Introduction to the use of gravity measurements

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Bertram Davidson
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1 Introduction to the use of gravity measurements Background & basic intuition Data reduction Use of the instrument See Reynolds, Chpt 2, for detailed quantitative discussion

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, magnetic ) Targets include: Topographic compensation - the crustal root (long λ) Fault structures/offsets (best for dip-slip faults or large offset strike-slip) Basin sediment fill depth (together with magnetic field variations) Aquifers Tunnels Buried bodies (tunnels, ore deposits, )

3 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

4 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

5 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!

6 Attraction from harmonic distribution of mass II We won t derive solutions today, but you can imagine integrating δg = Gδm r 2 over our given harmonic mass distribution. The vertical component is: where k = 2π/λ, h<<λ 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!

7 Attraction from harmonic distribution of mass III At long λ, k 0 g z = 2πGΔρh = 2πGΔσ No z-dependence Attraction from an infinite sheet of mass Will be used in making the Bouguer correction For z 0 If z increases (or k increases λ decreases): e -kz 0 Upward continuation: e.g., ΔZ=λ, e -2π =0.002 (faint signals) Downward continuation: e.g., ΔZ=-λ, e 2π =500 (amplification of short λ noise)

8 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. Latitude (proximity to Earth center, change in mass on ellipsoid, rotation) 88.9 mgal / degree latitude (we typically wont deal with this for our small surveys) 4. Free Air 5. Bouguer

9 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 h g 0 h 2g 0 R The L&R gravimeter has a sensitivity of ~3µGal h = δg R FA 1cm 2g 0 Careful with surveying (know your target) = mgals/m

10 Bouguer Correction Effect of mass (not distance) - Assume λ>>h, ignores dh/dx + g B = 2πGΔρh Δρ h h x What density to use? Standard is 2670 kg/m 3 You can also use this to estimate density (scramble the above) Nettleton s Approach - assume all variations are due to topography Clever differencing of gravity profiles is sometimes useful More sophisticated approaches Terrain corrections (graphical) Talwani (2D and 2.5D prisms - Matlab) Parker s Method (finite amplitude harmonic summation) Common Shapes: (Burger et al., Sec. 6.5) Vert/Horiz sheet/cylinder, quarter sheets, spheres, polygons

11 The 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) or use the feedback system to record relative values in mgals Reclamp Return to base station every ~30 minutes for tide/drift correction.

12 Homework 1) Considering the free air anomaly expression, use gravity to estimate the height of each floor of Milliken (use the stairwell). 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. Make sure to note the time. Take care to clamp the mass. 2) Given the coordinates of Caltech (34:09N, 118:10W, 250 m.a.s.l.) and our field area, and assuming all other factors are constant, which area do you expect will have a higher vertical component of the gravity field? (Hint: compare effects of latitude correction vs. FA correction.)

r 1 r 2 r 3 Dr is what matters!

r 1 r 2 r 3 Dr is what matters! 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 What and Why Gravity