Patent GB-174095-A (GB)
Title: Improvements in and relating to means for investigating the interior of the earth's crust
IPC: G01V1/16
Filed: 1920-07-09
Abstract: 174,095. Evans, J. W., and Whitney, W. B. July 9, 1920. . Prospecting and water-finding. - Means for carrying out a method of investigating the interior of the earth's crust based on the modifying influence of strata or surfaces of discontinuity upon sound waves or like pressure vibrations, comprise in combination means for generating the waves, a plurality of receiving apparatus, and one or more recording apparatus in connection, preferably electrically, with the generating apparatus and with the receiving apparatus. The system may be arranged so that the waves are received some before and some after passing through the stratum, or all after reflection or refraction at a surface of discontinuity, or after a number of reflections or refractions. The distance apart of generating and receiving stations will usually be large, s.ay one mile, and the receiving apparatus are preferably arranged on radial lines having the generator as centre and between 90 and 120‹ apart. The time of generation of the waves may be recorded by direct telegraphy. The waves may be generated, for example, by explosives or mechanical blows. The receiving instruments may comprise microphones, carbon, liquid or jet, telephone receivers, in combination if desired with an automatic make-and-break device or the special apparatus described later. The recording apparatus may comprise an Einthoven string galvanometer with automatic photographic recording device, apparatus based on the piezoelectric property of quartz, stethoscope, oscillograph &c. with automatic recording devices. The receiving apparatus may comprise a casing firmly embedded in the ground, an inertia mass resiliently supporting in the casing, and one or more pins or the like inserted in an electrical circuit between the mass and a microphone or like apparatus for detecting relative motion or variation in pressure between the mass and the casing. In one form, the pins may be replaced by a hemispherical elastic body. In one construction, Fig. 5, a heavy mass comprising a solid lead sphere 1 in a steel casing 2 is surrounded by a rubber envelope 3 enclosed in two hemispherical steel casings 4 bolted together so as to exert a slight pressure. The whole is enclosed in a concrete block 6 firmly embedded in the around. Steel pins 8 passing loosely through cylindrical holes 7 have their outer ends adjacent to a diaphragm of a sensitive microphone or a piezo-electric apparatus also firmly embedded in the concrete block. Variations in pressure between pin and diaphragm cause current variations which are recorded on a rotating drum. There may be one pin vertically above or below the centre of the sphere, or three pins in which case the other two are horizontal and at right angles, all three being connected to different microphones. Pairs of pins may be arranged opposite to one another, the opposite pins having a common microphone circuit, or, as an alternative, being independent. In addition to or in place of these, other arrangements of pins may be employed. The heavy mass 1 may also be of polydral form. In this case the pins are at the truncated angles of the polyhedron. The rubber support 3 may be replaced wholly or partially by springs, and for lateral preseure variation only, an inextensible cord may be employed. In another arrangement, Fig. 7, a casing a is divided into two parts between which is gripped a mica or insulated steel diaphragm b having at its centre an inertia, mass c of at least 8 ounces. An elastic body g whose surface may be coated with a layer of a substance with a high specific resistance, is fixed to the mass c, and an adjusting screw k carrying a shoe n also with a surface of high resistance is screwed down sufficiently to distort the diaphragm to such a degree that the shoe n and body q will always remain in contact whilst vibrations are being received. Oscillations of the casing cause changes in the area of contact of t:he shoe, n and body q, thus producing corresponding changes in resistance. The make-and-break or commutator device c is essential when the generator sends out waves at regular intervals of time, and may be arranged to break contact automatically at intervals corresponding to the arrival of the direct wave along the surface. This form of receiver may give quantitative results. In a modification, instead of the body q, the mass c may carry two platinoiridium or other metallic points, one on each side, placed just out of contact with two carbon or metallic shoes carried by two adjustable screws k. Oscillations set up in the casing cause. contact to be made between one of the pins and its shoe to complete a circuit through the recording instrument j. As an alternative, the pins and shoes may be adjusted so that a complete break occurs in the circuit when oscillations are received.