MXPA99001952A - Passive geophysical prospecting apparatus and method based upon detection of discontinuities associated with extremely low frequency electromagnetic fields - Google Patents

Abstract

An apparatus (200) for passively determining the depth and thickness of a subterranean geologic formation bearing hydrocarbons. The apparatus has an antenna (12) to pick up extremely low frequency signals naturally emanating from the Earth's surface. The received signal is amplified and filtered and then modulated onto a carrier wave. The modulated signal is then filtered to eliminate one of the sidebands. An oscillator (16) generates a tuning frequency which is then beat against the filtered, modulated signal in order to tune to a particular frequency. The oscillator sweeps through the range of frequencies. For each desired frequency within this range, the tuned signal is adjusted to the desired frequency and is sent to a voltage level detectoron and converting same to pulses. The pulses are counted over a desired time period to determine the pulse density.

Description

PASSIVE GEOPHYSICAL PROSPECTING APPARATUS AND METHOD IN BASE FOR THE DETECTION OF DISCONTINUITIES ASSOCIATED WITH ELECTROMAGNETIC FIELDS OF EXTREMELY LOW FREQUENCY BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an apparatus and method for passive geophysical prospecting. More particularly, the present invention relates to the detection on the surface of the Earth in a non-invasive way of underground discontinuities associated with electromagnetic fields of extremely low frequency. 2. Description of Related Art The technique is replete with various passive methods and associated apparatus for passive geophysical prospecting. There is a great motivation to discover a reliable method of this type, which is simple and therefore relatively inexpensive when compared with drilling or performing non-passive geophysical prospecting, for example, seismic measurements and associated computer manipulation and costly resulting data, in a known area having discontinuous strata or in an unknown area, ie, an exploration territory, to determine if some zones of a production formation were forgotten or just beyond the term of an existing well. Several passive methods use an antenna to collect frequencies that emanate naturally from the surface of the Earth. Typically, the received signal is amplified, filtered and detected. See, for example, U.S. Patent No. 5,148,110 to Helms, which detects a time-varying signal emanating from the surface of the Earth. U.S. Patent No. 4,686,475 to Kober et al., Detects the vertical electric field component of telluric currents using a special antenna and an RC filter selectable with the detection performed in an audio form using the ears of an operator. This method is subjective to the operator and therefore suffers from reliability and consistency. There is a lot of noise associated with or interfering with these low frequency signals. This is one reason why low-pass and high-pass filtering is employed after the initial amplification of the signal. However, due to the initial low frequencies of the signal, it is difficult to discern the valuable information that it carries and the conditioning is preferred. Those skilled in the art have long tried without great success to find the appropriate way to condition the received signal to discern this valuable information in a consistent and reliable manner. For this reason said passive techniques have not been accepted by the prospecting community. of hydrocarbons and relegated to the level of "dip bars".

One problem is that simple amplification is typically not simple to allow filters to operate effectively. For this reason, a frequency generated by an oscillator has been added to the signal in certain methods using a mixer to add the amplitude and terminate a reference frequency to filter the received signal. See, for example, US Pat. Nos. 3,087,111 to Lehan et al. (Amplifies the signal and then adds the oscillating frequency), 3,197,704 to Simon and others and 4,198,596 to Waeselync and others, (amplifies, filters, adds oscillator frequency and then low pass filters). For most, the oscillator controls the frequency of central passage of the filters that are being used. However, the quality of the received signal is not improved and the problems of reliability and consistency remain.

