Abstract: The invention relates to a method and an apparatus for detecting metal objects in a metal detecting apparatus using impulse techniques. The metal detector comprises a compensation circuit arranged in between the transmitter system (13) and the receiver system (14) in order to compensate the mutual inductance between the transmitter (11) and the receiver (12). The compensation circuit is advantageously operated so that the signal caused by the metal object in this detector is measurable during both at least two measuring sequences (21,22) located within the range of the cut-off peak of the transmitter current and during at least one measuring sequence (23) located outside the range of the cut-off peak of the transmitter current.

Abstract: A method for reducing noise in electromagnetic geophysical exploration data simultaneously measures three orthogonal components of the ambient noise to produce noise records and simultaneously measures three orthogonal components of the decay of current induced into the earth to produce response records. In measuring the records, the receiving antenna is positioned near the electromagnetic center of the transmitting antenna, whereby the maximum component of the induced current decay signal is vertical or perpendicular to the strata in the earth. The impulse response of the earth is determined from the noise records. The noise portion of the vertical response record is determined from the impulse response and the non-vertical response records, whereby the noise portion is removed from the vertical response record.

Abstract: An eddy-current loop detector operates to sense the presence of an electrically conductive object by applying D.C. onto a conducting loop so that a permanent magnetic field is induced, which field partly penetrates into the object; ensuring the quick cessation of self-inductive transient operation of the loop when the D.C. is switched off; and measuring the change in the part of the flux passing through the measuring loop arising from eddy-currents induced in the conductive object, and evaluating such change while the relatively slowly decreasing flux is maintained. By repeating these operations with the desired frequency, the eddy-current detector is well suited for continuous measuring of the position of the object.

Abstract: A frequency domain geophysical electromagnetic surveying system is provided in which intermittent primary fields having sharp terminations are generated in a cycle having a fundamental frequency by a transmitter, and a receiver includes a gate passing only secondary signals received during interruptions of the primary field, the gated signal being applied to filters tuned to harmonics of the predetermined frequency. The filter outputs are synchronously demodulated to provide signals corresponding to at least the in-phase component of the secondary signal at the fundamental frequency, and preferably in-phase and quadrature components at at least two harmonics. The system can provide improved resolution particularly of the characteristics of in-phase secondary signals received from terrain being surveyed.

Abstract: In a method of geophysical prospection a transmitter dipole and a receiver dipole which is sensitive to an electromagnetic field are positioned in cooperation with the subsoil. An impulsive excitation current is applied repeatedly to the transmitter dipole, the excitation current comprising a group of pulses of alternate polarity, each followed by zero current. The transient signal, which is produced thereby at the output of the receiver dipole, is repeatedly detected, and the transient signals are processed to obtain a figurative representation of the transient phenomenon which facilitates geophysical interpretation. Time intervals which are consecutive to excitation pulses and simultaneously with the excitation zero current are defined, these time intervals being of a predetermined duration, chosen to substantially cover all the useful parts of the transient phenomenon.

Abstract: In a method of geophysical prospection a transmitter dipole and a receiver dipole which is sensitive to an electromagnetic field are positioned in cooperation with the subsoil. An impulsive excitation current is applied repeatedly to the transmitter dipole, the excitation current comprising a group of pulses of alternate polarity, each followed by zero current. The transient signal, which is produced thereby at the output of the receiver dipole, is repeatedly detected, and the transient signals are processed to obtain a figurative representation of the transient phenomenon which facilitates geophysical interpretation. Time intervals which are consecutive to excitation pulses and simultaneously with the excitation zero current are defined, these time intervals being of a predetermined duration, chosen to substantially cover all the useful parts of the transient phenomenon.

Abstract: A transient electromagnetic signal transceiver for use in geophysical prospecting includes cultural noise and random noise reduction features. AC mains interference signals are minimized by synchronizing the reception of TEM signals with the mains frequency so that the mains interference components of alternate TEM signal measurements cancel. Other cultural noise is minimized by using low pass filters tuned to reject VLF radio signals. Common mode signal rejection, sferics suppression and signal stacking and averaging techniques improve the signal-to-noise ratio of the received TEM signals, while the use of a programmed microprocessor enables overall control of the transceiver to include the calculation and presentation of ground conductivity and other data.