WO2015167894A1

WO2015167894A1 - Heterodyned downhole source

Info

Publication number: WO2015167894A1
Application number: PCT/US2015/027048
Authority: WO
Inventors: Peter M. Eick; Joel D. Brewer
Original assignee: Conocophillips Company
Priority date: 2014-04-29
Filing date: 2015-04-22
Publication date: 2015-11-05

Abstract

Disclosed are devices for producing seismic signals for use in oil and gas exploration. In particular a downhole, passive sensor based on heterodyne signal generation is provided.

Description

HETERODYNED DOWNHOLE SOURCE

FEDERALLY SPONSORED RESEARCH STATEMENT [0001] Not applicable.

REFERENCE TO MICROFICHE APPENDIX [0002] Not applicable.

FIELD OF THE DISCLOSURE

[0003] The disclosure generally relates to downhole seismic surveying sources and, particularly, to sources with controllable seismic signal direction.

BACKGROUND OF THE DISCLOSURE

[0004] Geophysics is the study of the physics of the Earth, especially its electrical, gravitational, and magnetic fields, and the propagation of the elastic (i.e. seismic) waves within it. A major part of the search for oil and gas requires information concerning the geological features where hydrocarbons may be trapped.

[0005] Seismic surveying allows for mapping of subsurface distribution of different types of rocks and the fluids they contain. Oil field seismology, which accounts for approximately 95 percent of all geophysical activities worldwide, has traditionally been applied from the surface of the earth. The source and receivers of a surface seismic survey are usually positioned on the surface (or close to it), because surface equipment is the easiest to deploy. However, many advantages, including higher reservoir resolution, are achieved by placing the source and/or the receivers down inside the boreholes. This has been demonstrated over the last twenty years with experimental cross-well seismology— the most tested of the three new borehole seismic techniques.

[0006] Borehole seismic surveying can employ either active or passive sensing. Active sensing utilizes frequency ranges from about 50 to 1600 Hz and is most commonly used as an exploration tool for finding oil and gas containing hydrocarbon reservoirs. These frequencies are due to equipment capabilities and limitations. However, recent work has focused on passive sensing and trying to use the lower frequencies than were previously obtained. In passive sensing, low frequencies of less than 10 Hz are employed and they travel greater distances through the earth's crust with less attenuation than active frequencies. Furthermore, the information found in this frequency range has the potential to provide more accurate reservoir location information.

[0007] Passive or low frequency (LF) seismic analysis produces attributes that describe the variation of the naturally occurring seismic wave field below 10 Hz. A growing number of surveys over different oil and gas fields throughout the world have established the presence of spectral anomalies in the earth's ambient seismic wave field— microtremors— with a high degree of correlation to the location of hydrocarbon reservoirs. These anomalies may be directly related to the fluids inside the reservoir structure rather than to the reservoir structure itself. An analysis of the anomalies can therefore be used, together with other reservoir data prior to drilling, as an indicator for optimizing well placement during exploration, appraisal, development, and field extension, or reservoir management.

[0008] Conventional surface seismic data normally has a bandwidth of about 10 Hz to about 60 Hz for most land type sources. It tends to be higher if explosive sources are used and lower if vibratory sources are used. Thus, there is not a large overlapping frequency range between existing borehole sources and existing surface sources, making the comparison between the two elements difficult and subjective.

[0009] The potential benefits to the oil and gas company from low frequency seismic surveying are compelling. There is an increased probability of success in defining hydrocarbon zones prior to embarking on a drilling program, with fewer dry or nonproductive wells, lower drilling costs, and reduced health, safety, and environmental risk. LF seismic surveying also opens up opportunities that were previously considered "unexplorable." New LF technology developments allow the economic development and production of reserves that were thought to be uneconomic due to physical accessibility, field size, and/or geological complexity. Furthermore, LF data provides information on the subsurface fluids in complex geological settings and can be economically acquired in relatively small areas that are uneconomic for traditional seismic and other surveys prior to drilling.