COMPENDIUM OF THE INVENTION The present invention provides a surprisingly reliable and consistent method and apparatus for passive geophysical prospecting using the low frequency signals emanating from the surface of the Earth. As a result of extensive experimental testing, more than seventy percent (70%) of successful regimen has been experienced and the ability to reproduce results has a given location been achieved. This is particularly true to determine the absence of hydrocarbons or precious metals in underground formations, which is absolutely valuable in untested virgin areas. The apparatus and method are simple and relatively inexpensive and time-consuming when compared to methods that are currently being used commercially. The method and apparatus may be used only for the prospection of hydrocarbon or precious metal or together with the prospecting techniques currently available to verify the same or to identify promising and / or unpromising areas before making an effort and spending money to perform the most traditional prospecting techniques, time consuming, and costly. Accordingly, an apparatus and method is provided for passively determining the depth and thickness of an underground geological formation containing hydrocarbons, for example, oil and / or gas, or commercially important mines, eg, precious metals. In one embodiment of the invention, the apparatus has an antenna for collecting an extremely low frequency signal emanating from the surface of the earth. This signal is believed to be associated with Earth's electromagnetic fields. Each frequency of this signal corresponds to a certain depth on the Earth and carries information regarding the presence of said underground geological formations. Unlike the prior art, the novel apparatus of the present invention has a product detector, which receives the signal collected by the antenna and multiplies it with a frequency generated by an oscillator. The oscillator is capable of sweeping through the frequencies that correspond to the depths of interest. By "pressing" the frequency of the corresponding depth against the received signal, a selected signal is generated for each selection frequency by the product detector and contains the information corresponding to that depth. The selected signal can then be sent to a screen, recorder or a computer for further processing and evaluation. In order to improve the reliability and reproducibility of the apparatus according to the present invention, the apparatus preferably has a voltage detector. The voltage detector receives the selected signal from the product detector, detects the selected information carried by the selected signal and converts the selected information to pulses. The pulses are produced by the voltage detector. The pulses or output signal can be recorded digitally on a tape or in an analogous way on a strip recorder and / or sent to a computer to be presented on a CRT and then manipulated using methods and techniques that are well known in the field. pulses are counted during a desired period to determine the density of the pulse, that is, the number of pulses per unit of time It is observed that the function of the level detector can be performed through a computer acting on the selected signal received by the same in a real time or after the fact manipulating the selected signal information stored and generating equivalent data in the information content to the pulses generated by the voltage level detector. Either way, the pulse density can be determined from the pulse data using the computer. In addition, the pulse data can be manipulated using the computer, for example, by varying the specific unit of time, i.e., the elapsed time, to examine the pulse density information. Alternatively, with the voltage detector in place, the apparatus may further comprise a counter, which may be set to activate an output signal when a specific number of pulses per unit of time is obtained. The trigger point can be varied, thus varying the specific unit of time, i.e., the elapsed time, to examine the pulse density information. In view of the low frequencies and the low signal strength of the signals emanating from the surface of the Earth, the received signal is preferably conditioned to increase its frequency and the signal strength to improve detection of the selected signal information. In this way, in a preferred embodiment, the received signal is amplified and filtered. The signal is then modulated on a carrier wave. The modulated signal is then filtered to eliminate one of the sidebands, for example, the lower sideband. In this case, the oscillator generates a selection frequency, which is then pressed against the modulated signal, filtered in order to select a particular frequency. The oscillator scans through the frequency scale in the remaining sideband portion, for example, the upper side band portion, of the filtered modulated signal. For each frequency desired on this scale, a selected signal is generated corresponding to the desired frequency and, therefore, a certain depth on the Earth. As noted previously in the previous embodiment, the apparatus preferably has a voltage detector. The voltage detector receives the selected signal from the product detector, detects the selected information carried by the selected signal and converts the selected information to pulses. The pulses are produced by the voltage detector. The pulses or output signal can be recorded digitally on a tape or in an analogous way on a strip recorder and / or sent to a computer to be presented on a CRT and then manipulated. The pulses are counted during a desired period to determine the pulse density, that is, the number of pulses per unit of time. As noted above, the function of the level detector can be performed through a computer acting on the selected signal received by it in real time or after the fact by manipulating the selected stored signal information and generating data equivalent to the information content. the pulses generated by the voltage level detector. Either way, the pulse density can be determined from the pulse data using the computer. In addition, the pulse data can be manipulated using the computer, for example, by varying the specified unit of time, i.e., the elapsed time, to examine the pulse density information. Alternatively, with the voltage detector in place, the apparatus may further comprise a counter that can be set to activate an output signal when a specific number of pulses per unit of time is achieved. The trigger point can be varied, thereby varying the specified time unit, i.e., elapsed time, to examine the pulse density information. According to another embodiment of the present invention, a method for passive geophysical prospecting is provided, which comprises: receiving a signal emanating from the surface of the Earth with an antenna; generate a received signal that corresponds to the signal emanating from the surface of the Earth; generate a selection frequency; make a sweep of the selection frequency at least through the scale of frequencies contained in the received signal; multiply the received signal and the selection frequency to generate a product signal; synchronously select the product signal on the scale of frequencies contained in the received signal; and generate a selected signal containing selected information. In order to improve the reliability and reproducibility of the method of the present invention, the method preferably further comprises: converting the selected information to pulse information representative of the same. The pulse information can be manipulated using software on a computer or hardware to determine the pulse density. The pulse information can be generated by comparing individual pieces of information for a reference point, which can also be adjusted to effect changes in pulse density for evaluation purposes. The larger the pulse density, the greater the probability of a positive indication of the presence of, for example, the desired hydrocarbon or precious metal. In view of the low frequencies and the signal strength of the signal emanating from the surface of the earth, the received signal is preferably conditioned. Accordingly, a method for passive geophysical prospecting is provided, wherein the method comprises: receiving a signal emanating from the surface of the Earth with an antenna; generate a received signal that corresponds to the signal emanating from the surface of the earth; amplifying the received signal to generate an amplified signal; generate a carrier wave using an oscillator; modulating the carrier wave with the amplified signal generating a modulated signal having the carrier wave, an upper lateral band and a lower lateral band, wherein the modulation can be either amplitude or frequency modulation; preferably canceling the carrier wave of the modulated signal by generating an output signal having the upper and lower sidebands; and removing one of the sidebands using a filter, preferably removing the lower sideband using a high pass filter, which passes the high sideband but not the lower sideband. The method preferably further comprises: generating a selection frequency; make a sweep of the selection frequency at least through the scale of frequencies contained in the filtered, modulated signal; multiply the filtered, modulated signal and the selection frequency to generate a product signal; synchronously select the product signal at least on the scale of frequencies contained in the filtered, modulated signal; and generating a selected signal for each selection frequency, each of the selected signals having selected information. The selection frequency is preferably generated using an oscillator that can preferably increase (or decrease as desired) the selection frequency in increments ranging from 0.01 to 10 Hz. The sweep rate preferably ranges from 1 Hz per second to 200 Hz per second. Therefore, the method provides a record which relates the time to the selected frequency and consequently to the depth of the corresponding selected information, typically in the form of a field discontinuity or passing occurrence, which is indicative of the presence of a hydrocarbon or precious metals at that depth. The product signal contains the information selected in relation to the field discontinuities and can be recorded either in an analogous or digital form for processing and interpretation. Analogous detection of the tuned field discontinuities can be performed by feeding the heterodyne product signal to a level comparator for converting micro-field pulses, i.e., selected field discontinuities or passing occurrences, in, for example, representative pulses of 5 volts. The pulses of the comparator are counted and converted to a voltage to provide the number of pulses for a desired time to activate an analogous measuring device, for example, a pin recorder or a speed meter. The desired period can be from 0.01 seconds to 10 seconds. Preferably, a window detector using counter IC is used to count the number of micropulsations within a given period, i.e., a pulse density. This counter then generates an activation voltage when the count exceeds a previously specified number within a given period. These and other aspects and advantages of the present invention will be apparent from the following detailed description, wherein reference is made to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified functional representation of one embodiment of the present invention. Figure 2 is a graph that represents the depth to frequency ratio contained in the signal emanating from the surface of the Earth. Figure 3 is a simplified functional representation of another embodiment of the present invention. Figure 4 is a simplified functional representation of another embodiment of the present invention. Figure 5 is a simplified functional representation of another embodiment of the present invention, and Figures 6A to 6G are a detailed schematic representation of the embodiment shown in Figure 5.

