. Early geophysical papers of the Society of Exploration Geophysicists . Fig. 4. Gradient and gravity curves for assumed structure. resultant force and as the instrument is set exactly in line with this direction the measurement is the same. These surface masses make so much trouble for the torsion balance operator that it seems to be hard for him to believe that they do not disturb this instrument. The relation between the gravity anomaly, which is measured directly by the gravity balance, and the gravity gradient, which is measured by the torsion balance, is very clearly shown in a diagram i
. Early geophysical papers of the Society of Exploration Geophysicists . Fig. 4. Gradient and gravity curves for assumed structure. resultant force and as the instrument is set exactly in line with this direction the measurement is the same. These surface masses make so much trouble for the torsion balance operator that it seems to be hard for him to believe that they do not disturb this instrument. The relation between the gravity anomaly, which is measured directly by the gravity balance, and the gravity gradient, which is measured by the torsion balance, is very clearly shown in a diagram in Ambronn's Elements of Geophysics, which he has taken from a paper by von Eotvos. Figure 4 is derived from this diagram; the heavy curve representing the values of the gravity gradient and the light curve the gravity anomaly caused by a mass of heavy rock at varying depths below the surface. It will be noted that the curve representing the magnitude of the anomaly follows very closely the outline of the rock mass below while the curve representing the magnitude of the gradient bears no resemblance whatever to this outline and requires some study to interpret its meaning. In actual field work these curves are not ob- tained but only a rather small number of points on them. A comparatively 171
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