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Application of GPR for 3-D visualization of geological and structural variation in a limestone formation
Journal of Applied Geophysics 40 Ž1998. 29–36
Application of GPR for 3-D visualization of geological and structural variation in a limestone formation Thrainn Sigurdsson a , Torben Overgaard
b,)
a
b
TS Geokonsult, P.O. Box 44, S-443 21 Lerum, Sweden Faxe Kalk, P.O. Box 2183, DK-1017 Copenhagen, Denmark Received 13 February 1997; accepted 12 March 1998
Abstract Ground-penetrating radar ŽGPR. offers a simple and rapid means of providing valuable information for mapping of geological and structural variations in limestone. Data from closely spaced radar sections were gathered in two areas in Faxe Kalk’s limestone quarry on the island of Zealand, Denmark. A ‘pulseEKKO 100’ GPR system with 100 MHz antennas was used for data gathering. In this paper we describe the steps required to obtain three-dimensional Ž3-D. data. Data were collected in continuous mode with readings every 20 cm. The 3-D cubes were prepared for visualization by using several processing routines. Various data presentations, including chair views, and multiple and single slices, were generated from the 3-D cube to enhance the outline and distribution of flint-free limestone in the surveyed areas. Animation of 3-D data was found to be a very powerful way of visualizing the geological structures in the surveyed areas. The limestone formation in the quarry is of middle Danian age Žapprox. 63 million years old. and is divided into two main deposition facies—coral limestone and bryozoan limestone. The coral limestone appears as reef structures in the more abundant bryozoan limestone. In the bryozoan limestone frequent flint layers appear often delineating mound-like structures. Coral limestone and areas of bryozoan limestone without flint are quarried selectively and used for high quality products. Division of the limestone formation into areas of coral reefs and bryozoan mounds and separation of flint-free areas is made possible through distinctive reflection patterns and differing penetration depths between the rock units. The 3-D visualization allows estimation of potential volumes of limestone for selective exploitation. q 1998 Elsevier Science B.V. All rights reserved. Keywords: ground-penetrating radar; 3-D visualization; limestone; industrial mineral exploration; Denmark
1. Introduction Faxe Kalk is an industrial mineral exploration company dealing with products based on calcium carbonate. One of their limestone quarries is situated in the southernmost part of the island of Zealand, Denmark. Earlier groundpenetrating ŽGPR. works on the limestone in
)
Corresponding author.
this quarry ŽSigurdsson, 1993, 1994. have shown that distinction between some of the different limestone types in the quarry is possible. The implementation of GPR as an every day tool for exploration of flint-free limestone facies in the limestone quarry has been described by Overgaard Ž1995. . The geometry of the various limestone facies is fairly complicated making threedimensional Ž3-D. visualization a perfect tool for planning of selective mining of flint-free limestone types. An experimental 3-D GPR sur-
0926-9851r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 9 8 5 1 Ž 9 8 . 0 0 0 1 5 - 9
30
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
vey was therefore conducted in two promising areas in the bottom of the quarry. 1.1. Geological description Extensive bryozoan bioherms as well as local coral mounds developed during the Early and Middle Danian Ž Lower Palaeocene. in a narrow epicontinental seaway that followed the Thornquist–Sorgenfrei lineament ŽHakansson and ˚ Thomsen, 1979; Thomsen, 1989. Ž Fig. 1. . The stratigraphically lowest parts in Faxe Kalk’s limestone quarry are dominated by bryozoan bioherms, followed by an interval dominated by coral mounds Ž Willumsen, 1995a.. The bryozoan limestone and the coral limestone show a wide range of sub-facies and transitional facies as well as variations in diagenesis. The bryozoan limestone facies varies from mudstone through wackestone and packstone to grainstone going from completely un-cemented to well-cemented types ŽClassification according to Folk, 1962... Nodular or continuous layers of flint are very abundant in major parts of the bryozoan limestone. The flint is predominantly derived from silica fungi which were dissolved and for
unknown reasons concentrated in specific levels in the bryozoan limestone. The coral limestone can be described as a rudstone consisting of azooxanthellate coral-colonies. Degree of cementation and amount of matrix varies strongly in the coral limestone. Flint is not found in the coral limestone, but locally a weak silicification can be seen ŽFig. 1.. 1.2. Industrial implications All the limestone types are very pure typically consisting of more than 98% carbonates Žalmost purely calcite. when flint layers and flint nodules in the bryozoan limestone are excluded. From an industrial point of view, a division in three main types of limestone is required: Ž 1. coral limestone; Ž 2. bryozoan limestone with abundant flint layers; and Ž 3. bryozoan limestone with more than 1.5 m between flint layers Žhereafter named flint-free bryozoan limestone.. This division is based on the different products which can be manufactured from these limestone types, and the fact that bryozoan limestone with more than approximately 1.5 m between flint layers can be selectively
Fig. 1. Ža. Structural divisions in the Danish area. Žb. Facies distribution in the Danish area in the upper Middle Danian. ŽBoth figures are from Willumsen, 1995b.