DETAILED DESCRIPTION OF THE INVENTION Referring now to Figure 1, a simplified functional representation of an embodiment of an apparatus suitable for the purposes of the present invention can be seen. The apparatus 10 has an antenna 12 for receiving the low frequency signal emanating from the surface of the Earth. The emanation signal has a frequency scale on the electromagnetic frequency scale from 0 to approximately 5,000 Hz. Each of the frequencies on this scale is representative of underground information that corresponds to a certain depth below the surface of the Earth. See Figure 2 for a frequency versus depth relationship, the points representing real data and the dashed lines connecting the dots representing an interpolation of the data. More particularly, the frequency to depth relationship depends on the Earth's resistance capacity and electrical properties for a particular area. The actual data points in Figure 2 represent an average of the depth observed for the respective frequency from several well-known deposits. As noted above, the depths will vary from area to area for each particular frequency. Although the graph in Figure 2 can be used for estimation purposes, it is preferred that the depth / frequency ratio be determined for the particular area of interest. This can be done using the apparatus 10 to determine the locations of underground anomalies and to compare this data with the known information with respect to the area such as existing wells or seismic data, preferably, where the investigation was conducted using the apparatus 10. In this way , a frequency and depth correlation can be established in the particular area of interest. Referring again to Figure 1, the antenna 12 generates a received signal representative of the signal that emanates and contains information for all depths on the frequency scale of the signal. The antenna 12 is electrically connected to a product detector 14, which receives the received signal as a first input. The product detector 14 may be a demodulator, for example, LM1496 and LM1596, which may be balanced modular demodulators available from National Semiconductor, Santa Clara, California, which is configured as a demodulator. The apparatus 10 also includes an oscillator 16, which generates a selection frequency. Preferably, the oscillator 16 is capable of generating selection frequencies through the scale of frequencies contained in the received signal. Most preferably, the oscillator 16 is capable of sweeping through this frequency scale, preferably in increments or decrements from about 0.01 Hz to about 10 Hz. The sweep rate is preferably from about 1 Hz per second to about 200. Hz per second. The oscillator 16 is electrically connected to the product detector 14, which receives the selection frequency as a second input. The product detector 14 presses the selection frequency against the received signal to generate a selected signal for each selection frequency as an output. Therefore, each selected signal represents some depth within the Earth and contains information regarding underground anomalies, which may be a hydrocarbon, for example, oil and / or gas, or a mineral deposit, for example, precious metals. The product detector 14 can be connected to a recording device, such as a pin recorder of diagram 18 or to a storage and processing device, such as a computer 20. It is noted that this connection does not need to be a direct electrical connection, but it can be through a transmitter (not shown), which is electrically connected to the apparatus 10, which transmits the signal to a base location having a receiver (not shown) which in turn is electrically connected to the recorder 18 and / or computer 20. The computer 20 can store the raw data and manipulate the raw data in real time or at a later time. As the oscillator 16 sweeps through the frequency scale of the received signal, a curve is generated in relation to the frequency. The Curve is of signal strength at a particular depth against the frequency that represents the depth. Surprisingly, the curve is for the most part reproducible in a relative sense since the ratio of the relative resistance of the signal when compared to the different frequencies seems to be reproducible. In addition, the investigation performed with the apparatus 10 is an instantaneous aspect in time in the sense that a particular frequency is not maintained over a long period to see if there is a time variation of the signal resistance at the particular location. This avoids any fluctuation in the total resistance of the received signal, which can introduce an unnecessary error in the investigation. Accordingly, the research performed by the apparatus and obtained by the method of the present invention is distinguishable from that described in the patent of E. U. A. No. 5,148,110 of Helms. In addition, the investigation of the present invention is conducted while it is fixed. Helms recognizes that there will be no change in the received signal, but is not interested in the signal content to determine the character of the underground formation. Helms uses his invention in a fixed establishment only to detect a speed of change that occurs when a transient anomaly occurs near his detector, for example, when a seismic activity occurs or the passage of a mass of conduction or no large conduction happens beyond the detector, for example, a submarine. See column 7, line 63, column 8, line 15 of the mima. This is what Helms refers to as a time variation signal. Otherwise, Helms uses his invention as it traverses the surface of the Earth to detect changes at specific frequencies as a function of the distance traversed to locate anomalies underground. This is what Helms refers to as a signal of variation of location (surface). Referring now to Figure 3, a functional representation of another embodiment of the present invention is shown as apparatus 100. In order to improve the reliability and reproducibility of the apparatus according to the present invention, apparatus 100 has a voltage detector 22. Apparatus 100 is like apparatus 10, except that apparatus 100 also has voltage detector 22 and optionally has a counter 24 (shown in dashed lines). The voltage detector 22 receives the selected signal from the product detector 14, detects the selected information carried by the selected signal and converts the selected information to pulses. The pulses are produced by the voltage detector 22. The pulses or output signal can be recorded digitally on a tape or in an analogous manner on a tape recorder 18 and / or sent to a computer 20 to be presented on a CTR (not shown) and then manipulated. It is noted that the function of the voltage level detector 22 can be performed by the computer 20 in the apparatus 10 (Figure 1) by acting on the selected signal received by it in real time or after the fact by manipulating