r 1 r 2 r 3 Dr is what matters!

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Veronica Farmer
5 years ago
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1 Today s Agenda Gravity - Background & basic intuition - Data reduction - Use of the instrument - See textbook for detailed quantitative discussion - Measure height of Millikan or S. Mudd

2 What and Why Gravity measures the distribution of mass Variations in density Variations in interface locations Very non-unique => use in conjunction with other constraints (geologic, seismic, ) Targets include: Topographic compensation - the crustal root (long l) Fault structures/offsets (best for dip-slip faults or large offset strike-slip) Aquifers Tunnels Buried bodies (tunnels, ore deposits, ) Basin sediment fill depth (together with magnetic field variations)

3 r 2 r 1 r 1 r 2 r 1 r 3 r 2 Dr is what matters!

4 Data Availability Global: Satellite tracking/altimetry (over water)/other ~2 km resolution over oceans ~100 km resolution from harmonic models None of these useful for our purposes Regional: Carry the gravimeter around Land, boat, plane Arbitrary resolution

5 Basic intuition Everything we need can be worked out from basic potential field theory (see Reynolds or Turcotte & Schubert): 2 V = 0 where V is the gravitational potential g z = V z where g z is the vertical component of the gravity field Units: 1 mgal = 10-5 m/s 2 1 g.u. = 0.1 mgal = 10-6 m/s 2

6 Attraction from harmonic distribution of mass I Z Δρh k cos( kx) = Δσ k cos( kx) Why? Intuition Decompose any mass distribution into individual harmonic (Fourier) contributions and sum linearly Example: Topography, density variations on a 2D or 3D grid X

7 Attraction from harmonic distribution of mass II We won t derive solutions today, but you can imagine integrating over our given harmonic mass distribution. The vertical component is: where k = 2p/l, h<<l δg = Gδm r 2 ˆ r g z = 2πGΔρh k cos( kx)e kz = 2πGΔσ k cos( kx)e kz The e -kz term corresponds to the 1/r 2 decay with distance (the peaks and troughs tend to cancel when viewed from increasingly large distances) Z Δρh k cos( kx) = Δσ k cos( kx) X

8 Attraction from harmonic distribution of mass III g z = 2πGΔρh k cos( kx)e kz = 2πGΔσ k cos( kx)e kz At long l, kþ0 g z = 2πGΔρh = 2πGΔσ No z-dependence Attraction from an infinite sheet of mass Will be used in making the Bouguer correction For z¹0 If z increases (or k increases Û l decreases): e -kz Þ0 Upward continuation: e.g., DZ=l, e -2p =0.002 (faint signals) Downward continuation: e.g., DZ=-l, e 2p =500 (amplification of short l noise)

9 Measurements Absolute Needs known reference station to tie to Relative Small scales are fast (what we typically do) Common corrections before getting to interesting signal 1. Instrument drift: return to base station regularly 2. Tides: return to base station regularly 3. Centrifugal acceleration: keep track of latitude (we wont) 4. Free Air 5. Bouguer

10 Free Air Correction Bring all observations to a common horizontal reference surface (E.g., a datum or geoid) Effect of distance not mass Results in Free Air Gravity Commonly used for airborne and marine data g 0 = Gm R 2 ; g h = Gm (R + h) 2 δg FA h = g g h 0 h 2g 0 R The L&R gravimeter has a sensitivity of ~3µGal h = δg FAR 1cm 2g 0 Careful with surveying (know your target) = mgals/m

11 Bouguer Correction Effect of mass (not distance) - Assume l>>h, ignores dh/dx + g B = 2πGΔρh Dr h h x What density to use? Standard is 2670 kg/m 3 You can also use this to estimate density Nettleton s Approach - Assume all variations are due to topography Clever differencing of gravity profiles is sometimes useful - Over hill vs through roadcut (removes regional trend)

12 More sophisticated approaches Terrain corrections (graphical) Parker s Method (finite amplitude harmonic summation) Common Shapes (Burger et al., Sec. 6.5): - Vert/Horiz sheet/cylinder - quarter sheets - spheres, polygons - Talwani (2D and 2.5D prisms - Matlab)

13 r 2 r 1 r 1 r 2 r 1 r 3 r 2 Dr is what matters!

15 Locoste & Romberg Model G Gravimeter Love and Care $$,$$$ Needs power to keep isothermal - keep plugged in somewhere Highly sensitive spring and mass system (read Reynolds) - Do not walk around without clamping the mass Getting data Level gravimeter (you want all of g z ) Unclamp Get a reading by adjusting the spring s.t. mass is in the right place - can be tedious if you have changed location a lot A manual raw data point is derived by reading off the counts and converting to mgals using a calibrated table (or Matlab) Typically, take a manual reading at a base station - use force-feedback system to record relative values - Careful with on/off of feedback during a survey Reclamp (DON T BE SHY ABOUT CHECKING YOUR COLLEAGUES) Return to base station every ~30 minutes for tide/drift correction.

16 Homework 1. Read the chapter on gravity in Reynolds 2. Use gravity to estimate the height of each floor of Millikan or S. Mudd. Make sure to correct for any tidal/drift effects by interpolating differences in the base station measurement at the beginning and end of the survey. Since tides/drift are slowly varying over hours, you can empirically correct for them by remeasuring a base station at different times to estimate the correction as a function of time (by interpolation). Note: Tidal variation +/- ~0.3 mgal or 300µGal 3. Given the coordinates of Caltech (34:09N, 118:10W, 250 m.a.s.l.) and Wallace Creek/Carrizo Plain, and assuming all other factors are constant, which area do you expect will have a higher vertical component of the gravity field (show your analysis)? (Hint: compare effects of latitude correction vs. FA correction).

Introduction to the use of gravity measurements

Introduction to the use of gravity measurements Background & basic intuition Data reduction Use of the instrument See Reynolds, Chpt 2, for detailed quantitative discussion What and Why Gravity measures