[0010] Ideally, low frequency or passive surveying can be coupled with a borehole to provide the critical depth and velocity parameters needed to link surface seismic data with downhole log and well data. However, the major impediment to widespread use of borehole seismic techniques has been the lack of a suitable, commercially available, downhole seismic source that has the bandwidth to cover from a few Hz up to about 100 Hz where existing borehole seismic sources take over and generate sufficient signal strength. This is especially true of passive seismic sensing. Passive seismic sensing is necessary for the low frequency spectral region because man-made generators of seismic energy (explosions or sledgehammer-like vibrators) do not produce enough energy at the low frequencies needed.

[0011] Thus, what is needed in the art is a reliable downhole wellbore source capable of passive surveying, but also able to generate sufficient energy up to approximately 100 Hz where existing borehole vibratory sources take over. Ideally, this source will allow for controllable and directional surveying.

SUMMARY OF THE DISCLOSURE

[0012] The present disclosure describes a downhole seismic source wherein transducer technology is combined with heterodyning techniques to impart higher levels of seismic energy into the formation. Two or more sub-sources, such as marine sonar transducers, are heterodyned to create controlled pressure pulses in a fluid media or at a fluid-earth boundary. The interference pattern created by the sub-sources is the source for the seismic energy for the survey. The pressure waves created herein are akin to those created by propeller driven airplanes during take off or a beat frequency oscillator in a ham radio. The pulsing and 'beat' frequency that occurs while the airplane control mechanism syncs up to the phase and speed of the rotation of the propellers is analogous to the pressure waves created herein.

[0013] "Heterodyning" is a radio signal processing technique invented in 1901, in which new frequencies are created by combining or mixing two frequencies. In the most common application, two signals at frequencies fl and £2 are mixed, creating two new signals, one at the sum fl + £2 of the two frequencies, and the other at the difference fl - £2. These new frequencies are called heterodynes. Typically only one of the new frequencies is desired, and the other signal is filtered out of the output of the mixer.

[0014] Heterodyning is not a new concept to seismic surveying, particularly in the oil and gas exploration area. US3 16632 describes the use of resonator rods to impress the signal on the ground for seismic surveying. In particular, first and second mechanical sonic generators, which generate high level sonic energy at frequencies substantially higher than and which differ by a desired seismic transmission frequency, have their outputs coupled to the resonant vibration system rods. The outputs of the resonant vibration systems are mixed in a nonlinear device, i.e. heterodyned, to obtain a beat frequency difference signal at the desired seismic transmission frequency. US3302745 describes heterodyning waves produced from a seismic transducer to produce low frequency seismic waves. However, these are surface based systems, and to date, heterodyning has not been used for a downhole system, nor with passive surveying.

[0015] In the present method, a first sub-source emits a constant frequency, while at least one additional sub-source sweeps around that constant frequency, e.g., from the constant frequency to a lower frequency. As the frequency of the additional sub-source changes, the signals begin to go out of phase and develop the heterodyning effect. This method will allow for the establishing the constant frequency at some useful level for maximum power output, then sweeping the additional sub-source around the constant frequency to find acoustic heterodynes with the best transmission ability. The interference pattern between the sub-sources will impinge the wellbore or hydrocarbon fluid at some common focal point. The heterodynes at the impingement will serve as the source for the seismic survey.

[0016] The constant frequency chosen for the first sub-source can be a single frequency from e.g., 10 kHz-50 kHz, or thereabout. This frequency is convenient because existing commercially available high power output marine sonar transducers are capable of this frequency at the lower end of their range. However, many other high frequencies can be used as well since the heterodyne will use the difference frequencies. Furthermore, if a transducer can be provided with sufficient power at LF ranges (0-10 Hz) or higher (10-1000 Hz), then additive beats could also be used.