the stored selected signal information. and generating equivalent data in information content to the pulses generated by the voltage level detector 22. Preferably, the pulses are counted during a desired time to determine the density of the pulse, that is, the number of pulses per unit of time. If the computer 20 in the apparatus 10 or the voltage level detector 22 in the apparatus 100 generates the pulse data, the pulse density can be determined from the pulse data using the computer 20. In addition, the data from The pulse can be manipulated using the computer 20, for example, by varying the specified unit of time, i.e., the elapsed time, to examine the pulse density information. Alternatively, as shown in Figure 3, with the voltage level detector 22 in place, the apparatus 100 may further comprise a counter 24 (shown in dashed lines), which may be set to activate an output signal when a specific number of pulses per unit of time is obtained. The trigger point can be varied, thus varying the specified unit of time, i.e., the elapsed time, to examine the pulse density information. The output of the voltage level detector 22 can be received by the counter 24 instead of the recorder 18 and the computer 20. Alternatively, if desired, the recorder 18 and the computer 20 can receive the outputs of both the level detector and the detector. voltage 22 as the counter 24. Referring now to Figure 4, a simplified functional representation of another embodiment of the present invention is shown. In view of the low frequencies and the low signal strength of the signals emanating from the surface of the Earth, the received signal is preferably conditioned to increase its frequency and the signal strength to improve the detection of the selected signal information. Thus, in Figure 4, an apparatus 200 is shown having an antenna 12 for receiving the signal emanating from the surface of the Earth and generating a received signal. The antenna 12 is electrically connected to an amplifier 26, which in turn is electrically connected to a filter 28. The amplifier 26 may be, for example, a double operational amplifier TL072 available from Texas Instruments. Accordingly, the received signal is amplified and filtered resulting in a filtered, amplified signal. The filter 28 is electrically connected to a modulator 30. The modulator 30 is preferably a sideband modulator, for example, LM1496 and LM1596, which are balanced modulator demodulators available from Semiconductor, Santa Clara, California. The modulator 30 is also electrically connected to an oscillator 32, which generates a carrier wave. The oscillator 32 can be any sine wave oscillator. The signal is then modulated on the carrier wave generating a modulated signal. The modulated signal is then filtered by the filter 34 to eliminate one of the sidebands, for example, the lower sideband, which is a mirror image of the upper sideband and, therefore, contains redundant information in relation to the upper lateral band. The modulator 30 can also be configured and used as an individual sideband suppressed carrier demodulator, wherein the resulting signal has only one of the sidebands. The resulting signal can be used in other passive geophysical prospecting devices of the prior art, such as those identified in the background section of the present, since the resulting signal has been reinforced with power and filtered, and therefore has a better signal-to-noise ratio than signals that have been conditioned by methods in the prior art. However, as shown in Figure 4, the apparatus 200 preferably has a product detector 14, an oscillator 16 and a recorder 18 and computer 20. In this case, the oscillator 16 generates a selection frequency, which is then pressed against the filtered signal, modulated through the detector of the product 14 in order to select a particular frequency. The oscillator 16 scans through the frequency scale in the upper sideband portion of the filtered, modulated signal. For each desired frequency on this scale, a selected signal is generated corresponding to the desired frequency and, therefore, to a certain depth on the Earth. The output of the product detector 14 is then sent to the recorder 18 and / or computer 20. Alternatively (not shown), the detector output of the product 14 can be sent to a voltage detector 22 and then to the recorder 18 and / or the computer 20 as shown in Figure 13. As a further alternative, the voltage detector output 22 may be sent to a counter 24 and then to the recorder 18 and / or the computer 20, again as shown in FIG. Figure 3 (using the striped line configuration). As discussed with respect to Figure 3, if the apparatus 200 has a voltage detector 22, the voltage detector 22 receives the selected signal from the product detector 14, detects the selected information carried by the selected signal and converts the selected information to pulses. The pulses are produced by the voltage detector 22. The pulses or output signal can be recorded digitally on a tape or in an analogous manner on a tape recorder 18 and / or sent to a computer 20 to be presented on a CTR (not shown) and then manipulated. The pulses are counted during a desired period to determine the pulse density, that is, the number of pulses per unit of time. As noted above, the function of the voltage level detector 22 can be performed through a computer 20 using methods and techniques known to those skilled in the art acting on the selected signal received by it in real time or after the fact by manipulating the selected signal information stored and generating equivalent data in information content to the pulses generated by the voltage level detector 22. Either way, the pulse density can be determined from the pulse data using the computer 20. In addition, the pulse data can be manipulated using the computer 20, for example, by varying the specified unit of time, i.e., the elapsed time, to examine the pulse density information. Alternatively, with the voltage level detector 22 in place similar to Figure 3, the apparatus 200 may further comprise a counter 24, which may be set to activate an output signal when a specific number of pulses per unit of time it is obtained. The trigger point can be varied, thus varying the specified unit of time, i.e., the elapsed time, to examine the pulse density information. Referring now to Figure 5, a functional representation of another embodiment of the present invention is shown. An apparatus 300 is illustrated having an antenna 12, a first amplifier 26a, a first high pass filter 28a, a low pass filter 28b. a second amplifier 26b, a modulator 30, an oscillator 32, a second high-pass filter 34, a demodulator configured as a synchronous product detector 14, an oscillator 16, a low-pass filter 14, a third amplifier 36, a meter 38, a voltage level detector 22, a speed meter 40 and a pattern detector 42.

