Download Lab 3

EAS-450 – Physics and Chemistry of the Earth
Lab exercise – Gravity Anomalies
Objective: Understand Bouguer corrections and anomalies. Manipulate isostasy.
1. What is the interest of using sea-floor gravimeters in the oceans rather than gravimeters on
ships, or classical gravimeters on-land rather that airborne gravimeters (on airplanes) inspite
of the fact that, in both cases (sea-floor and on-land) one will collect much less
measurements?
2. With respect to a station located at M, what will be the difference in gravity at:
a. A at the top of a 80 m tall tower (assuming the tower has no mass)?
b. B on a 80 m high plateau (very far form its edge, assuming a density of
2700 kg/m3)?
c. C at the bottom of a 80 m deep well in the plateau (very far form its edge)?
What is the gravity difference between the top and the bottom of the well?
Gravity measurements at various elevations in wells provide a way to determine the density
of the medium. How is that possible?
B
A
M
C
3. The measured gravity at the Chamonix train station (French Alps) is 980332.9 mGal. The
elevation of the station is 1038 m, its latitude 45o54’ N. Assuming that c, = 2800 kg.m-3 and
a terrain correction (in mGals) of 12.73 x 10-3 mgal/[kg/m3] x c calculate:
a. The terrain correction
b. The plate correction
c. The free-air correction
d. The Bouguer anomaly
4. The mean elevation of the Tibetan plateau is 5000 m.
a. Calculate the thickness of the root assuming Airy isostasy (use c=2800 kg.m-3,
m=3300 kg.m-3).
b. Calculate the density of the crust in the mountain assuming Pratt isostasy and
assuming the crustal thickness outside the Tibetan plateau is 35 km. What does
this suggest about the validity of Pratt versus Airy models for mountain ranges?
c. Let’s assume that a sudden climatic changes increases erosion, resulting in a
decrease of the mean elevation of the plateau by 2 km. When the new isostatic
equilibrium is achieved, what is the elevation of the plateau? Assume the Airy
model for isostasy and an unknown crustal thickness.
d. How much material has to be eroded to bring the plateau down to sea level (use
c=2800 kg.m-3, m=3300 kg.m-3)?
5. Let’s consider a passive continental margin is in isostatic equilibirum (Airy-type) and it has
the following densities: oceanic crust = 2800 kg.m-3continental crust = 2700 kg.m-3water =
1030 kg.m-3mantle = 3300 kg.m-3. Knowing that the oceanic crust is 6.6 km thick and that
the water depth is 5.5 km, what is the thickness of the continental crust?
2700
?
3300
1030
5.5 km
2800
6 km