Note: Descriptions are shown in the official language in which they were submitted.
CA 02510536 2005-06-22
ELECTROMAGNETIC llVIPULSE SURVEY APPARATUS AND METHOD
Field of the Invention
[0002] The present invention relates to an apparatus and method for performing
passive geophysical
prospecting. More particularly, the present invention relates to an improved
apparatus and method for
locating and identifying selected subsurface Earth material deposits or
geologic formations bearing
hydrocarbons, e.g., oil and/or gas, or commercially important ore deposits,
e.g., precious metals, as a
function of ionospheric impulse discontinuities in the near surface
atmosphere.
Background of the Invention
[0003] It is well known that petroleum deposits, ore bodies, and other
valuable Earth materials are found
at various locations and depths in the Earth, and that these Earth materials
are often difficult if not
impossible to fmd with the naked eye. Accordingly, it is also well known that
many different exploration
techniques and systems have been developed to provide a reliable indication of
the presence of these
commercially important deposits.
[0004] It is conventional, of course, to drill test holes at locations of
particular interest, and to recover
samples of Earth materials at various depths, to determine the actual
character of the Earth materials. If
cost were not a factor, drill holes such as for oil and gas would be cored
throughout their entire length.
This is not feasible, however, for reasons of economy. As such, cheaper
procedures have been developed
and utilized.
[0005] It is also conventional to measure topographical irregularities in
order to obtain an indication of
the existence of subsurface structures of particular interest. Similarly, it
is conventional to measure
diiTerences in seismic reverberations, and to measure variations in
gravitational pull at selected locations.
Although such measurements are often used with success to locate faults, traps
and other subsurface Earth
structures wherein oil and other valuable minerals could be found, most strata-
graphic traps and the like do
not contain such minerals, and therefore such measurements are most useful for
eliminating unlikely areas
of interest rather than to detect actual deposits of minerals.
[0006] More recently, procedures for subsurface prospecting have been
developed which measure
CA 02510536 2005-06-22
electromagnetic radiation emitted by the mineral-bearing formations. It is
known, of course, that this
planet itself constitutes and functions as a generator of electromagnetic
radiation which, in turn, creates
current flows within the Earth. Accordingly, measurement techniques such as
those described in U.S. Pat.
No. 3,679,978 have been developed to detect and analyze these magneto-telluric
currents within the Earth
bed adjacent the surface, as a direct indication of selected minerals of
interest. Although effective in
locating and measuring the extent of ore bodies, such techniques do not
indicate the type of minerals
present.
[0007] It is apparent that if the planet is a generator of electromagnetic
radiation within itself, these
current flows within the Earth will include both AC and DC currents which will
be functionally related to
both the individual mineral-bearing formations and their contents.
Furthermore, it will be apparent that
current flows within but adjacent the surface of the Earth will inherently
create functionally related
electrical fields adjacent but above the surface of the Earth. These
electrical fields are composed of carrier
waves having frequencies characteristic of the type of mineral in that
formation.
[0008] Several techniques have been developed to measure these electrical
fields which exist near but
above the surface of the Earth. U.S. Pat. No. 4,507611 to Helms describes a
method of traversing the
surface of the Earth and recording "solar wind" activity of sufficient
strength to detect anomalies related to
surface and subsurface mineral deposits. This apparatus uses the root mean
square (RMS) method to detect
increases or decreases in the Earth's electrical fields. U.S. Pat. No.
3,942,101 to Sayer describes a
prospecting apparatus that utilizes a distortion of the atmospheric electro-
static potential gradient, which is
suggested to be a result of the Nernst effect. Sayer teaches that the
distortion provides a means for locating
subterranean sources of geothermal energy. Alterations in the Earth's magnetic
fields known as "magnetic
noise" is described by Slichter in U.S. Pat. No. 3,136,943, which discloses
that such noise is primarily the
product of lightening discharges. However, because many of these methods and
apparatus are based on the
AC components of the electrical fields, the techniques are more effective and
reliable depending upon the
size or area extent of the mineral deposit of interest. More particularly, the
techniques based on the AC
components are less sensitive and effective in detecting the presence of
smaller mineral deposits. To
overcome the shortcomings of the AC measurements, U.S. Pat. No. 4,841,250 to
Jackson provides a
2
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technique utilizing the DC components of the electrical fields.