[0017] To successfully operate the source in diverse geological and reservoir conditions it is important to generate a wide bandwidth signal. High seismic frequencies (500-1,000 Hz) are necessary to achieve higher resolution images of small geologic features, but low seismic frequencies (1-100 Hz) are necessary for long range surveys and surveys in low Q sediments. Thus, preferably the difference heterodynes generated are in both these frequency ranges.

[0018] The second sub-source (preferably identical to the first sub-source) can sweep around the constant frequency, either up or down, depending on the value of the constant frequency and whether difference or additive beats will be used. Once the best heterodyne signals are obtained, the second sub-source can simply operate at that second frequency if desired, but additional information may be available if the second sub-source sweeps an optimal frequency range.

[0019] To perform single well seismic imaging of geologic features far away from the borehole, the source must not produce strong tube waves. This can be achieved by clamping the source to the borehole wall rather than coupling the seismic energy to the formation via the borehole fluid. Thus, clamping to the wellbore is a preferred embodiment.

[0020] Because liquid is required to efficiently propagate the heterodyned energy, the sub- sources will need to be water-proof and capable of enduring the conditions found in a typical oil well. As such, the preferred sub-source are marine sonar transducers that can handle high temperatures and brine, water or oil filled boreholes, but the sub-sources can be any small piezo or electrostatic/mechanical sonar source.

[0021] In order to more easily maintain a defined orientation for each sub-source, they may be contained within a housing or framework, sufficient to fix the orientation of each sub-source. In some embodiments, the orientation can be changed from the surface, by sending signals to the housing to rotate one or more of the sub-sources, which would be mounted e.g., on a controllable gantry. In such an embodiment, an accelerometer can be added to the source to monitor and/or control the rotation.

[0022] The housing can either be fluid filled to allow for wave propagation, but more simply is perforated or of framework construction so that oil, brine/water and/or mixes thereof in the wellbore fills the space between the sub-sources providing a medium for wave propagation.

[0023] In one embodiment of the present device, two or more sub-sources are configured such that they point at the same focal area on a well-bore wall and transmit through a fluid in contact with the well-bore wall. By varying the frequency emitted by one of the transducers, a heterodyne pulse will be created at the well-bore wall. By quickly altering the transducer's frequency, the heterodyned pulse will be a signature series of pulses or beats that will propagate through the earth as a seismic signal. Because the transducers are pointed at the same target area, or focal point on the well-bore wall, the emitted seismic signal is directional.

[0024] In another embodiment of the present device, two or more sub-sources are configured such that they point at each other. The assemblies are mounted perpendicular to the well-bore and transmit in a fluid media (e.g., hydrocarbon fluid in the wellbore) at the focal point; however, the transducers are separated so that a heterodyne pulse will occur between them. Thus, the heterodyne pulse is triggered by the transducers emitting the correct frequencies. By "turning on and off (either by frequency shift or on/off power change) the heterodyne effect, a signature series of pulses are created. Because the focal point of the heterodynes is between the sub-sources, the interference pattern will be felt by the entire well-bore wall as opposed to a specific focal point in the above embodiment. Such a configuration would result in an omnidirectional seismic signal.

[0025] In yet another embodiment, sub-sources are arranged in a circular pattern in a closed system and are pointed at a central location, e.g., in the middle of the sub-sources. The transducers are located within a liquid cavity that includes the central point such that each transducer is pointing at the liquid environment. This will facilitate the propagation of energy to the wellbore, and thence to the reservoir. Such a configuration would also result in an omnidirectional seismic signal.

[0026] In the above embodiments, the frequencies of the pressure pulse at the heterodyning point, i.e. the transducer's focal point, will be selected to produce a heterodyned signal with the maximum power output. The interference pattern at the focal point on the well bore or in the fluid in the wellbore will create the seismic waves that will then be propagated to deform the strata. Receivers or sensors located on the surface or within the wellbore measure the reflections of the seismic waves. The receiver data is processed by computer to construct 3-D maps of the underlying strata in the wellbore.

