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Instruments and Gravity Processing Drift and Tides Gravity 5 Objectives Instruments Gravity Gravity 5 Corrections Instruments and gravity processing Drift and Tides Latitude Free Air Chuck Connor, Laura Connor Atmosphere Simple Bouguer Summary Potential Fields Geophysics: Week 5 Further Reading EOMA Gravity 5 Objectives for Week 5 Gravity 5 Objectives Instruments • Gravity Learn about gravity Corrections instruments Drift and Tides • Learn about Latitude processing of gravity Free Air data Atmosphere • Make the Simple Bouguer corrections to Summary calculate a simple Further Bouguer anomaly Reading EOMA Gravity 5 The pendulum The first gravity data collected in the US were Gravity 5 obtained by G. Putman working for the Coast and Geodetic survey, around 1890. These gravity data comprised a set of 26 measurements made along a roughly E-W transect across the entire continental Objectives US. The survey took about 6 months to complete and was designed primarily to investigate isostatic Instruments compensation across the continent. Gravity Putnam used a pendulum gravity meter, based on the Corrections relation between gravity and the period of a pendulum: Drift and Tides 4π2l g = Latitude T 2 Free Air where: l is the length of the pendulum Atmosphere T is the pendulum period Simple Example Seems easy enough to obtain an absolute gravity Bouguer reading, but in practice pendulum gravity meters are Assuming the period of a pendulum is known to be 1 s Summary problematic. The length of the pendulum can change exactly, how well must the length of the pendulum be with temperature, the pendulum stand tends to sway, known to measure gravity to 10 mGal precision? Further air density effects the measurements, etc. The Reading Mendenhall pendulum (around 1910) handles some of T 2∆g these problems, for instance by using a vacuum, but l = = 2:5µ EOMA basically one cannot see subtle variation in gravity 4π2 with a pendulum apparatus! Gravity 5 The zero-length spring gravimeter The first truly portable gravimeter was invented Gravity 5 by Lucien LaCoste in the mid-thirties, when he was in his twenties.This gravimeter makes relative measurements rather than absolute measurements. It does not provide any information about the absolute Objectives acceleration due to gravity, only the relative change in gravity from place to place. These types of meters are Instruments still the work-horses of the gravity world. Most gravity measurements you will use are made using this type of Gravity instrument, now manufactured by Burris. Corrections This gravimeter contains a mass attached to a Drift and cantilevered beam and suspended with a metal or Tides quartz spring. The tension on the spring can be Latitude adjusted to bring the beam to a null position. The force required to move the beam to the null position is Free Air proportional to the change in gravity. The apparatus The main features of the gravimeter design have not is called a zero-length spring meter because the spring changed from LaCoste's early concept. Improvements Atmosphere is pre-stressed: if the mass were removed altogether, have been made to electronically determine the null the spring would contract to \zero" length. Simple position of the meter, using a servo motor interfaced Bouguer to a PDA. A number of features of the Burris gravity To make accurate measurements, the instrument must meter are designed to minimize instrument drift, Summary be level (aligned with the vector of the Earth's gravity including a heater to maintain constant temperature, field), in a place quiet enough to avoid vibrations a sealed case to minimize the speed of pressure Further (trucks rumbling by and earthquakes are a problem!), change inside the gravimeter, and electronic Reading and given sufficient time to be in thermal and monitoring of the instrument level. In ideal conditions, mechanical equilibrium (avoiding sharp changes in gravity measurements with this instrument are good EOMA temperature that effect the instrument's metal parts, to 1{10 µGal. etc.). Gravity 5 Absolute gravimeter Gravity 5 An absolute gravimeter, like a pendulum apparatus, measures gravitational acceleration directly, rather then the relative change in gravity from Objectives place to place. These instruments are \sort of" portable (weighing around 50 kg), can Instruments make accurate determinations of the gravity field within one hour, and are Gravity increasingly field-worthy, although still Corrections quite expensive (>100K USD). Drift and Tides The current generation of absolute gravity meters are free-fall devices, simply Latitude measuring the time it takes an object to fall a given distance in a vacuum. Free Air Improvements in the instrument have involved improving the timing of the fall Atmosphere (using an atomic clock) and the distance of the fall (using a high-precision laser), the Simple quality of the vacuum, a dampening system Bouguer to lessen the effect of vibrations, and the Summary ease with which free-fall measurements are repeated. Further Reading The absolute precision on