[0009] It is also known to utilize the electromagnetic radiation emitted by
the mineral-bearing formations
to create radioactivity "logs." In oil-field terminology, a "log" is a report
that furnishes information
regarding the geological formations. A radioactivity log includes the gamma-
ray log, gamma-gamma log,
neutron-gamma log and neutron-neutron log. The gamma-ray log records the
natural radioactivity in the
form of gamma-rays in the bore hole emanating from the formation. The most
abundant radioactive
isotope is K4~, which occurs in potassium-bearing minerals and is especially
abundant in clay minerals.
Therefore, the gamma-ray log distinguishes shale beds from non-shale beds by
recording a high gamma
radiation. In the gamma-gamma log, the radiation is induced by bombarding the
bore-hole walls with
gamma rays. The amount of back-scatter is recorded. Because the more dense
atoms resist the
bombardment, the back-scatter is greater. Accordingly, the amount of back-
scatter is directly related to the
bulk density of the formation and to the porosity.
[0010] In the two neutron logs, the formation is bombarded with neutrons. The
neutron-gamma log
measures the induced gamma radiation from the heavier atoms. In this reaction,
hydrogen ions absorb the
neutron particles, and reduced gamma radiation indicates the relative
abundance of hydrogen, which may
exist largely in the fluids of pores. Therefore, the induced gamma radiation
is inversely proportional to the
porosity of the formation. The neutron-neutron log measures neutron capture
within the formation, which
again is proportional to the hydrogen density and therefore to the porosity or
bulls density of the formation.
[0011 J A limitation of the radioactivity log is that they cannot distinguish
between water and
hydrocarbons, e.g. oil. Both would indicate a relative abundance of hydrogen
and, therefore, the presence
of porous formations. These logs could not distinguish between these two. The
use of radioactive
detection at the Earth's surface or near surface has been well known for many
years and known as
radiomeirics, which is a method to log variations in the Earth's natural
radioactive emissions as one
traverses the surface on land or by plane in order to measure decreases and
increases in these emissions in
order to locate oil, gas and mineral deposits.
[0012] These disadvantages of the prior art are overcome with the present
invention, and improved
methods and apparatus for passive geophysical prospecting are provided for
obtaining a more sensitive and
3
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precise measurement of the presence and character of relatively small deposits
of valuable materials, e.g.,
hydrocarbons, ore deposits, and precious minerals.
Theory of the Invention
[0013] As discussed above, a primary magnetic field of electromagnetic energy
is generated by the Earth
itself and exists in the near surface atmosphere. Within the primary magnetic
field exist random occurring
impulses of energy. These impulses, which occur within the audio frequency
range, exist in the random
vertical components of the Earth's primary magnetic field.
[0014] Secondary magnetic fields result from the current flows associated with
the radiation emanating
from the hydrocarbon accumulation within the Earth as the result of the
chemical release of electrons
during a redox reaction. [cite Mr. S.J. Peirson of Te University of Texas]. As
the random occurring
impulses in the primary magnetic field interact with the secondary magnetic
fields, energy is transferred to
the secondary fields creating an impulse. The number of impulses is related to
the strength of the
secondary magnetic fields.
[0015] At present, the source of these random occurring impulses is
speculative. However, it is widely
believed that the impulses are related to lightning activity around the Earth.
One study conducted by S.H.
Ward showed the relationship between lightning activity and resulting changes
in the measured electrical
fields. [citation]. AF-MAG - AIRBORNE AND GROUND", Geophysics, No. 4, Oct.
1959, pp. 761-789
describes measuring lightening activity in the audio range of frequencies near
Kitwe in northern Rhodesia
during the months of July, August, September and October of 1957. Another
study concluded that
lightning discharges in the Earth-ionosphere cavity would propagate with a
horizontal traverse magnetic
field that is perpendicular to the direction of prepagation. [citation].
However, regardless of source, the
existence of random occurring impulses is recognized.
4
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Summary of the Invention
[0016] A passive geophysical prospecting method is provided. The method
comprises:
traversing above the surface of the Earth with an antenna; detecting
electromagnetic radiation naturally
emanating from the Earth's surface with the antenna; generating an electrical
signal from the detected
electromagnetic radiation while performing the traversing step; filtering from
the electrical signal
frequencies below 65 Hz, preferably below 100 Hz and more preferably below 800
Hz, and above 12, 000
Hz, preferably above 8,000 Hz, to generate a filtered signal; converting the
filtered signal to a prospecting
voltage signal; and comparing the prospecting voltage signal to a set voltage
(or reference voltage) and
generate an output signal which provides information regarding the presence or
absence of the deposit of
interest. The output signal is preferably in the form of a voltage or as
counts in analog or digital format.
