables from GRAVPAC or computed on a handheld computer.
One of the larger sources of error in a field survey comes from large and irregular meter drift. The main cause of the problem is rough treatment of the meter. The more vibration and bumps, the more irregular the drift. Certain vibration frequencies are worse than others. Snowmobiles and helicopters may cause severe shortterm drift. The smoother the ride, the better the meter performance. For very accurate work, it may be necessary to carefully hand carry the meter. If transported in a vehicle, it may be necessary to handhold the meter off the seat and minimize the vibration that is transmitted from the vehicle to the instrument. The selection of a field vehicle with softer suspension is usually worthwhile. A vibration isolation carrying case rack is used by some observers.
If the meter goes above or below its thermostated temperature, there will be intolerable instrumental drift. Each instrument is thermostated at a different temperature. Unless especially designed, the thermostated temperature will be between 47°C and 55°C. It is best to use a meter thermostated at the lower temperatures when working in a cold climate, as it will conserve battery power. It is best to use one thermostated at the higher temperatures when working in a hot climate, as it will be less likely to go over temperature.
When the meter is placed in much colder air, often it will make several loud cracking sounds. This is due to contraction of the fiberglass housing or other superficial parts. Be sure to check the liquid levels after these noises occur.
Spirit levels may change somewhat when the meter is in cold air. They may require readjustment after the meter has been in the cold. Electronic levels are within the insulation and are not affected by changes in ambient air temperature, a distinct advantage when working in cold weather. In severe cold weather, electronic levels are required for accurate and efficient work.
Keep the baseplate cold when operating on ice or frozen ground. A warm baseplate will thaw the ground and it will be impossible to keep the meter level long enough for a reading. If the plate is warm, you may use a board or chips of wood between the baseplate feet and the frozen ground. In snow you may wish to use a special plate with long wooden legs (see options).
In severe cold, the standard battery will not last for a full
day of field work. This is due in part to the extra power
consumed by the meter to keep it at its thermostated temperature.
The main limitation is the battery not releasing as much energy
when it is cold. Try to keep the battery and spare battery warm
so they will yield more energy. With severe cold, some operators
use the power extension cable and a larger battery such as a
sealed motorcycle battery.
NiCad batteries are generally more expensive, have a shorter life and are more troublesome to work with than are gelatin stabilized leadacid batteries. However, they are not affected as severely by cold.
For high precision work in cold climates, a double oven meter option is available.
There are several methods for keeping the meter cool in hot weather. Avoid allowing the sun to shine directly on the carrying case. A towel or cloth with a short slit cut in its middle can be placed over the carrying case and allow the carrying handle to come through the towel. In dry hot climates the towel can be moistened. Evaporation will provide some cooling. Try to minimize or eliminate use of the reading lamp. It adds considerable heat to the system. In severe heat, a block of frozen gelatin in a well sealed plastic pouch can be carried on the battery side of the carrying case. These are commercially available. If condensation accumulates in the carrying case, it should be dried periodically.
It may be necessary to conduct field work at night and in the early morning hours when air temperature is cooler and when radiation from the sun can be avoided.
Leaving a meter in a parked vehicle with closed windows in a hot climate is a likely way to drive the meter over temperature.
The LED reading lamp option will reduce the internal heat generated if the reading lamps must be used.
The semiproportional heater control option allows the minimal amount of energy flow to the meter when the ambient temperature is approaching the thermostated temperature. It is a distinct advantage if the meter is tending to go above its thermostated temperature.
The CPI electronics generate about 1-1/2 watts of heat and are located against the side of the sensor. Half of that heat is generated by a single component on the PC board. This component can be mounted on long wires and placed just below the black lid, keeping the heat away from the sensor.
Each meter is normally thermostated at the temperature which gives the smallest change in reading for a given small change in internal temperature. Under prolonged severe heat, if the above field methods do not work, the thermostated temperature can be set to a higher temperature. This will decrease the accuracy of the meter as small changes in internal temperature from heater cycling or use of the reading lamp will have greater effects on the meter.
However, for regional gravity surveys these errors may be well within the tolerance of the survey.