Antenna 12 in one embodiment is a metal core made of a metal that has a high susceptibility to magnetic fields but low retention time. The metal can be, for example, Permalloy (nickel and iron alloy). The metal core is then wound with, for example, copper magnet wire. The assembly is then placed inside a protective sheath, for example, a PVC tube. In a particular embodiment, the Permalloy core has a diameter of approximately 0.9525 cm for a length of approximately 10.16 to 15.24 cm, a single layer of 16-gauge copper magnet wire is wound around the circumference of the metal core about 2/3 of the length of the nucleus. Having this type of antenna relatively short in core length, the creation of inductance is reduced to a minimum, thus conserving the received signal. During use, the antenna can be laid on the surface of the Earth with the main axis of the antenna substantially parallel to it, or more specifically, substantially perpendicular to an imaginary ray extending from the center of the Earth to the surface of the Earth . In a specific embodiment the received signal was then amplified by the amplifier 26a using a double operational amplifier TL072. The high-pass filter 28a was an active high-pass filter consisting of a 9-pole drop, -120db at 60 Hz using a double operational amplifier TL072. The low pass filter 28b consisted of a 2-pole drop-60db at 8,000 Hz using a double operational amplifier TL072. The combination of the high-pass filter 28a and the low-pass filter 28b resulted in a frequency step band of about 90 Hz to about 8,000 Hz. The passed band signal is the one amplified using the amplifier 26b and sent to the input modulation of the sideband modulator 30, which was a balanced modulator demodulator LM1496. The carrier wave was supplied using a sine wave oscillator 32 set at 20,000 Hz. The modulated signal is filtered by a high pass filter using a 2 poles drop-40db at 20,000 Hz, active filter 34, comprising an operational amplifier LM356 . The resulting output has a frequency bandwidth of about 20,000 Hz to about 28,000 Hz, which was the upper sideband of the modulated signal. This signal is then sent to the signal input of a balanced modulator modulator LM1396 of National Semiconductor configured as a synchronous product detector 14 producing the product of the input signal and the output signal of the selection oscillator 16. The sweep oscillator selection 16 generated a sine wave from 20,000 Hz to 25,000 Hz. Sweep oscillator 16 also had the ability to do a continuous sweep across the bandwidth from 20,000 Hz to 25,000 Hz at several fixed continuous scan speeds. 1 Hz per second at 50 Hz per second. The detected signal resulting from the product detector 14 was then sent to a passive low pass filter 44 set at 10,000 Hz, and then amplified using an audio amplifier 36 LM380 from National Semiconductor. The output of the audio amplifier 36 contained the valuable information demodulated for the selected frequency. This output signal was then sent to the mounting level detector 22 comprising a LM311 voltage comparator from National Semiconductor. This detector 22 compared the signal input thereof against a selectable DC voltage to detect scalar voltage potentials in the selected demodulated signal. The reference DC voltage level should be used by adjusting a potentiometer to a desired level to increase or decrease the sensitivity of the detector 22. For example, the reference DC voltage level can be set to a value such as 4.95 volts so that slight variations above this level will be recognized on the signal scale of interest. The comparator, ie, the detector 22 was configured to produce pulses of 0 to 5 volts representing the detected scalar voltages that represent the important information with respect to underground geological formations and their contents, i.e., hydrocarbons or precious metals. The output of the comparator 22 was sent to a speed meter 40 which will be converted from pulses per second to a corresponding voltage. The voltage output of speed meter 40 was used to establish a baseline reference for recording purposes. The output of comparator 22 was also sent to a pattern detector 42, which connected the number of pulses in a given period and produced a response to a recorder 18 (not shown in Figure 5) when the preset number of pulses during a given period was found or exceeded. The preselected number of pulses in a given period is preferably adjustable. This variable can be adjusted based on the activity found in the signal of interest. This difference in activity may be due to the difference in the material being explored, for example, oil versus gold and / or the amount of such material found in the underground formation. An output of the comparator 22 is also available to the computer (not shown in Figure 5), wherein the pulses are digitized and processed using methods and techniques known to those skilled in the art to determine the pulse density during a selected period or unit of time. The processed information can then be printed using a printer (not shown). Figures 6A-6G with minor differences (details in parentheses when appropriate) are a detailed schematic representation of the embodiment shown in Figure 5. Figures 6A-6C illustrate antenna 12 and circuit system for amplifier 26a, the high pass filter 28a, low pass filter 28b, amplifier 26b, modulator 30, oscillator 32 and high pass filter 34 (an operational amplifier LM356 is named in the discussion of Figure 5 and a pair of operational amplifiers TL072 are identified in Figure 6C). Figures 6D and 6E illustrate the circuit system for oscillator 16 (referred to in Figures 6D and 6E, 16A and 16B). Figure 6F illustrates the circuit system for amplifier 46 (specifically not discussed with respect to Figure 5, but shown in dashed lines in Figure 5, although present in Figure 6F), product detector 14, filter 44 and amplifier 36 (the discussion of Figure 5 indicates an LM380 and Figure 6F identifies LM384). Figure 6G illustrates the circuit system for the voltage level detector 22, the speed meter 40 and the pattern detector 42. The following is a list of the components in Figures 6A to 6G. With respect to adjustable resistors or potentiometers, "(adj.)" Is indicated after the maximum resistance. "TL072" and "TL082" refer to operational amplifiers available from Texas Instruments. "LM1496", "LM384", "LM555" and "LM381" refer to products available from National Semiconductor. "ML2036" is a programmable sine wave generator available from Micro Linear. "SPG 8540 AN" is a crystal oscillator available from Analog Devices. "2n 7000" is a transistor, where "G" represents a gate, "D" represents a drain, and "S" represents a source. "4516", "165", "4013", "4020", "4024" and "4060" are generic chips known to those skilled in the art. "OP 290" is an operational amplifier.