The output signal can be recorded and can also be converted from a voltage to
counts.
[0017] A passive geophysical prospecting apparatus is also provided. The
apparatus comprises:
an antenna for detecting electromagnetic radiation naturally emanating from
the Earth's surface and
generating an electrical signal from the detected electromagnetic radiation
while traversing above the
Earth's surface; a filter for filtering from the electrical signal frequencies
below 65 Hz, preferably below
100 Hz and more preferably below 800 Hz, and above 12,000 Hz, preferably 8,000
Hz to generate a
filtered signal; a converter for converting the filtered signal to a
prospecting voltage signal; a level detector
for comparing the prospecting voltage signal and a set voltage and generating
a difference signal; and
means for recording the difference signal.
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Brief Description of the Drawings
[0018] FIG. 1 shows a block diagram of an embodiment of the electromagnetic
impulse survey apparatus
according to the present invention.
[0019] FIG. 2 shows a block diagram of another embodiment of the
electromagnetic impulse survey
apparatus according to the present invention.
[0020] FIG. 3 shows a log obtained using an electromagnetic impulse apparatus
according to the present
invention.
[0021 ] FIG. 4 shows a block diagram of another embodiment of the
electromagnetic impulse survey
apparatus according to the present invention.
[0022] FIGS. SA and SB is a detailed schematic of the embodiment shown in FIG.
4.
[0023] FIG. 6 shows another log obtained using an electromagnetic impulse
apparatus according to the
present invention.
[0024] FIG. 7 shows another log obtained using an electromagnetic impulse
apparatus according to the
present invention
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Detailed Description
[0025] The following detailed description illustrates the invention by way of
example and not by way of
limitation. Fig. 1 shows a block diagram of a preferred embodiment of the
electromagnetic impulse survey
apparatus 100 according to the present invention. The apparatus 100 comprises
an antenna. 110 for
detecting electromagnetic impulses originating from subsurface formations and
for producing a signal
which is amplified in an amplifier 120. The amplified signal is then passed
through a band pass filter 130
to selectively eliminate frequencies below a certain frequency level and also
frequencies above a specific
frequency. This may be accomplished by a single device or a high pass filter
in combination with a low
pass filter. The signal exiting the band pass filter 130 is then sent to a
level dectector 140. The level
detector 140 compares the signal to a set output voltage reference. When the
signal exceeds a set voltage
reference of the level detector 140, a pulse or count is output to an analysis
device 150. Typically, counts
per second are output to either a computer for digital counting and alarm
detection or to a voltage detection
circuit for recording the counts per second on a chart recorder.
[0026] Referring to Fig. 2, a block diagram of a preferred embodiment of the
present invention is shown.
The electromagnetic impulse survey apparatus 200 comprises an antenna 210 for
detecting the
electromagnetic noise of the formations traversed. The sensed signal is then
passed from the antenna 210
to a buffer amplifier 220. The signal is then sent to the band pass filters
230 which may be set to pass
different bands of frequencies and eliminating those frequencies outside their
respective ranges. For
example, the band pass filters 230 may be set to pass a frequency range from
about 65 Hz to about 12,000
Hz, for example from about 800 Hz to about 8,000 Hz, to yield overall
formation noise. Frequencies
below and above the specified ranges would be eliminated to provide the sought
after information.
[0027] In a preferred embodiment of the present invention, the band pass
filtering stage is preferably
within the audio fi~equency range. The band pass filtering stage more
preferably having at least one
channel wherein the channel filters out frequencies below 65 Hz, preferably
below 100 Hz and more
preferably below 800 Hz, and above 12,000 Hz, preferably above 8,000 Hz, to
provide hydrocarbon
information.
[0028] As shown in Fig. 2, the band pass filters 230 of a preferred embodiment
has a high pass filter 232,
7
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a first gain amplifier 234, a low pass filter 236, and a second gain amplifier
238. The output of the high
pass filter 232 is amplified in the first gain amplifier 234 and then sent
through the low pass filter 236. The
resulting signal is again amplified in the second gain amplifier 238 and sent
to a level detector 240 which
compares the resulting noise signal to a voltage reference. In one embodiment,
the reference voltage was
2.SV. When the signal exceeds the reference voltage level of the level
detector 240, a pulse or count is
output to an analysis device 250.