The procedure for changing the thermostat setting is given in the Meter Details section of this manual.
The continental crust of the earth is an efficient wave guide for seismic surface waves of certain wave lengths. When ocean waves of longer period strike the coast and reflect seaward, the resulting standing waves can massage the continental shelf and initiate these crustal seismic waves. In North America they can travel half way across the continent. Winter storms in the north pacific are the major causes in western and central North America. After severe storms in the distant North Pacific, gravity observations may be difficult in western states and provinces.
Microseisms usually have a period of about 4 to 5 seconds and, because of constructive and destructive interference, usually build up then die away with about five to twelve cycles per packet of waves.
To observe gravity during times of large amplitude microseisms, the variable damping option can be used. If the meter has the CPI option, another equally satisfactory solution is a simple RC filter and external galvanometer or digital multimeter. A third option is to use electrostatic feedback nulling with or without additional filtering. Since the timeconstant of the feedback loop is set near 90 seconds, the system effectively averages high to medium frequency seismic noise. This can average the oscillations of the microseism.
Local earthquakes seldom are a problem during gravity observations. They occur and are gone before they are any inconvenience. This may not be the case with volcanic tremors. In the vicinity of a very active volcano, there may be frequent small tremors. The observer may wait for quiescent periods to observe. If available, the variable damped meter or meter with electrostatic nulling may be of assistance.
Distant earthquakes can delay observations for several hours. This is particularly true for large earthquakes on the opposite side of the earth. Their dispersive waves form a very long wave train when at a great distance from the source. A great earthquake with several major aftershocks can hamper careful gravity observation for days; three days following the 1975 China earthquake, for example.
Often observers have not identified the oscillation of a distant earthquake as such. They often believe the oscillation of the meter to be a malfunctioning of the meter. There is nothing in the design of the meter that could internally provide continuous oscillation. The electrostatic nulling options are an exception. If in doubt about the occurrence of an earthquake, you are welcome to telephone our lab. Usually we have at least one meter recording.
Though the frequencies may be different than those of microseisms, the solutions to observing may be the same; the variable damping , the RC filter on the CPI option, electrostatic nulling option. At higher frequencies and larger amplitudes, the RC filter may be a better solution and the electrostatic nulling option is the best solution. When the heavy damping is engaged on meters with variable damping, there is about 25 times normal damping.
If there is some nonlinearity in the air damper and the vibration is faster in one direction than the other, there may be some error due to "pumping". Pumping is defined as the displacement of the meter's mass above or below a rest equilibrium position due to vibration.
Lateral vibration has little or no effect on the meter. The reason for this insensitivity is explained in the section Physics of the Sensor.
Wind movement of the meter is particularly troublesome. It causes two effects that are difficult to separate. It tips the meter out of plumb, this reducing the observed gravity. It also rocks the meter back and forth causing the beam to oscillate. If practical, place the meter in a location sheltered from the wind. If this is not feasible, place the meter so the wind direction is normal (90°) to the direction of the beam. Averaging the oscillations of the beam or galvanometer will not give accurate results, as the meter is out of level a portion of the time and at those times, is not observing the full force of gravity. Strong wind dictates shelter or curtailing gravity observation. The gravity observer can take advantage of these times to reduce data or help the survey crew establish station positions and elevations.
In selecting the location of a local basestation, a site near the center of the survey is best. Less time is wasted returning to the base and the meter undergoes less vibration traveling to and from the base. It is possible to return to the base more frequently and overall accuracy is improved. If the survey area is large, several local bases can be established. The local bases are tied to the main base or regional base several times.
Try to select quiet locations for bases. Also, select locations that will be easy to relocate years later in case the survey may be extended or tied to other surveys in the region. Good field notes describing bases are an indication of a professional observer.
A sheltered base is a great advantage for closing a loop of observations, if weather becomes severe during the survey.
The standard Model D has a range of 205 to 250 milligals, depending on the meter's calibration factor. This is enough to work over elevation differences of about 1,200 meters (4,000 feet).