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
quarried without enhancing the silica content in the otherwise pure limestone. Coral limestone is mainly used for making burnt lime, which among a variety of products, can be used in the production of PCC ŽPrecipitated Calcium Carbonate.. The flint-free bryozoan limestone is used as an industrial limestone for making different kinds of fillers and for making burnt lime. The bryozoan limestone containing abundant flint layers can, because of the enhanced silica content Žaround 3%. , only be used as an agricultural limestone.
2. Methodology Subsurface profiling with GPR uses electromagnetic ŽEM. wave energy reflection for detecting material variations. The reflection is caused by contrasts in the dielectric constant of different materials. The velocity and attenuation of the radar wave are derived from the electromagnetic wave equation allowing formulation of the conditions for the wave propagation.
31
2.1. Field methods A ‘pulseEKKO 100’ GPR system with 100MHz antennas was used for data gathering. The data were collected in step mode using an odometer wheel to trigger readings every 20 cm in a grid with 1-m spacing between the individual measuring lines. Data were collected in two separate areas in the southern part of the quarry. In test area 1, the surface geology showed presence of an area containing flint-free bryozoan limestone followed by coral limestone. In test area 2, a drill hole indicated presence of 8 m of flint-free bryozoan limestone. 2.2. 3-D Õisualization and data processing The 3-D visualizations of GPR data were performed in a program called Slicer, from Spyglass. Gathered GPR data were pre-processed and converted into a special format to fit into the Slicer program. The following processing
Fig. 2. Two-dimensional view of line 6 from test area 1. In the right hand side of the radar section the complicated reflection pattern seen in the coral limestone is displayed. In the bottom left hand side and central part of the radar section flint-free bryozoan limestone is indicated by few and weak reflections. The strong and continuous reflections seen on the top left hand side of the radar section correspond to flint-rich bryozoan limestone.
32
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
routine was applied to the collected data, as follows: location of timezero; dewow correction; timezero points in data file shifted to the same point; fill in of gaps in collected data; data decimated in time direction; data resampled in time direction; data chopped to the same length; data converted from 16 bit to 8 bit data.
In Slicer the GPR data can be displayed as a 3-D data cube which can be rotated, squeezed, stretched, sliced and coloured. This can greatly simplify the interpretation of large volumes of data. To control the ‘natural’ variation between reflectors from different interfaces ŽSigurdsson, 1993., the raw data are usually processed in a relative simple way. The processing of data was made with the Spreading and Exponential Compensation Ž SEC. gain in combination with low-
Fig. 3. Three-dimensional views and geological interpretations of the data cube from test area 1. Both views of the data cube show cutouts of the reflection patterns from approximately 1 m to 11 m depth. Fig. 2a shows the whole data cube from line 0 to line 45. In the geological interpretation of the cube, all the three main types of limestone found in the quarry can be seen. Fig. 2b shows a cutout of the data cube from line 35 to line 45. The geological interpretation shows that only two of the main limestone types are present in this cutout of the data cube.
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
33
Fig. 3 Žcontinued..
pass filtering. The SEC gain will compensate for the amplitude fall off with reflection depth. The down the trace average operation performs the signal averaging by replacing the amplitude by the average over a range centred about that point. The processed GPR sections allow division of the measuring results into areas with: Ž a. relative high amplitude attenuation, weak and limited numbers of reflections, Žb. irregular reflection pattern and Žc. relatively regular and frequent reflection pattern.
ences between the layers. The thickness of the flint layers as well as the inclination of the layer will also influence the strength of the reflection. Reflection and refraction of EM waves are described by Snell’s law and Fresnel’s equation is well-known. The angle of incidence and reflection is: sin u i s sin u t
and for a given angle of incidence and angle of refraction: sin u i sin u t
3. Theoretical considerations The electromagnetic wave reflection from interfaces between flint layers and the bryozoan limestone is dependent on the dielectric differ-
Ž ui s ut .
s
kt ki
where k is the magnitude of the wave vector for layer n s i and n s t, respectively. The amplitude reflection coefficient, r, is given as a characteristic impedance of two media. The connection between the reflection coefficient and the
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
34
dielectric constant is for perfect dielectric materials defined by Fresnel as follows: 1
sin u i sin u t
k2
s
s
yj vm 2 Ž s 2 q j ve 2 .