[0029] The analysis device 250, typically a computer for digital counting and
alarm detection and
processing or a voltage detection circuit for recording counts per second on a
chart recorder, compares the
counts per second registered by the level detector 240 with a base count. The
base count can be established
by traversing the antenna 210 across the surface in the near Earth surface
atmosphere over an established
homogeneous area and determining the number of impulses encountered per second
within the desired
frequency range. The counts per second are directly proportional to the
strength of the secondary fields.
As the antenna 210 approaches an area where an increased secondary field
exists, the count rate will
increase. Thus, for example, because the size of a hydrocarbon accumulation
determines the strength of
the magnetic field, as the antenna 210 approaches an area of large hydrocarbon
accumulation, the count
rate will increase.
[0030] In Fig. 3, there is shown a log obtained using an electromagnetic
impulse apparatus like that of
apparatus 200. The field test demonstrates the value and effectiveness of the
apparatus and method of the
present invention. Specifically, the log based on impulse counts per second is
able to discern the
presence of oil producing zones.
[0031] The antennas 110 and Z10 are preferably a wire having a length of at
least 1 foot. 16 gage wire in
lengths of 2 ft. to 24 ft. have been used, for example, under the fuselage of
an airplane in an airborne
embodiment of the present invention. Preferably, between the antenna and the
respective amplifier 120 or
220 is a load resistor attached to ground (for example, a 500 ohm pot) for
impedance purposes.
[0032] The level detector 140 and 240 with associated pulse density circuitry
is like that used in US
5,777,476 to Jackson, incorporated herein by reference.
[0033] In a preferred embodiment of the present invention, a method of
locating subterranean
8
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accumulations of hydrocarbons or other valuable minerals is descn'bed with
reference to Fig. 2. The
antenna 210 is traversed over a homogeneous area. The antenna 210 can traverse
the area by attachment to
a mobile surface vehicle or traverse via an airborne craft. The signals
received by the antenna 210 are
amplified by the buffer amplifier 220 such that the signal gain is between 100
and 500, for example. The
signal is then passed through high pass filter 232 with a frequency cut off at
65 Hz, preferably 100 Hz and
more preferably 800 Hz, for example, with a 120 db drop off. After passing
through the first gain
amplifier 234, the signal passes through the low pass filter 236 with a
frequency cut off at 12,000 Hz,
preferably 8000 Hz, for example, with a 120 db drop off. The resulting signal
is a filtered response of, for
example, from 800 Hz to 8.000 Hz. The filtered signal is then buffered and
gained using the second gain
amplifier 238.
[0034] Once the filtered signal is obtained, it is input into the level
detector 240. The level detector 240
has a selected voltage reference. The signal is increased with the second gain
amplifier 238 until the
threshold voltage reference is exceeded, which outputs a pulse count. The
signal is again increased until
the pulse count per second falls between 60 and 150 counts, for example. The
number of counts per
second over the homogenous area becomes the base count.
[0035] After determining the base count, the area is traversed and the antenna
210 continues to receive
signals. As the antenna 210 approaches an area where an increased secondary
field exists, the count rate
will increase, that is, exceed the base count. The increase is directly
proportional to the strength of the
secondary fields, which are directly proportional to the hydrocarbon
accumulation. Thus, once the base
count is determined, increases representing hydrocarbon accumulations can be
easily and readily identified.
It should be noted that, because the secondary magnetic fields exist over
water as well as land, surveys for
hydrocarbons utilizing the present apparatus and method can readily be
conducted over the oceans that
cover the Earth. Unlike radioactivity logs which cannot distinguish between
water and hydrocarbons, the
present invention does readily distinguish between these two and the
identification of hydrocarbons is not
affected or inhibited by the presence of water.
[0036] Now referring to FIG. 4, there is shown a functional representation of
another embodiment of the
present invention. An apparatus 300 is depicted having an antenna 110, a first
amplifier 120, a high pass
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filter 130a, a low pass filter 130b, a second amplifier 136, a meter 138, a
voltage level detector 140, a rate
meter 160a and a pattern detector 160b. Outputs are provided for a chart
recorder 160c (not shown) and to
a computer 160d (not shown). The analysis device 160 of FIG. 2 are devices
160a, 160b,160c and 160d,
each of these either used alone or in any combination thereof.
[0037] In a specific embodiment, amplifier 120 using a LF442CN dual
operational amplifier available
from National Semiconductor then amplified the sensed signal from antenna 110.