[0027] Marine sonar transducers are suitable for use herein and operate in a 30 kHz -200 kHz range with the limited ability to tune the frequency output. This particular range is beyond what is usable for seismic surveying. However, the heterodyne process will allow for syncing of the frequency of multiple transducers to create usable pressure pulses. For marine sonar transducers, the summation frequencies will usually be filtered out because the frequency is too great. Thus, typically only the difference frequencies will used in the seismic mapping.

[0028] Any commercially available marine sonar transducer can be used in the present device. Preferably, the selected transducer is capable of emitting waves in the 0 to 400 KHz range with a power rating of 2 kWatts. Power output must be high enough to achieve a seismic transmission distance of 1,000 - 2,000 meters (3,000 - 6,000 feet) between wells or between a well and the surface. [0029] Furuno™ offers a series of marine transducers emitting waves of 50 or 200 kHz with a power rating of 0.6-2 kWatts. Raymarine™ offers transducers that are tuned to either 50 or 200 kHz. Alphatron Marine™ offers single or dual frequency units (for frequencies of 30-210 kHz) with up to four transducers. Garmin™, Airducer™, and Humminbird™ also offer marine transducers.

[0030] Many marine sonars are available with more than one transducer or with single or dual frequency settings. In the present disclosure, the marine sonar can have any number of transducers as long as it fits in the borehole. Furthermore, while a dual frequency sonar offers greater resolution, the single frequency sonars are more than adequate for the sweeping and heterodyning output in the present application and are more cost effective. However, both are acceptable. Dual frequency sonars are still considered to be first and second sub-sources herein.

[0031] The preferred transducers are able to withstand the pressure and temperature of deployment at depth, and the housing materials are resistant to oilfield chemicals and conditions. For example, in an active area of exploration in south central Texas, the production occurs at pressures of 10,000 PSI at 300°F, and the hydrocarbons may have corrosive components. The sensor would need to be operational under these environmental conditions. Other potential sub- sources include small piezo and electrostatic/mechanical sub-sources.

[0032] The generated sonar waves will be mixed to produce the desired heterodynes. The ability to isolate signals, or reject unwanted ones, is a function of the heterodyne receiver bandwidth. For example, the band-pass filter in the receiver is what isolates the desired signal from the adjacent ones. However, in this particular application, superheterodyning, or using the lower side-band (i.e. F2-F1), is the preferred method of frequency selection. The advantages of superheterodyning include reduction of the signal from very high frequency sources where ordinary components would not work and improve signal isolation by arithmetic selectivity.

[0033] The resulting "beat" frequency from the lower-side band heterodyne will be a pressure pulse that will induce deformation in the strata of the well bore. Thus, this beat acts as a seismic source. By creating a seismic source this way, the power in the marine sonar signal is transferred to the 'beat'. As such, the present device results in a powerful, yet low frequency wave, needed for passive seismic surveying.

[0034] One requirement of using heterodyned beats as a source signal is the need for a fluid to propagate the signal. Fluid is normally present in the wellbore, and the sub-sources will thus normally contact fluid. For configurations where the sonars are pointed at a particular focal point of the well-bore wall, the beat will be efficiently transmitted as long as the fluid or strata contacts the same well-bore wall. Thus, the controlled pulses are created in the fluid-solid boundary.

[0035] It should also be noted that high power output marine style transducers require the fluid to maintain coupling and avoiding burnout. This is a common problem with ultrasonic cleaners, which are another version of the same basic concept. Thus, preferred heterodyned signal sources will have a perforated or framework housing allowing each transducer to contact fluid or be housed in a fluid filled cavity.

[0036] Any seismic detector capable of sensing the reflected low frequency seismic waves, but rugged enough for borewell use can be used with the present device. MET Technology™ offers a low frequency sensor MTLF-1040. Other suitable seismometers include Geospace Technologies's™ GS-1 and Sercel's L-4.

[0037] As used herein, "omnidirectional" means the signal propagates in all directions surrounding the focal point of the device.