If a survey area is mostly at a higher elevation than a base, it might be wise to rerange with the counter set about 1500.00. If the survey area is mostly at a lower elevation than the base, the meter could be reranged with the counter set at about 0500.00.
Once a survey is under way, The meter should not be reranged except at a station or base where gravity has already been measured.
When observing on asphaltic pavement, the standard baseplate will gradually sink into the pavement. In all but the coldest weather, this will prevent the meter from remaining level enough for a reading. If you cannot find a convenient location away from the asphalt, distribute the load of the three legs by placing a board or other flat rigid material between the tripod and the asphalt.
If the tripod is above freezing temperature, it will gradually sink into the ice or frozen soil. Instrument leveling will be difficult or impossible. If convenient, keep the tripod cold; outside the vehicle and out of the sun. If this is not convenient, the tripod can be placed atop a board or other rigid item with a low coefficient of thermal conductivity. Even if the nonmetal item is warm it will not melt an appreciable amount of ice or soil.
When operating near powerful radio transmitters or when twoway field radios are very close to the gravity meter, they can cause the CPI electronics and/or the galvanometer to be affected. Under these field conditions, revert to the use of the microscope to determine beam position. The optical system is not disturbed by a strong EM field.
The standard charger is designed to keep the gravity meter supplied with power when there are failures of the AC power supply. As long as there is a standard L and R battery connected to the charging portion of the charger/eliminator, the meter will remain on power. When AC power goes off, a relay in the charger/eliminator drops and connects the meter to the battery that was charging. When AC power returns, the relay is energized and reconnects the meter to the eliminator (regulated 12volt supply) and reconnects the battery to the charger.
If AC power is not available, the L and R 12volt charger can be used. This optional charger can be powered by unregulated 12volt DC power or by AC power. At remote locations it can be attached to the battery of a vehicle and used to recharge standard L and R batteries. It does not use a great deal of power and should not excessively draw down a vehicle battery overnight. Connecting to the vehicle's power via the cigarette lighter socket is not wise as most modern vehicles ignition switch disconnects the lighter.
Where there is no AC or DC power available, solar/electric cells can be used to recharge the batteries if there is enough sunlight. A simple modification to the standard L and R charger will permit a panel of solar cells to energize the charger.
Leaving the standard leadacid battery in a discharged state for a prolonged period drastically reduces its capacity. The practice of using several standard batteries in remote areas and waiting many days before they can be recharged is workable but costly.
There are many reasons for measuring gravity below the earth's surface. If gravity is measured in a mine drift and on the surface, areas of greater density rock can be located and the optimum location selected to stope the mine.
If the ceiling is not too high, the meter can be placed halfway between the floor and ceiling. The significant local anomaly from the missing mass of the mine will cancel itself at the mid elevation. A special baseplate is available for this purpose. It has a flat surface on the center of its bottom and has a standard threaded hole (5/8inch N.C.) for mounting atop a surveyor's tripod. The baseplate still has the standard three feet and can be used as a normal baseplate.
For more details there are several technical papers with case
histories and more details. A recent paper is:
Casten U., and Gram, Chr., Recent developments in underground gravity surveys, Geophysical Prospecting, Vol. 37, pp 7390, January 1989.
If the ground surface is irregular, large errors are introduced into vertical gradient observations. Where the ground surface is smooth, vertical gravity gradients may be a useful technique for investigating shallow targets. A meter of high precision is required, such as a Model G with the electrostatic nulling option or a Model D. The meter must be in good repair, especially the hysterisis compensator.
A special tripod can be used that has an upper and lower baseplate with a constant separation between the two plates. This has the advantage of not having to carefully measure the difference in elevation between the upper and lower reading. If a special tripod is not available, a standard baseplate can be used on the ground. After finishing the ground reading, the plate can be left in place while a surveyors tripod is positioned above it and the height above the ground baseplate can then be carefully measured. A special tripod baseplate can then be affixed to the tripod and the upper observation made.
If more than one meter is used on a survey, it is best not to intersperse the work of two meters. The effects of any minor irregularities in calibration can be minimized by assigning to each meter its sector of survey area.
There are many considerations in performing a competent periodic gravity survey.