The high pass filter 130a
was an active high pass filter using a Burr Brown UAF42 Universal Active
Filter configured as a
Butterworth filter. The low pass filter 130b used a Burr Brown UAF42 Universal
Active Filter configured
as a Butterworkh filter. The combination of the high pass filter 130a and low
pass filter 130b resulted in a
pass band of frequencies from about 800 Hz to about 8,000 Hz to yield overall
formation noise. The band
passed signal was then amplified using a National Semiconductor LM380 audio
amplifier 136. The output
of the audio amplifier 136 was then sent to the voltage level detector 140
comprising a National
Semiconductor LM311 voltage comparator. This detector 140 compared the signal
input thereto against a
selectable reference DC voltage to generate a difference signal that is
proportional to the secondary fields
of interest. The reference DC voltage level can be adjusted using a
potentiometer to a desired level to
increase or reduce the sensitivity of the detector 140. For example, the
reference DC voltage level can be
set to a value corresponding to a known area devoid of hydrocarbons so that
slight variations above this
level will be recognized in the signal range of interest. The comparator,
i.e., detector, 140 was configured
to output pulses of from 0 to 5 volts representing the important information
about subterranean geologic
formations and their contents, i.e., hydrocarbons or precious metals. The
output of the comparator 140 was
sent to a rate meter 160a to be converted from pulses per second to a
corresponding voltage. The voltage
output of the rate meter 160a was used to establish a base line reference for
recording purposes. The output
of the comparator 140 was also sent to a pattern detector 160b which counted
the number of pulses in a
given period of time and outputs a response to a recorder 160c (not shown in
FIG. 4) when a preselected
number of pulses for a given time period was encountered or exceeded. The
preselected number of pulses
in a given time period is preferably adjustable. This variable may be adjusted
based on the activity
encountered in the signal of interest. This difference in activity may be due
to the difference in material
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being prospected, e.g., oil versus gold, and/or the quantity of such material
encountered in the subterranean
formation. An output from the comparator 140 was also available to a
computer160d (not shown on FIG.
4) where the pulses are digitized and processed using methods and techniques
lmown by those skilled in
the art to determine the pulse density over a selected period or unit of time.
The processed information may
then be printed using a printer (not shown) or shown on a computer screen (not
shown).
[0038] FIGS. SA and SB with minor differences (detailed in parentheticals
where appropriate) are a
detailed schematic of the embodiment shown in FIG. 4. FIG. SA depicts the
antenna 110 and the circuitry
for amplifier 120, high pass filter 130a (two UAF42's instead of one), low
pass filter 130b and amplifier
136 (which includes a dual op amp LF442CN prior to the LM380). FIG. SB depicts
the circuitry for the
voltage level detector 140, rate meter 160a, pattern detector 160b and
computer 160d.
[0039] The following is a list of the components in FIGS. SA and SB. In
regards to adjustable resistors or
potentiometers, "(adj.)" is indicated next to the maximum resistance. "UAF42"
refers to a universal active
filter available from Burr Brown. "LF442CN", "LM1496", "LM311", "LM380",
"LM555" and "LM331"
refer to products available from National Semiconductor. "2n 7000" is a
transistor wherein "G" stands for
gate, "D" stands for drain and "S" stands for source. "4024" and "4060" are
generic chips known to those
skilled in the art. "OP 290" is an operational amplifier.
[0040] Capacitors:
Cl-lmuF
C2-lmuF
C3-O.lmuF
C4-0. lmuF
CS-lmuF
C6-lmuF
C7-O.lmuF
C8-lmuF
C9-lmuF
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C 10-0. lmuF
C39-lmuF
C40-33muF
C41-lmuF
C42-0.01F
C43-0.01 F
C44-0.1 F
C45-0.01F
C47-4.7muF
C48-O.OlmuF
C49-470pF
C50-.1 F
C51-lmuF
C52-0.0047muF
C53-0. lmuF
C54-0.1 muF
C55-l OmuF
C56-0.01muF
C57-O.OSmuF
[0041 ] Resistors:
Rl-20K ohm
R2-20K ohm
R3-1K ohm
R4-SOK ohm
RS-20K ohm
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R6-1K ohm
R7-SOK ohm
R8-28K ohm
R9-1K ohm
R10-93.1K ohm
Rl l-93.1K ohm
R12-191K ohm
R13-191K ohm
R14-34K ohm
R15-34K ohm
R16-lOK ohm (adj.)
R17-lOK ohm (adj.)
R18-lOK ohm (adj.)