the best absolute gravimeters is about 1 µGal. EOMA Gravity 5 Gravity from space Gravity 5 Space-based gravity observations generally rely on accurately measuring a satellite's position and comparing it to its theoretical position given a model Objectives of the Earth's (or other planet's) gravity field, assuming no anomalous mass distribution. In early Instruments efforts, this involved tracking a single satellite in its orbit and the departure of its true position from an Gravity expected position. Corrections Drift and Alternatively, the exact altitude of the satellite could Tides be compared to its expected altitude, by measuring altitude using a satellite-mounted laser altimeter. Latitude Variations in altitude measured by laser are a particularly good way to track variation in gravity at Free Air sea. Because the altimeter is measuring the ocean height variation, this method essentially maps gravity Atmosphere variation on the ocean surface rather than at the height of the satellite, so relatively small anomalies Simple can be detected. Bouguer Modern gravity missions, such as GRACE and GRAIL, map variation in the gravity field by placing two Summary satellites in orbit along the same orbital track. Gravity anomalies cause the distance between the two Further satellites to deviate from their expected distance of separation. The size of the gravity anomaly (or the Reading excess mass that will be detected) depends on the altitude of the satellites and the distance between them. EOMA Gravity 5 Gravity corrections Gravity 5 Putman had a big job on his hands after he • Tidal corrections, to account for the time finished gathering his gravity data. He knew that the varying gravitational acceleration due to the gravity varied for many reasons other than lateral motion of the Sun and Moon variations in mass in the subsurface. We have already Objectives • Theoretical gravity correction, to account for seen that gravity varies with latitude due to the shape the shape and rotation of the Earth and rotation of the Earth, the tides, and with Instruments variations in elevation ( dg ). • Free air correction, to account for variations dR in gravitational acceleration with elevation Gravity Corrections Gravity corrections involve reducing the effects of • Atmospheric correction, to account for varying these features of Earth/planetary gravity in order to density of the atmosphere with elevation Drift and isolate the effects of lateral variations in mass. These • Simple Bouguer correction, to account for the Tides corrections are not a trivial exercise. Some of the average density of rocks as a function of corrections are mathematically complex (e.g., the Latitude elevation (sometimes called the Bullard A effect of latitude). By making the corrections in a correction) Free Air step-wise fashion, removing one effect at a time systematically, we often can learn more about the • Spherical cap correction, to account for the change in the Bouguer correction due to the Atmosphere interior of the Earth than we could if we simply make the corrections all at once. roughly spherical shape of the Earth (also Simple known as the Bullard B correction) Bouguer The usual corrections made to gravity data include: • Terrain correction, to account for the exact form of the terrain and its influence on density • Instrument drift, associated with variations in Summary distribution around the gravity station (also gravity only due to the fact that the known as the Bullard C correction) Further gravimeter registers different readings with Reading time, due to mechanical, thermal, and • Isostatic correction, to account for broad electrical changes in the instrument (long wavelength) variations in gravity due to EOMA isostatic compensation of the crust. Gravity 5 Instrument drift and the tides Gravity varies over time at a specific location for several reasons: Gravity 5 1 The instrument reads out different values of gravity over time because of changes in the instrument itself. For example, changes in Objectives temperature might cause thermal expansion of the beam, changing the tension on the Instruments \zero-length" spring. This type of variation with time is referred to as instrument drift. Gravity Corrections 2 Gravity varies due to tidal effects. For any location at the surface of the Earth, the Drift and distance to the Sun and Moon change Tides continually with time, changing the gravity field. Latitude 3 On short time scales, gravity varies due to Free Air ground vibrations, microseismic events, or teleseisms (ground motion due to distant Atmosphere earthquakes). The instrument may also go Simple out of level during measurements due to Bouguer settling on soft ground. These types of effects are usually referred to as noise and are Summary minimized by adapting field procedures. Further 4 Gravity varies with time for geological reasons. Reading The elevation of the gravimeter might change A drift curve measured with a LaCoste \zero-length" due to subsidence or inflation of the site. spring instrument in the USF lab.
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