[0038] As used herein, "directional" means the majority of the signal propagates in a 0±30° direction from the focal point of the device.

[0039] As used herein, the terms "heterodyning point" or "focal point" are used interchangeable to mean the point at which the heterodyned wave is tuned to pulse. When multiple transduces are pointed at the same point, the heterodyne can be tuned to pulse at that distance from the transducers to create the pressure wave that is subsequently transmitted to the earth.

[0040] As used herein a "heterodyne source signal" is the beat produced by heterodyning and which serves as the source of seismic waves for seismic mapping.

[0041] As used herein a "sub-source" is one of at least two signal sources needed to create the heterodyne source signal.

[0042] As used herein a "heterodyne-source" requires at least two sub-sources to make a heterodyned beat. The two sub-sources can be contained in the same housing however, and need not be separate.

[0043] As used herein, "low frequency" or "LF" seismic surveying is 0-10 Hz. [0044] The use of the word "a" or "an" when used in conjunction with the term "comprising" in the claims or the specification means one or more than one, unless the context dictates otherwise.

[0045] The term "about" means the stated value plus or minus the margin of error of measurement or plus or minus 10% if no method of measurement is indicated.

[0046] The use of the term "or" in the claims is used to mean "and/or" unless explicitly indicated to refer to alternatives only or if the alternatives are mutually exclusive.

[0047] The terms "comprise", "have", "include" and "contain" (and their variants) are open- ended linking verbs and allow the addition of other elements when used in a claim.

[0048] The phrase "consisting of is closed, and excludes all additional elements.

[0049] The phrase "consisting essentially of excludes additional material elements, but allows the inclusions of non-material elements that do not substantially change the nature of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0050] FIG. 1. displays an embodiment of the presently described device wherein the transducers are pointing at the same point on the wellbore.

[0051] FIG. 2. displays one embodiment of the presently described device wherein the transducers are pointing at each other.

[0052] FIG. 3. displays another embodiment of the presently described device wherein multiple transducers are pointing at a central point in the wellbore.

DETAILED DESCRIPTION

[0053] The disclosure provides a novel device for creating a low frequency downhole source for passive seismic surveying, methods of using the device, and systems including the device.

[0054] Specifically, the signals from two or more sub-sources, such as marine sonar transducers, are heterodyned in a well bore containing a fluid. The heterodyning creates a pulsed signal that the fluid and/or wellbore feels as a pressure wave capable of being transmitted through the earth as a seismic signal.

[0055] The first sub-source is maintained at a constant frequency that will provide maximum power output. The second sub-source will then sweep frequencies within 1000 or 500 Hz of the constant frequency, or within 100 Hz, or within 10 Hz of the constant or base frequency established by the first sub-source, depending on survey needs. The results beats will be LF, e.g., <10 Hz, and/or intermediate or higher e.g., 10-100 or 101-1600 Hz. Multiple first and second sub-sources can contribute to the strength of the source signal.

[0056] The actual source of the seismic energy for surveying purposes will be the interference pattern that is created by the sub-sources at the point of impingement of the wellbore wall or fluid, as appropriate. This energy will be transmitted to the strata and variably reflect back towards a receiver in the wellbore or above ground. The receiver can remove non-useable waves using a low pass filtering prior to digitization. Because the constant frequency is maintained at a frequency of maximum power output, the seismic energy waves produced by the current device will have more power and discriminatory abilities than previous downhole seismic sources.

[0057] Depending on the configuration of the sub-sources, a directional or omnidirectional signal is possible. For the directional signal, the device can be rotated within the wellbore or held stationary, as desired if the device is also equipped with directional control means.

[0058] The invention includes one or more of the following embodiments, in any combination thereof:

[0059] A downhole heterodyne seismic signal source device, said device comprising: housing shaped to fit inside a borehole, said housing having: a first sub-source transducer, wherein said first sub-source transducer emits at a constant frequency, and a second sub-source transducer, wherein said second sub-source transducer emits at a sweeping frequency; said first and second sub-source transducers producing a heterodyne source signal at a common focal point; a hanging means for lowering said housing into a wellbore attached to said housing; and optional means for attaching said device to another object on said housing or on said hanging means.