R55-1K ohm
R56-S.1K ohm
R62-1K ohm
R63-lOK ohm (adj.)
R64-lOK ohm (adj.)
R72-SOK ohm
R73-2 meg ohm
R74-lOK ohm
R75-SK ohm (adj.)
R76-lOK ohm
R77-lOK ohm
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R78-100K ohm
R79-100K ohm
R80-100K ohm
R81-100K ohm (adj.)
R82-20K ohm
R83-470K ohm
R84-1K ohm
R85-100K ohm
R86-6.8K ohm
R87-lOK ohm
R88-SOK ohm
R89-lOK ohm
R90-lOK ohm (adj.)
R91-lOK ohm
R92-1K ohm
R93-lOK ohm (adj.)
R94-1K ohm
R95-1K ohm (adj.)
R96-lOK ohm
R97-SK ohm (adj.)
8105-1K ohm
8106-20K ohm (adj.)
[0042] Chips and Operational Application:
U1-UAF42
U2-UAF42
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U3-UAF42
U12-LF442CN
U13-LM380
U20-LF442CN
U21-LM311
U22-2n7000
U23-4024
U24-LM555
U25-4060
U26-LM331
U27-OP290
U28-OP290
[0043] Switches:
SW3
[0044] Referring now to FIG. 6, there is shown a log obtained by using the
device of FIGS. SA & SB
using a chart recorder. The device was carried on a small plane with the
antenna trailing below and behind
the airplane. The plane flew at a speed of 120 mph at an altitude of 300 feet.
This airborne run was
performed by flying from South to North (S to N) for about 6 miles and then
making a u-turn flying North
to South (N to SO for about 6 miles retracing the initial S to N flying route
over the Big Finn Valley Reef
Oil Field in Alberta, Canada (Big Valley notation on the log).
[0045] The S to N mile markers 07 through 14 and N to S mile markers 15
through 20 are shown on FIG.
6. GPS waypoints were saved at each mile mark to correlate the signal strength
(counts) with the GPS
positions. The notations "wells" and "Hy" (for highway) were made on the log
for additional reference
points. Increased pulse or count density (signal strength) was encountered
between markers 11-14 and on
CA 02510536 2005-06-22
the return trip at 15 - 18, with the wells notation at mark 11.
[0046] Though not shown, a map of the area can be scanned into GPS mapping
software on a computer,
for example, Des Newman's OziExplorer Version 3.84.2, which works with
Magellan, Garmin,
Lowarance, Eagle and MLR GPS receivers. Then, the stored waypoint data in the
GPS receiver can be
downloaded into this software to indicate the positions where the airborne
survey was conducted.
[0047] In this case, a Garmin receiver was used. The waypoints for the route
flown at the markers
identified on the log were downloaded into the software program. The count or
signal strength were noted
on the map to identify areas of greatest interest.
[0048] The map can be an underground topographical map from 3-D seismic
showing the strata
formations which are conducive to the accumulation hydrocarbons. The count
data can be positioned on
the map to identify areas of greatest interest for additional airborne
surveys. Further, the survey can be
performed where the fly routes are a series of parallel routes at
predetermined spaced distances or a grid
pattern and the data placed on the map by relative strength indications using
different colors on the map,
thereby corresponding the data to the geology of the area to identify areas of
greatest interest for additional
airborne surveys to fme tune the map and/or pinpoint potential drilling sites.
[0049] Rather than or in addition to the chart recording, the data can be
captured on a computer and
displayed on the computer screen along with the GPS location (that is also
downloaded to the computer) in
real-time.
[0050} Referring now to FIG. 7, there is shown another chart recorded log of
an airborne survey using the
device of FIGS. SA & SB performed over another portion of the Big Finn area.
At about the same altitude
and speed, the airborne survey was conducted over a pinnacle Devonian reef
over a 10 to 12 mile line.
Between markers 260 to 262, here is shown a significant increase in count
activity corresponding to the
Devonian LeDuc Reef. Such reefs are difficult to locate using conventional
seismic techniques since they
are so small, relatively speaking to the types of formations typically
detected by seismic.
[0051 ] To this point, out of 14 airborne surveys used to locate potential
drilling sites in wildcat territory
in South Texas, all 14 have resulted in wells capable of producing in
commercial quantities of
hydrocarbons, that is, gas and/or oil.
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[0052] Many methods can be employed to indicate count rates exceeding that of
the base count. In a
preferred embodiment, an alarm similar to a radiation detector triggers a
response once the base count has
been exceeded.
17
