Continuity logging using differenced signal detection

A method of determining the continuity of a lithographic layer located between two vertical boreholes is disclosed. A seismic source is lowered in the first borehole while simultaneously a receiver pair, preferably a pair of "vertical" geophones spaced apart by about two feet, are lowered in the second borehole to develop a "differenced signal". The recording of high amplitude signals within a layer is an indication of a continuous, low velocity layer. The middle of a layer can also be discovered using a single vertical receiver and finding the place of phase reversal.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention pertains to continuity logging and more particularly to the 
detection of guided waves in lithographic layers located between two well 
bores as a measure of continuity. 
2. Description of the Prior Art 
Well logging by utilizing various devices is a time honored means of 
recording physical measurements of the formations transversed by the 
borehole as a function of depth in the hole. These measurements are 
commonly used to identify the formations and to determine the amount and 
type of fluid in the formations. Such measurements are typically 
restricted to the region immediately adjacent to the well bore. 
In addition to well-logging techniques, well bores have also been used in 
seismic exploration. The majority of seismic gathering procedures utilize 
the positioning of seismic sources and receivers on the land surface or in 
shallow, below-surface locations. However, when they are available, well 
bores have been employed in seismic exploration activity since there are 
some advantages afforded by having a well bore in existence. Probably the 
main advantage is the ability to place the source or the receiver or both 
closer to the reflecting geological interfaces than is possible from using 
surface locations alone. 
When more than one borehole exist, then it is possible to utilize spaced 
apart boreholes for seismic exploration not possible with a single 
borehole. For example, U.S. Pat. No. 4,214,226, Narasimhan, et al., issued 
July 22, 1980, describes a procedure for producing a tomograph of the 
subsurface structure between boreholes by placing spaced geophones in one 
borehole and by producing a sequence of shots at spaced apart locations in 
the other borehole. The arrival times or amplitudes for transmitted 
primary (compressional) waves which travel directly between shot and 
geophone pairs are used to produce an image of the structure between the 
boreholes. In another example, U.S. Pat. No. 4,298,967, Hawkins, issued 
Nov. 3, 1981 a procedure for providing a horizontal profile of a reflector 
located between boreholes by using multiple shot and receiver positions in 
the boreholes. In this case, the arrival times from reflected primary 
waves which travel from the source to the reflector and then to the 
geophones are used to image the reflector. 
Another type of subsurface seismic exploration involves placing sources and 
receivers within a coal seam and along roadways in mines. In this 
application, guided waves, which are trapped within the coal, and which 
are typically called seam waves or channel waves, are used to determine 
the location of faults in the seam. Unlike body waves such as primary and 
secondary (compressional and shear) waves, guided waves are confined in 
space. These guided waves are trapped in low-velocity layers such as coal 
by critical reflections of the wave at the upper and lower boundaries of 
the layers. Because these guided waves have their largest amplitudes in 
the coal, they can be used to investigate discontinuities in the coal. 
Guided waves are often used to measure discontinities in three-foot coal 
seams even though the wavelength of the body waves generated from the same 
source may be 100 feet long. The resolution of seismic imaging using body 
waves is limited to one half of a wavelength which is often much larger 
than the coal seam. 
A number of techniques have been developed to enhance the detection of 
guided waves in coal seams. These techniques require the detectors to be 
deployed in a mine at specific location within the coal layer. For 
example, U.S. Pat. No. 3,352,375, Krey, issued Nov. 14, 1976, uses 
geophone pairs with one geophone located near the upper boundary of the 
seam and the other located near the lower boundary of the seam. The 
signals from the two geophones in each pair are subtracted. In another 
patent, U.S. Pat. No. 3,858,167, Stas, et al., issued Dec. 31, 1974; the 
geophones are located in the center of the seam and oriented in a specific 
direction to provide maximum sensitivity to the seam waves. The technique 
described in U.S. Pat. No. 4,351,035, Buchanan, et al., issued Sept. 21, 
1982, requires an array of geophones located along a roadway to generate a 
holograph of the seam. 
The development of a procedure for ascertaining the existence of continuity 
in one or more layers as opposed to discontinuity existing in other layers 
can be extremely useful for both exploration and production of oil and 
gas. In exploration, for example, continuity logging can be useful for 
constructing geologic models, correlating formation properties between 
boreholes, and determining the presence of faults. In production, 
continuity logging can be used to indicate the continuity of the reservoir 
between the boreholes and the presence of permeability barriers. In 
addition, continuity logging can be useful in planning the mining of coals 
and other minerals. 
Therefore, it is a feature of the present invention to provide an improved 
method of detecting the presence of a continuous layer between boreholes 
in the subterranean lithography therebetween. 
It is another feature of the present invention to provide an improved 
method of detecting the presence of guided waves in specific lithographic 
layers as a means of determining the continuity of such layers. 