[0060] A method of providing a downhole heterodyne seismic signal source for oil and gas seismology, said method comprising: lowering a heterodyne signal source into a hydrocarbon well-bore having a fluid in contact with a well-bore wall, said heterodyne signal source comprising first and second sub-sources; emitting a first constant frequency from said first sub- source and sweeping over a frequency range from said second sub-source so as to generate a heterodyne source signal. [0061] A method of using the heterodyne seismic signal source device for oil and gas exploration, comprising: lowering a device as herein described into a fluid in a well-bore by said hanging means; creating a heterodyned beat seismic wave with said first and said second sub- source transducers; and collecting data from the reflection of said seismic waves by the earth using a passive frequency receiver.

[0062] An improved method of generating a downhole source signal for seismic surveying, said method requiring a downhole source to generate a seismic signal, the improvement comprising a first downhole sub-source that provides a constant frequency, and a second downhole sub-source that sweeps from said constant frequency to a second frequency, said first and second sub-source generating a heterodyning beat that serve as the source signal for the seismic survey. Preferably, the constant frequency is an intermediate frequency for example of 3500 Hz, and said second downhole sub-source sweeps a variable sweep of around ± 100-1000 or ±100-500 Hz to create a high frequency beat in the 100-1600 Hz range, a 10-100 Hz sweep will create an intermediate beat between 10 and 100 Hz, and a ± 0-10 Hz sweep will create a low frequency sweep in the range of about 10 Hz. These ranges can of course be variously combined, as desired by the surveyor.

[0063] A system for passive downhole sensing, said system including the device as herein describe, plus receiver for receiving data and a computer processor for processing said data.

[0064] A system for passive downhole sensing, said system including the device as herein describe, plus a mixer, a filter, receiver, a computer processor and data display means.

[0065] The device, method or system as herein described, wherein said first and said second sub-source transducers are pointed at each other.

[0066] The device, method or system as herein described, wherein said common focal point is located on the wellbore wall.

[0067] The device, method or system as herein described, wherein said sub-source transducers are marine sonar transducers.

[0068] The device, method or system as herein described, said first sub-source transducer emitting at a frequency between 10 and 50 KHz or about 30 KHz.

[0069] The device, method or system as herein described, said second sub-source transducer sweeping at frequencies around the constant frequency, e.g. within 500 or 1000 Hz of the constant frequency or within 10 Hz for LF surveys. [0070] The device, method or system as herein described, wherein said first and said second sub-source transducers are in contact with a liquid.

[0071] The device, method or system as herein described, wherein the device comprises a plurality of second sub-source transducers and wherein said first and said second sub-source transducers are arranged in a circular configuration and pointed at a common focal point.

[0072] The device, method or system as herein described, wherein said first and said second sub-source transducers are enclosed in a liquid-filled housing, said liquid-filled housing being in contact with wellbore wall.

[0073] The present device includes two or more sub-sources attached to a housing that is preferably coupled to the wellbore and is contained entirely within the wellbore. The sub-sources are configured to point at one particular focal point on the well bore or at a focal point central to each sub-source. The directional control of the resulting signal is dependent on the sub-sources' configuration.

[0074] FIG. 1 depicts a seismic source device 1000 having two or more sub-source transducers 1005 are aimed at the same point 1010 on the well bore wall. The transducers 1005 are attached to a housing 1002 that can be attached to the well bore or freely hanging to allow for rotation of the device 1000. Such configuration results in a directional signal. The advantage of the directional signal is that seismic information in one direction can be obtained, or in the alternative, the device can be rotated in the well to obtain information for any given direction. Ideally, a means for lowering the device, such as a rope or cable, can be used to rotate the device as well. This can be beneficial when trying to focus on a suspected hydrocarbon pocket.

