Patent GB-1105491-A (GB)
Title: System for measurement of static force field gradients
IPC: G01V3/165
Filed: 1965-05-17
Abstract: 1,105,491. Measuring magnetic and gravitational fields electrically; measuring static electric field gradient. HUGHES AIRCRAFT CO. May 17, 1965 [June 5, 1964], No. 20716/65. Headings G1N and G1U.  The gradient, with respect to distance, of a force field such as gravity is measured by an apparatus including a member consisting of, for example, an elongate strip 31, Fig. 3, carrying masses 41, 42, at its ends, which member is rotated on a shaft 32 (or caused to perform a rotary reciprocating motion, or a linear reciprocating motion parallel to or perpendicular to its length) at a frequency which makes the member vibrate at its natural resonant frequency in response to a net difference in gravitational forces acting upon the masses 41, 42, the amplitude and phase of the natural oscillation being indicative of the magnitude and direction of the field gradient. First and higher order derivatives of the gradient may be measured by appropriately selecting the frequency of rotation (or the like) of shaft 32 so that the resonant frequency is a predetermined  harmonic thereof. Piezo-electric or capacitative strain gauges 34, 35, measure the amplitude of the resonance in the member and their output is connected to a preamplifier 36, a narrow band amplifier (37, Fig. 2, not shown), and via slip rings 43 to an oscilloscope or a recorder (40). The vibrating member may be contained in an evacuated, magnetically shielding container (30, Fig. 1, not shown). The elongate strip 31 may be of aluminium steel, or quartz, and in the last case electrodes may be mounted directly on the strip to replace the strain gauges. The slip rings may be replaced by a radio device. In alternative embodiments of the resonating member: (a) The plane of vibration of the member may contain the shaft 32, Fig. 5 (not shown) instead of being at right angles to it, Fig. 3. (b) The distributed mass of a single strip (31, Fig. 6, not shown) may be used instead of having two separate masses 41, 42. (c) The member may comprise springs (67, 68) and masses (65, 66, Fig. 11, not shown) so that vibration is along the axis of the member. (d) The member (31, Fig. 10, not shown) may be mounted at its centre in a bifurcated yoke (63) to be rotated about its own longitudinal axis, which is a continuation of the axis of the shaft (32). (e) Permanent magnets may be used as the member. In operation, a small mass of dense rocks (54, Fig. 9, not shown), which produces a slight variation in gravitational force is detected by rotating the member 31, Fig. 3, at a frequency F slightly less than that corresponding to resonance (Fr, Fig. 7, not shown) so that if the member has a narrow response curve (51) the effect of the general background mass of rocks upon the vibration of the member (at 2F) is substantially reduced. The force due to the dense rocks, although very small (at frequency 2F-f), acts upon the member at a frequency which is shifted towards the resonant frequency by virtue of the constant velocity (proportional to f) of the aircraft in which the member is carried, so that its effect upon the resonance of the member is much enhanced. The resulting signal, Fig. 8 (not shown) is then a 2F C. W. (52) due to the background mass, plus a modulation f signal (53) due to the dense rocks being investigated. Measurement of higher order gradients improves rejection of signals due to surrounding earth mass. Magnetic fields are measured by mounting coils (80, 81, Figs. 13-17, not shown) at opposite ends of the rotating member (90), connecting them differentially, and feeding the difference signal to a resonant circuit (85, 86) and amplifier arrangement (36-40).  Electric fields are measured by mounting a dipole (100, 101, Figs. 18, 19, not shown) on the end of a rotating shaft (110), connecting the two halves thereof via a transmission line to a resonant circuit (102) which feeds an amplifier arrangement (36-40), and thereby deriving a signal proportional to the difference in field strengths over the extent of the dipole. The magnitude and phase of the derived signal gives the magnitude and direction of the gradient.