Crystal: Crystal of C27 - 4 194 mega Hz Capacitors C1 - 100 pF C18 • - 25μF C36 - 5μF C2 - 1μF C19 - 25μF C37 - 0.1μF C3 - 0.1μF C20 - 0.1F C38 - 220μF C4 - 0.1μF C21 - 0.1F C39 - 1μF C5 - 0.1μF C22 - .002F C40 - 33μF C6 - 0.068μF C23 - 0.1μF C41 - 1μF C7 - 0.068μF C24 - 0.001F C42 - 0.01F C8 - 0.68μF C25 - 0.001F C43 - 0.01 F C9 - 0.68μF C26 - 1μF C44 - 0.1F C10 - 0.1μF C28 - 0.1μF C45 - 0.01F C11 - 0.1μF C29 - 0.1μF C46 - 0.001F C12 - 0.68μF C30 - 0.01F C47 - 4.7μF C13 - 0.68μF C31 - 0.01F C48 - O.OIμF C14 - 0.1μF C32 - 0.01F C49 - 470pF C15 - .001F C33 - 1μF C50 - .1F C16 - .001F C34 - 0.1μF C51 - 1μF C17 - 0.1μF C35 - 5μF Resistors R1- 500 ohms (adj) R25 - 100 ohms R49 - 1K ohms R2 - 1 meg ohms R26 - 100 ohms R50 - 1K ohms R3 - 100 ohms R27 - 13K ohms R51-10K ohms (adj) R4 - 1 K ohms R28 - 10K ohm R52 - 50K ohm R5 - 100 ohm R29 - 1K ohm R53 - 50K ohm R6 - 50K ohm (adj) R30 - 1K ohm R54 - - 1K ohm R7 - 1K ohm R31 - 820 ohm R55 - • 1K ohm R8 - 10.2K ohm R32 - 1K ohm R56 - -1K ohm R9 - 17.4K ohm R33 - 1K ohm R57 - - 1K ohm R10 - 7.5K ohm R34 - - 10K ohm (adj) R58 - - 5K ohm (adj) R11 - 61.9K ohm R35 - • 380K ohm R59 - - 1K ohm R12 - 17.4K ohm R36 - - 1K ohm R60 - 6.8K ohm R13 - 14K ohm R37 - - 1K ohm R61 - 50K ohm R14 - 3.32K ohm R38 - - 10K ohm (adj) R62 - 3.9K ohm R15 - 113K ohm R39 - 380K ohm R63 - 3.9K ohm R16 - 2.72K ohm R40 - 5K ohm (adj) R64 - 1K ohm R17 - 14K ohm R41 - 3K ohm R65- 10K ohm (adj) R18 - 1K ohm R42 - 1 meg. Ohm R66 - 1K ohm R19 - 10K ohm R43 - 20.43K ohm R67 - 1K ohm R20 - 10K ohm R44 - 3.26K ohm R68 - 20K ohm R21 - 10K ohm R45 - 3K ohm R69 - 5K ohm (adj) R22 - 50K ohm R46 - 10K ohm R70 - 1 meg ohm R23 - 10K ohm R47 - 10K ohm (adj) R71 - 2.7 ohm R24 - 100 ohm R48 - 10K ohm R72 - 50K ohm R73 - 2 meg ohm R82 - 20K ohm R91 - 10K ohm R74 - 1K ohm R83 - 100K ohm R92 - 1K ohm R75 - 5K ohm (adj) R84 - 1K ohm R93- -10K ohm (adj) R76 - 10K ohm R84 - 100K ohm R94 - 1K ohm R77 - 10K ohm R86 - 6.8K ohm R95 - 1K ohm (adj) R78 - 100 K ohm R87 - 10K ohm R96 - 10K ohm R79 - 100K ohm R88 - 68K ohm R97 - 5K ohm (adj) R80 - 100K ohm R89 - 10K ohm R81 - 100K ohm (adj) R90 - 10K ohm (adj) Chips and Operational Application: U 1 - TL072 U12 - TL072 U24 - LM555 U2 - TL072 U13 - TL072 U25 - 4060 U3 - TL082 U14 - TL072 U26 - LM331 U4 - TL082 U15 - TL072 U27 - OP290 U5 - TL082 U16 - TL071 U28 - OP290 U6 - TL082 U17-LM1496 Z1 - 4516 U7 - TL072 (fix pin 1 to 400mV Z2 - 4516 U8 - TL072 and leg 8 to 300) Z3 - 4516 U9 - LM1496 U18 - TL072 Z4 -4516 (fix pin 1 to 325 mV U19 - TL072 Z5 - 165 and leg 8 to 10 mV) U20 - LM384 Z6 - 165 U 10 - SPG 8540AN U21 - LM311 Z7 - 4013 (Base Clock) U22 - 2n7000 Z8 - 4013 U11 - TL072 U23 - 4024 Z9 - 4020 Z10 - 4516 Z11 - ML2036 Switches SW1 SW2 SW3 In each of Z1, Z2, Z3 and Z4, legs 10 and 16 are connected to Vcc and legs 8 and 9 are connected to ground. In each of Z5 and Z6, the leg 16 is connected to Vcc and the leg 8 is connected to ground. In Z8, legs 6, 7 and 8 are connected to ground. In Z9, legs 8 and 11 are connected to ground. In Z10, legs 1, 3, 4, 5, 8, 9, 12 and 13 are connected to ground. In Z11, legs 11 and 12 are connected to ground.