[0075] The strongest point of focal imaging is in the line of plane, but there will also be off- axis alignment points of the heterodyned energy. Thus, for a receiver/mixer located at the focal points, off-axis heterodyned energy would have to be suppressed to reduce interference and vice versa.

[0076] The device in FIG. 1 will produce pulsed pressure waves and the resulting pressure waves will be transmitted to the earth strata in the form of seismic energy. The reflected seismic waves can be detected by one or more seismic receivers located on the earth's surface or in the well bore. To produce the pulse pressure waves, one sub-source will emit a constant frequency while the additional sub-sources emit a sweeping frequency. This will result in an off/on pattern of the heterodynes, thus creating an interference pattern at the well-bore wall with pulses and beats. This pattern at the well-bore wall is the source of seismic energy for the survey.

[0077] The constant frequency can be any frequency with maximum power output, but is estimated to be around 10-50 KHz or 30 KHz when a commercial marine sonar transducer is used. The sweeping frequencies of the second transducer can be within a 1000 Hz, 500 Hz, 100 Hz, or within 10-20 Hz of the constant frequency. Most preferred the sweep is within 10 Hz of the constant frequency, providing a LF passive survey, but this can also be combined with intermediate and/or higher frequency sweeps, proving an active survey. Any non-usable waves can be removed via low-pass filtering at a receiver, or other suitable device.

[0078] FIG. 2 depicts a closed seismic source device 2000 having an omnidirectional signal configuration. In this configuration, the sub-source transducers 2005 are aimed at each other. The transducers 2005 are attached to a housing using clamps 2002. The housing is a closed system with a liquid cavity 2001 between the tranducers. The liquid cavity is necessary for the propagation of the heterodyne beat. Specifically, the pressure wave will be created in this liquid cavity and transmitted to the reservoir strata through the normal borehole fluids present in the well.

[0079] The device 2000 can be lowered in the well using a e.g. rope or cable attachment and subsequently fixed to the well bore wall by attachments 2010 such as e.g. clamps.

[0080] FIG. 3 depicts a third configuration 3000 when multiple sub-source transducers 3005 are pointed at and heterodyned to one focal point 3010. Such a configuration also results in an omnidirectional signal and requires a liquid-filled cavity at the focal point. The primary advantage of this system configuration is having more power and sources available for penetration into the formation. It could also be used as a beam forming or beam guiding source that would aim the energy in a desired direction for analysis.

[0081] Any piezo or electrostatic/mechanical sub-source can be used in the present devices. However, due to the well-bore environment and need for waterproof sub-sources, marine sonar transducers are the preferred source.

[0082] Any standard seismic receiver can be used with the present device and may be located in the well-bore with the source or on the surface. A low-pass filter can be used to remove non- useable waves or heterodynes created by the device. [0083] The heterodyne signal source can be used in any method known in the art, see e.g., FIG. 4 showing varying placement of sources and receivers.

[0084] While the invention is described above in detail, it should be understood that various changes, substitutions, and alterations can be made without departing from the spirit and scope of the invention as defined by the following claims. Those skilled in the art may be able to study the preferred embodiments and identify other ways to practice the invention that are not exactly as described herein. It is the intent of the inventors that variations and equivalents of the invention are within the scope of the claims while the description, abstract and drawings are not to be used to limit the scope of the invention. The invention is specifically intended to be as broad as the claims below and their equivalents.

[0085] The following references are incorporated by reference in their entirety.

US3416632

US3302745

Andre Poon, Passive Broadband Seismometers Sample Natural Seismic Wave Field To Acquire Low-Frequency Data, The American Oil & Gas Reported, January 2010.

