Question 3 of 10: Planning and Executing a Ground Gravity Survey — Drift and Earth-Tide Corrections

Question 3: Planning and Executing a Ground Gravity Survey — Drift and Earth-Tide Corrections (Choose 6 of 10 – equal value)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Planning and executing the survey

Planning begins from the target's expected density contrast, depth and size, which set the required station spacing (typically no more than one to two target depths, so the anomaly is sampled by several stations) and the survey grid orientation relative to expected geological strike. A stable local datum (benchmark or GPS base) is established for station ELEVATIONS, since the free-air and Bouguer corrections are far more sensitive to elevation error than almost any other survey parameter (a few centimetres of elevation error can rival the target anomaly itself). A gravity BASE STATION, tied where possible to a national/regional absolute-gravity network, is selected, and the field crew designs a LOOP: a closed sequence of station occupations that starts and ends at the same base station (or a common tie point) within a short time window, so that any change in the reading between the loop's start and end can only be instrument drift plus tidal effects, not a real change in subsurface gravity.

In EXECUTION the crew reads the base station first, then occupies each station in the planned loop order, logging the exact clock time of every reading (the tidal correction is a function of time, so a mis-set or unlogged clock is unrecoverable), taking two or three repeat readings per station and averaging them to suppress instrument noise and microseismic disturbance, and levelling and re-seating the meter carefully at each set-up. The meter is transported clamped and kept thermally stable, since rough handling and temperature shock are what turn an otherwise well-behaved linear drift into an erratic one. Station elevations are surveyed as the stations are occupied rather than in a separate later campaign, so every gravity reading has a matching elevation of known quality before the crew leaves the site.

Instrument drift: monitoring and removal

Modern spring gravimeters (e.g. LaCoste–Romberg or Scintrex CG-type instruments) exhibit a slow, roughly LINEAR mechanical drift of the reading spring over the course of a survey day, typically a few hundredths of a mGal per hour. This is monitored by re-occupying the base station (or any previously-read tie station) at the start and end of each loop, and periodically within a long loop; the difference between the repeat reading and the original reading, divided by the elapsed time, gives the local drift rate, which is then removed from every intervening station by LINEAR INTERPOLATION between the two ties, weighted by each station's own observation time.

Earth tides: physical cause and removal

Superimposed on instrument drift is the SOLID-EARTH TIDE: the same lunar and solar gravitational forcing that raises ocean tides also periodically stretches and relaxes the solid Earth itself, changing the local value of g by amplitudes of roughly ±0.2–0.3 mGal over a smooth, predictable, semi-diurnal-to-diurnal cycle. Because the tidal forcing is fully deterministic (it depends only on the Sun's and Moon's known positions, which are given by astronomical ephemerides), the tidal correction is NOT read from repeat station occupations at all but is instead COMPUTED directly from a harmonic tidal-prediction formula (e.g. the Longman 1959 formula) for the station's latitude/longitude and the exact time of each reading, and subtracted before the drift correction is applied. Because the tidal correction is computed rather than measured, it removes an effect that a simple linear-drift interpolation alone would otherwise only partially capture (the tide is not linear in time over a long loop), and most modern gravimeters compute and apply it automatically in real time from a built-in ephemeris.

Typical values at a range of locations