EXAMPLES EXAMPLE 1 In this example, an apparatus according to that shown in Figures 6A-6G was tested to determine the accuracy of the apparatus in relation to known underground hydrocarbons. As the test site, the Channing gas field was selected over the Morrow formations in eastern Colorado. As the apparatus sweeps through the frequencies of passive electromagnetic fields emanating from the earth, the selected field activity reacts to discontinuities caused by underground hydrocarbons and was expressed as a barb or a series of barbs on the opposite recorder at a given frequency. Frequency refers to depth using Figure 2. Production formation in this field was located approximately 1402.08 m below the surface of the land. The equipment was easily fixed to a small carrier box, which was easily fixed on the back of a sport installation vehicle. A total of fourteen (14) readings in different locations were taken, but two were in invalid as they were taken outside the preferred time window in the morning at sunset or due to atmospheric disturbances such as thunderstorms. Eight (8) of the remaining readings were on the formation that takes oil and four (4) outside the limits of the formation. Of the eight (8), six (6) readings gave positive indications of the presence of hydrocarbons at the depth of interest. This represents a 75% success scale for detecting hydrocarbon-bearing formations. One (1) of the negative readings on the field after was in the Springer well 1-33. It is believed that this negative reading resulted due to the thin formation that has a thickness of approximately 152.4 m in this area and the effect of a nearby production well decreased the formation at this point. It should be noted that partially depleted fields in Texas such as the Delhi field and the Katy field gave good responses when investigated through this apparatus, but these fields produce from a thicker hydrocarbon range. Similar false readings can occur in training areas, which have been flooded with water, particularly if the zones are relatively thin. One (1) of the positive points was repeated later. These two readings were very similar and showed a good repeatability achieved by the device. Of particular interest were the four (4) readings taken outside the formation limits. All these readings gave negative indications for the presence of hydrocarbon. Therefore, the apparatus obtained a 100% success scale to determine the absence of hydrocarbons in underground formations. Three other fields in the Morrow formation were tested outside the limit of hydrocarbon bearing areas in them and obtained a success rating of approximately 93%. These results they make the apparatus absolutely valuable as a prospecting tool in exploration areas, at least to regulate areas where multiple readings are taken and a substantial portion of it provides negative indications for the presence of hydrocarbons EXAMPLE 2 In this example, the apparatus used in Example 1 was used for prospection of gold-bearing formations. An area of interest was analyzed. Based on the underground samples, the apparatus with multiple readings methodically located around the area being explored successfully located a large gold carrier deposit and I provide the thickness and boundaries of the vein and the inclination angle of the same