Claims

What Is Claimed Is:

1. A downhole heterodyne seismic source signal device, said device comprising: a) a housing shaped to fit inside a borehole, said housing having: i) a first sub-source transducer, wherein said first sub-source transducer emits at a constant frequency, and ii) a second sub-source transducer, wherein said second sub-source transducer emits at a sweeping frequency of similar frequency range to the first sub-source transducer; iii) said first and second sub-source transducers producing a heterodyne source signal at a common focal point, said common focal point comprising a liquid; b) a hanging means for lowering said housing into a wellbore attached to said housing; and c) optional means for attaching said device to another object on said housing or on said hanging means.

2. The device of claim 0, wherein said heterodyne source signal is between 1-1000 Hz.

3. The device of claim 0, wherein said first and said second sub-source transducers are pointed at each other.

4. The device of claim 0, wherein said common focal point is located on the wellbore wall.

5. The device of claim 0, wherein there are multiple second sub-sources, and wherein said first and said second sub-source transducers are arranged in a circular configuration and said common focal point is inside said wellbore.

6. The device of any of claims 0 - 5, wherein said sub-source transducers are marine sonar transducers.

7. The device of claim 1 , said first sub-source transducer emitting at a frequency between

10 and 50 KHz.

8. The device of claim 1, said first sub-source transducer emits at a maximal power level.

9. The device of claim 8, said second sub-source transducer sweeping at frequencies within 1000 Hz of the constant frequency.

10. The device of claim 8, said second sub-source transducer sweeping at frequencies from ±0-10 Hz of the constant frequency.

11. A method of providing a downhole heterodyne seismic signal source for oil and gas seismology, said method comprising: a) lowering a heterodyne signal source into a fluid in a hydrocarbon well-bore, said heterodyne signal source comprising first and second sub-sources; b) emitting a first constant frequency from said first sub -source and sweeping over a frequency range from said second sub-source so as to generate a heterodyne source signal.

12. The method of claim 11, wherein said first and said second sub-source are pointed at a common focal point away from each other on said well-bore wall.

13. The method of claim 11, wherein said first and said second sub-source are pointed at a common focal point between each other.

14. The method of claim 11, wherein there are multiple second sub-sources, and all sub- sources are arranged in a circular configuration and pointed at a common focal point in the middle.

15. A method of using the heterodyne seismic signal source device of claim 1 for oil and gas exploration, comprising: a) lowering said device into a fluid in a hydrocarbon well-bore by said hanging means; b) creating a heterodyned beat seismic wave with said first and said additional sub-source transducers; and c) collecting data from the reflection of said seismic waves by the earth using a passive frequency receiver.

16. The method of claim 15, wherein said first and said second sub-source transducers are pointed at a common focal point away from each other.

17. The method of claim 15, wherein said first and said second sub-source transducers are pointed at a common focal point between each other.

18. The method of claim 15, wherein the device comprises a plurality of second sub-source transducers and wherein said first and said second sub-source transducers are arranged in a circular configuration and pointed at a common focal point.

19. The method of claims 17 or 18, wherein said first and said second sub-source transducers are enclosed in a liquid-filled housing, said liquid-filled housing being in contact with a wellbore wall.

20. The method of claim 15, wherein said first and said additional sub-source transducers are marine sonar transducers.

21. An improved method of generating a downhole source signal for seismic surveying, said method requiring a downhole source to generate a seismic signal, the improvement comprising a first downhole sub-source that emits at a constant frequency, and a second downhole sub-source that emits at one or more frequencies within 1000 Hz of said constant frequency, said first and second sub-source generating a heterodyning beat that serve as the source signal for the seismic survey.

22. The improved method of claim 21 , wherein said constant frequency is about 3000 Hz, and said second downhole sub-source sweeps over a low frequency range within 10 Hz of the constant frequency.

23. The improved method of claim 21 , wherein said constant frequency is about 3000 Hz, and said second downhole sub-source sweeps over a frequency range within 1000 Hz of the constant frequency.

24. A system for seismic mapping, said system comprising the device of claim 1, plus one or more receivers for receiving reflected seismic waves, plus a processor for analyzing said reflected seismic waves.