Gravity Field How-to

Precautions

You will be operating and handling a LaCoste-Romberg gravity meter. It is extremely sensitive, extremely fragile, and extremely expensive. Be very gentle with it, and above all never move it unless the beam is clamped (clockwise = locked).

The Instrument

The gravity response system consists of a weight on the end of a horizontal beam supported by a zero-length spring (a zero-length spring is defined as one in which the tension is proportiona1 to the actual length of the spring, that is, if all external forces were removed the spring would collapse to zero length).

The gravity meter can detect very small changes in gravity by measuring the restoring force necessary to bring the horizontal beam to a reference position. It is important to note that the instrument does not measure the total force of gravity, only changes in gravity. The accuracy of the instrument is about .01 mgal, perhaps .005 if great care is exercised. The temperature inside the instrument is carefully controlled by a heater and thermostat, because changes in temperature affect the physical properties of the spring.

Taking a reading (G meter):

1) Place baseplate in the ground, approximately level. Try and find a spot away from severe local topography.

2) Place gravimeter on the baseplate. Check that the temperature is correct and that the instrument still has power (flip on switch and see light in bubble level and in gauge).

3) Level gravimeter. Level the meter using the leveling screws located beneath the meter. Move the two screws on the left side of the meter in unison and in opposite directions. Move the screw on the right side by itself. Watch the bubble level.

4) Unlock gravimeter. Release the beam of the gravity meter by turning the arrestment knob counter-clockwise to the limit.

5) Make a reading. Each gravimeter uses a different reading line, so you need to know the one for your gravimeter (ours is 3.2). If the heavy line in the eyepiece is much to the right of 3.2, you need to lower the value; if it is to the left, you need to increase the value. Occasionally the meter can get slightly stuck on one side; if you suspect this is the case, you can gently brush the side of the instrument (a very very light tap). The reading is made when your dark line just touches the measuring line coming from the right side, extinguishing the narrow line of light between the dark line and the measuring line. Too rapid an attempt to make the measurement will result in overshoot. Long, slow movement of the line back and forth can occur from a distant large earthquake--you cannot make a measurement for awhile if this occurs (trains and some trucks can do this too). Check that the meter is still level before proceeding.

Turn the measuring screw clockwise, so that the crosshair moves from left to right. Stop turning it when the crosshair is just to the right of 3.2

Advanced hint: If you know the approximate difference in elevation and latitude from your last station, you can guess a starting value equal to the last measurement (in mGal) - 0.2 * (new elevation - old elevation in m) + 0.8 * (distance in km new station is north of old near 40°N). This can speed things up as the approximate starting point can be dialed in while the meter is locked.

6) Report the reading. You will get seven digits from the meter. Read the first four from the counter, and the last three from the dial on the measuring screw. For example, if the counter reads 26543 and the dial setting is 364, the meter is reading 2654.364. To convert the counter units to milligals, multiply by 1.0554.

7) Lock the instrument, dial up about 10 mGal, then unlock the instrument and repeat (5) & (6) twice for a total of three readings.

8) LOCK THE INSTRUMENT. Double-check this. Turn off the instrument light. Pack up the instrument in its case, making sure the power cable is not caught in the lid.

When carrying the gravimeter, you should put two hands under the case. Do NOT rely on the handle on top--the clasps occasionally undo themselves.

Log entries:

The beginning of the log should include the names and duties of the field party (e.g., "John Jones scribe, Bob Smith reader, Jim Brown GPS"), the id of the instrument being used (including any GPS unit, if applicable), the time zone being used, and the date. A brief summary of the day's agenda doesn't hurt.

For each station, include the station id, the time, location information (GPS file name if doing differential GPS, an estimate of lat and lon and elevation from GPS if applicable, elevation and position difference from previous station if applicable), meter temperature, the measurements from the reader. An estimate of B and C ring terrain corrections should be made if necessary. The location of the station should be placed on a map as precisely as possible even if post processing of GPS is anticipated. For GPS measurements, UTM northing and easting can be a help in getting our elevations from a digital elevation model. Making each gravity station a waypoint in the handheld GPS can also work well.

Terrain corrections:

Effects from terrain within about 50 m need to be estimated in the field (see p. 331-335 of Burger or p. 31 of Milsom). For our level of accuracy, variations from the base under about 2 m within zone B (2-16 m) and 7 m in zone C (16-50 m) need not be noted. When necessary, either a small sketch of rings B and C as in a Hammerchart on p. 31 of Milsom should be drawn with topographic differences indicated (in m), or if on a planar slope, the dip angle in that distance range can be indicated.