Claims (12)

1. - An apparatus for passive geophysical prospecting based on the detection of discontinuities associated with low frequency electromagnetic fields, the apparatus comprises: an antenna for collecting an extremely low frequency signal emanating from the surface of the Earth, where the signal it contains a frequency scale and where each of the frequencies contained in the signal corresponds to a certain depth on the Earth; an oscillator that generates a frequency of selection, in which the oscillator is capable of generating and sweeping through the frequencies contained in the signal; and a product detector that receives the signal collected by the antenna and multiplies it with the selection frequency generated by the oscillator generating a selected signal, the selected signal containing information with respect to underground geological formations at a certain depth corresponding to the frequency of selection.

2. The apparatus according to claim 1, characterized in that it comprises: means for digitizing the selected information contained in the selected signal.

3. The apparatus according to claim 2, wherein the digitizing means comprises a voltage detector, wherein the voltage detector receives the selected signal from the product detector, detects the selected information carried by the selected signal and converts the information selected in pulses.

4. The apparatus according to claim 3, wherein the digitizing means further comprises a counter for counting the pulses received from the voltage detector and generating an output signal corresponding to the number of pulses counted during a specified time unit. .

5. The apparatus according to claim 2, wherein the digitizing means comprises a computer programmed to detect the selected information carried by the selected signal and convert the selected information to pulses.

6. The apparatus according to claim 1, characterized in that it also comprises means for conditioning the received signal.

7. The apparatus according to claim 6, wherein the conditioning means comprise: an amplifier and a first filter that in combination amplify and filter the received signal and generate an amplified, filtered signal: a modulator to modulate the amplified signal , filtered on a carrier wave and generating a modulated signal having the carrier wave, an upper sideband and a lower sideband; a second filter to eliminate one of the sidebands; and where the oscillator sweeps through the frequencies contained in the remaining sideband.

8.- A method for passive geophysical prospecting, the method includes: receiving a signal that emanates from the surface of the Earth with an antenna; generate a received signal corresponding to the signal emanating from the Earth's surface, the received signal containing a frequency scale; generate a selection frequency; make a sweep of the selection frequency through at least the scale of frequencies contained in the received signal; multiply the received signal and the selection frequency to generate a product signal; synchronously select the product signal on the scale of frequencies contained in the received signal; and generate a selected signal containing selected information.

9. The method according to claim 8, characterized in that it further comprises converting the selected information to pulse information representative of the same.

10. The method according to claim 9, wherein the conversion step comprises comparing the selected information with a reference point and generating pulse information in relation to the reference point.

11. - The method for passive geophysical prospecting, the method includes: receiving a signal that emanates from the surface of the Earth with an antenna; generate a received signal that corresponds to the signal emanating from the surface of the earth; amplifying the received signal to generate an amplified signal; generate a carrier wave using an oscillator; modulating the carrier wave with the amplified signal generating a modulated signal having the carrier wave, an upper sideband and a lower sideband; optionally canceling the carrier band of the modulated signal by generating an output signal having upper and lower sidebands; and eliminate one of the sidebands and generate an individual sideband signal.

12. The method according to claim 11, characterized in that it further comprises: generating a selection frequency; to sweep the selection frequency through at least the scale of frequencies contained in the individual sideband signal; multiplying the individual sideband signal and the selection frequency to generate a product signal; synchronously selecting the product signal on at least the scale of frequencies contained in the individual sideband signal; and generating a selected signal for each selection frequency, each of the selected signals having selected information relative to a particular depth on the surface of the Earth.