SUMMARY OF THE INVENTION 
The preferred method of continuity logging in accordance with the present 
invention involves the utilization of two boreholes spaced apart and 
progressing downwardly past the lithography to be explored by continuity 
logging. A seismic source is located in the first borehole in the vicinity 
of low-velocity layers in the lithography as previously determined, such 
as from previous conventional logging of the well. Such layers are 
generally less dense or more porous than the layers on either side. At the 
same time a combination of two substantially identical geophone receivers 
are lowered into the second borehole, the two geophones being fixed 
together a short distance apart, normally at a distance of about two feet. 
Therefore, in the borehole, one receiver is located about two feet above 
the other. 
Seismic sources can either be the kind that produces a continuous acoustic 
wave over a relatively long period of time or preferably the kind that can 
be repeatedly triggered to produce a pulse-like seismic signal. In either 
event, for purposes herein the produced wave motion imparted into the 
formation will be referred to as producing "impulses". Suitable seismic 
sources for practicing the invention are well-known in the art. 
The electrical signal output of the two receivers are combined so that the 
output of one is subtracted from the other, thereby producing a 
"differenced signal". The source is lowered in the first borehole to be 
located opposite a first layer and the receiver combination is lowered in 
the second borehole about the same distance so as to be presumptively 
located opposite the same layer. The source is then actuated. The 
differenced signal that is produced by the two-receiver combination 
reduces the values of the individual body waves (both primary and 
secondary waves) that are present while enhancing the presence of guided 
waves. Thus, if there is an appreciable signal present or detected, it is 
an indication of the presence of guided waves, and, hence, the presence of 
continuity in the layer. If, on the other hand, there is only a small 
differenced signal present or none at all, then there is no or a 
negligible amount of wave guide action present, and hence there is 
discontinuity in the layer. The entire range of low-velocity layers can be 
surveyed in like fashion by simultaneously lowering the source and 
receiver combination in their respective borehole and repeating the above 
procedure. 
When vertical motion sensing geophones or other receivers are employed for 
the two receivers in the receiving combination, some additional 
information is provided. A vertical motion sensing geophone is sensitive 
to the detection of a symmetric guided wave that oscillates in the 
formation in such a fashion that a null or zero-crossover point appears in 
the layer at about its mid-point (assuming a homogeneous layer). 
Therefore, when such geophones are used for developing the guided wave 
differenced signal as noted above, a phase reversal will occur in such 
signal when one of the geophones is lowered past the mid-point of 
continuous low-velocity layer. Thus, not only can the presence of the 
layer be determined, but its mid-point can be determined by the presence 
of polarity or phase reversal.

DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
Definitions of "body waves" and "guided waves" are set forth below as aids 
in understanding the invention herein described. 
Body waves are defined as disturbances which can travel throughout a 
medium. Body waves are not confined in space. In a solid medium, two types 
of body waves can exist: namely, compressional waves and shear waves. One 
type of wave which is not a body wave is a surface wave; the amplitudes of 
surface waves decrease rapidly with distance from the surface. 
Under certain circumstances wave energy may be trapped within a layer which 
is then known as a wave guide. Guided waves have normal amplitudes in such 
layer, whereas the amplitudes in the material surrounding the layer 
rapidly approach zero with increasing distance from the layer. The guided 
waves can be trapped or partially trapped in a low-velocity layer by 
critical reflections of the waves at the upper and lower boundaries of the 
layer. Because guided waves have their largest amplitudes in the 
low-velocity layer, they can be used to investigate discontinuities in the 
layer. Guided waves are used to measure discontinuities in three-foot coal 
beds even though the wavelength may be 100 feet long. In coal geophysics, 
these guided waves are often called seam waves or channel waves. 
Now referring to the drawings, and first to FIG. 1, a low-velocity layer or 
coal seam 10 is shown between a source borehole 12 and a receiver borehole 
14. The boreholes are spaced at a distance from each other of several 
hundred feet, for example, 2000 feet. The low-velocity layer is known to 
exist, at least approximately, from existing well logging or other data at 
the depth shown. For purposes of illustration, the low-velocity layer 
sustains seismic waves travelling therein at 9900 ft/sec., whereas all 
other layers in the vicinity sustain seismic waves from the same source at 
faster velocities. 
Seismic source 16 in borehole 12 is a typical mechanical seismic source, 
such as a vibrator, that can be actuated upon command in traditional 
fashion well-known in the art to established acoustic waves in the layers 
adjacent borehole 12 where the source is located. Geophones 18 and 20 are 
located in borehole 14 at approximately the same depth as source 16 at any 
given time. Each time the source is actuated the source imparts 
substantially the same signal into the formation, these wave travelling in 
the intervening layers between the boreholes. 
Geophones 18 and 20 both include a motion sensitive sensing or receiving 
element of the same kind. That is, both are either "horizontal" geophones 
or "vertical" geophones, depending on the directionality of the sensing 
element. The two geophones are spaced apart by a linkage 22 so as to 
maintain the two geophones at the same separation. Typically, the 
separation is two feet. Ordinarily, each geophone develops an electrical 
voltage signal representative of the acoustical wave that is sensed by the 
geophone. In discussing the wave patterns presented in FIGS. 3-4 and 6-7 
below, it is assumed that only one geophone is operational. However, in 
accordance with the invention, both geophones are operable and are 
electrically connected in such a manner that the output of one of the 
geophones is subtracted from the other. This signal development produces a 
"differenced signal", as more completely discussed below with respect to 
the discussion of FIGS. 5 and 8. 
