Questions: Gravimeter Types, Calibration, and Field Operations

Question 1

A gravity survey uses a relative gravimeter and returns to a base station four times throughout the day in addition to visiting field stations. What is the primary purpose of these repeated base-station occupations?

• A. To improve GPS positioning accuracy by averaging multiple readings at a well-surveyed location
• B. To characterize and remove the instrument's drift — the slow creep of the spring that causes readings at a fixed location to change over time even when gravity does not
• C. To verify that the base station has the highest gravity value in the survey area, ensuring it is a valid reference point
• D. To average out random measurement noise by accumulating multiple readings at the highest-precision site

Answer: (B)

Spring-based relative gravimeters drift: the spring slowly relaxes (viscous creep), causing the instrument to read different values at the same station across hours even when gravity hasn't changed. Returning to the base station provides known time-stamped readings; the difference between successive base-station readings divided by the elapsed time gives the drift rate. This drift is then interpolated across the intervening time and subtracted from field readings. Without base-station reoccupations, drift would be indistinguishable from real geological gravity signals. Option D is a misconception: averaging multiple readings at one site reduces random noise, but that is not the purpose of cycling back to the base station throughout the day.

Question 2

For which application would you prefer an absolute gravimeter over a relative gravimeter, despite the absolute instrument being heavier, slower, and more expensive?

• A. A rapid regional survey requiring 200 stations covered in a week
• B. Monitoring gravity changes at a volcano over many years to detect subsurface magma movement
• C. Establishing a benchmark station with a well-known absolute gravity value to anchor a regional relative survey network
• D. A marine gravity survey conducted from a moving ship

Answer: (C)

Absolute gravimeters determine the actual value of g from first principles (free-fall timing), need no external reference, and do not drift. This makes them ideal for establishing benchmark stations that anchor entire survey networks — all relative measurements in the region can be tied to the absolute value at the benchmark. For rapid dense surveys (option A), the slow measurement rate of absolute instruments (minutes per reading) makes them impractical; relative gravimeters cover many more stations per day. For volcanic monitoring over years (option B), both types are used, but if the monitoring network needs to be self-consistent across decades without drift accumulation, absolute gravimeters have advantages for periodic reoccupation.

Question 3

A relative gravimeter can determine the absolute value of gravitational acceleration at a field station with the same precision it uses to measure gravity differences.

• T. True
• F. False

Answer: False

A relative gravimeter measures gravity *differences* between stations — its output is a reading on a scale, not an absolute acceleration in m/s². To convert these differences to absolute values, the instrument must be tied to at least one station where absolute g is already known (a benchmark established by an absolute gravimeter). The precision of the difference measurement (microgals) is maintained in the final absolute values, but the absolute value itself cannot be determined without the external reference. A relative gravimeter operating entirely alone can only give you a self-consistent map of differences, not absolute gravity.

Question 4

A sudden unexplained step offset ('tare') in repeated base-station readings from a relative gravimeter permanently corrupts most data collected after the tare occurs.

• T. True
• F. False

Answer: False

A tare appears as a discrete step change in base-station readings at a specific time. If the survey returns to the base station frequently enough, the tare can be detected as an abrupt jump (rather than the smooth linear drift expected). Once detected, the data can be split into segments before and after the tare, and separate drift corrections can be applied to each segment. The tare's magnitude is estimated from the step size and subtracted from subsequent readings. Frequent base-station returns are precisely what makes tares detectable and correctable rather than permanently corrupting the data.

Question 5

Explain why spring-based relative gravimeters drift, and how gravity survey design accounts for it.

Model answer: Spring-based gravimeters measure gravity differences by monitoring how much a test mass deforms a spring. The spring is subject to viscous creep — slow, time-dependent deformation — so the spring constant gradually changes, causing the instrument to read slightly different values at the same location over hours even when gravity is constant. This drift is typically close to linear over the timescale of a field day. Surveys account for it by returning the gravimeter to a base station at regular intervals throughout the day. The difference between successive base-station readings reveals how much the instrument drifted over that time interval. Assuming linear drift between reoccupations, the drift can be interpolated and subtracted from all field readings collected between those times. The result isolates real spatial variations in gravity from the instrument's temporal drift.

Drift correction is one of the most important processing steps in gravity surveying because the geological signals of interest are often only a few milligals — comparable in magnitude to instrument drift over a field day. The survey design (how often to return to base) is directly driven by the instrument's known drift rate.