Now referring to FIG. 3, a wave pattern development is shown as it relates 
to a signal produced by a single horizontal geophone, oriented for 
receiving waves in line with direction 24 established by boreholes 12 and 
14. The boreholes are known to be 1940 feet apart. A low velocity layer is 
known to be within 60 or so feet represented by the diagram. That is, each 
vertical line separation represents a distance of one foot. The velocity 
characteristics of the layers traversed over the range of inquiry are 
shown at the top of the diagram in feet per second, although it is assumed 
that these layers are not precisely known in the beginning. What is known 
is that a low velocity layer of interest lies somewhere within the overall 
range. 
The source and the receiver are located in their respective boreholes at 
the same approximate depths and sourcing is initiated. In this case, there 
is only one operable receiver and that receiver is a "horizontal" 
geophone. The receiver output recording is shown for the first depth on 
the left side of the drawing. The source remains fixed in the layer and 
the receiver is lowered one foot and the source is again actuated to 
produce a signal on the second line. This procedure continues for each 
successive line until the whole diagram is developed. 
There are two series of waves developed, the first of which can be 
identified with the primary (compressional) waves and the second with the 
secondary (shear) waves. There is no significant difference in appearance 
of the low velocity layer in the middle and that of the neighboring 
layers. That is, the signals recorded for the low velocity layer are quite 
similar to those recorded above and below the layer and the layer is not 
readily identified. 
FIG. 4 is developed in the same manner as FIG. 3, except a "vertical" 
geophone is employed as the receiver. Again, there are two groups of wave 
patterns, but the appearance of the patterns are not significantly 
different for the low velocity layer and the layer above and the layer 
below such layer. It may be seen, however, that there is a phase reversal 
in the middle that approximately identifies the low velocity layer. 
Moreover, as will be seen, this part of the signal will be enhanced when 
the differenced signal is developed. 
Now referring to FIG. 5, a series of signals are developed in much the same 
manner as for FIGS. 3 and 4, except that in this case, two "vertical" 
geophones connected together two feet apart are used as a single receiver, 
the output therefrom being the "differenced signal". The low velocity 
layer is now sharply defined for both set of waves. Furthermore, the 
entire boundary of the layer is defined, not just the middle or center 
thereof. What is shown by the diagram is not just that there is a low 
velocity layer in the middle but that layer sustains the waves and 
therefore indicates continuity of the layer. 
A similar series of diagrams to that of FIGS. 3, 4, and 5 are shown in 
FIGS. 6, 7 and 8. In the latter case, the structure under investigation is 
a shale structure, rather than a coal structure. In this case the source 
borehole and the receiver borehole are only about 300 feet apart. Thus, 
the primary and secondary waves are not separated in time. Otherwise, the 
conditions are the same. That is, FIG. 6 shows the results of using a 
single horizontal geophone as the receiver; FIG. 7 shows the results of 
using a single vertical geophone; and FIG. 8 shows the results of using a 
pair of vertical geophones spaced apart at a two-foot distance, the output 
being the differenced signal. In the case of a shale layer, the velocity 
characteristics of the entire layer may not be homogeneous; however, the 
shale is comprised of lower velocity material than its neighboring layers. 
Therefore, as is true for the coal model, the large differenced signals 
for the shale model also sharply define the edges of the layer (FIG. 8) in 
a manner not evident by the single receiver diagrams (FIGS. 6 and 7). 
Furthermore, each of the three sublayers of the principally detected layer 
are also identified. 
It should be noted that the data shown in FIGS. 3-8 was actually recorded 
with the source fixed in one location, with the receiver being located at 
different positions or locations relative to the layer. This demonstrates 
that the technique described herein is able to distinguish cases where the 
source is in the layer but the receivers are not in the layer. This is 
important because the location of the layer may not be precisely known. 
Moreover, the layer can in some cases intercept the boreholes at different 
depths rather than at the same depth, as heretofore assumed. 
Referring back to FIGS. 4 and 7, it will be seen that the single vertical 
geophone in each case develops a phase reversal in the approximate center 
or mid-section of the detected layer. Thus, by the series of signals, one 
is able to distinguish the boundaries and the center section of the 
continuous low velocity layers in the formation. 
Although FIGS. 5 and 8 were developed using vertical geophones, similar 
diagrams are possible by using a pair of horizontal geophones. A vertical 
pair is preferred, however, because they do not specifically have to be 
aligned in their respective horizontal orientations to be in line with the 
formation direction between the boreholes. 
Moreover, hydrophones or receivers having pressure sensing elements could 
also be employed, if desired, under proper environmental conditions for 
their operations, such as with a fluid-filled borehole. 
While two particular preferred embodiments of the invention have been shown 
and described, it will be understood that the invention is not limited 
thereto, since many modifications may be made and will become apparent to 
those skilled in the art.