2 1
k1 yj vm 1 Ž s 1 q j ve 1 .
2
if the conductivity is zero and the magnetic permeability insignificant varying the equation can be written as: sin u 1 sin u 2
s
k2
s
k1
e2
1 2
e1
Finally the power reflection coefficient, R is: Rs
(e (e
( q (e
1
y e2
1
2
The indices 1 and 2 refer to the layers on both sides of the reflecting interface. The bryozoan limestone is usually strongly bedded with thin layers of flint Ž 0.05–1.00 m thick. alternating with thicker layers of limestone Ž0.2–10.0 m thick.. The flintrlimestone contacts act as excellent reflectors depicting smaller and larger mound shapes in the limestone. The reflection pattern is therefore characterized by medium to strong, mainly continuous reflections caused by these flintrlimestone contacts. Flint nodules can often be seen as short discontinuous reflections. The strength of the reflectors depends mainly on the thickness and the dip of the flint layers. Smaller flint nodules, thin flint layers and strongly dipping flint layers will give comparatively faint reflections. Larger areas in the bryozoan limestone without flint layers show practically no reflections, and can therefore easily be detected. The penetration depth in the bryozoan limestone is usually between 7 and 11 m. The coral limestone seldom has a visible internal bedding, but consists of rather porous rock with differing degrees of cementation and matrix contents. The reflection pattern is generally characterized by strong and numerous dis-
continuous reflections often dominated by overlapping hyperbolas. These are probably caused by the variations seen in cementation and matrix content of the coral limestone leading to areas with higher air or water content and thereby acting as point reflectors. Steep bedding with slopes ranging from 448 to 648 have, though, been observed in some coral mounds. The layering arises from repetitive shifts between matrix-rich and matrix-poor coral limestone ŽWillumsen, 1995a.. The penetration depth in the coral limestone is usually better than in the bryozoan limestone, normally more than 10 m and in some instances, up to 18 m. This is probably caused by a high degree of cementation leading to a lower content of fines in the coral limestone than in the bryozoan limestone.
4. Results and interpretation The two-dimensional Ž2-D. radar section in Fig. 2, shows the GPR data from line 6 in test area 1. The radar section has a length of 70 m and a depth of 400 ns corresponding to 14 m using a velocity of 0.07 mrns. This section displays the typical reflection patterns of the main limestone types and therefore explains the general principle behind the interpretation of the data cubes shown in Fig. 3. The coral limestone is indicated by the complicated reflection pattern seen on the right hand side of the radar section. The central part of the radar section contains relatively few and weak reflections, which indicate the presence of flint-free bryozoan limestone. In the top left hand side of the radar section bryozoan limestone with numerous flint layers are indicated by strong and continuous reflections. 4.1. Test area 1 The horizontal extension of this test area is 45 m by 70 m. The 3-D data cube can be divided into three main limestone domains, sim-
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
ilar to the interpretation of the two-dimensional radar section in Fig. 2. Two views of the data cube and their interpretations can be seen in Fig. 3. The views show data from approximately 1 m to 11 m depth. In Fig. 3a, a reef of coral limestone is seen in the right hand corner of the cube. Alternating
35
matrix-rich and matrix-poor layers causes the dipping, semi-continuous reflectors seen in parts of the interpreted coral reef. The top section of the cube is dominated by bryozoan limestone with abundant flint layers, and the bottom and central parts of the cube consist of a flint-free bryozoan limestone type called wackestone.
Fig. 4. Three-dimensional views of the data cube from test area 2. All the views show cutouts of the reflection patterns from approximately 1 m to 12 m depth. Flint-free bryozoan limestone is seen as areas in the cube with few and weak reflections. Fig. 3a shows the whole data cube as seen from line 31. In Fig. 3b, the front right hand corner has been cut out of the cube. Fig. 3c shows the data cube from line 14 to line 0, and Fig. 3d shows a horizontal cut of the data cube, i.e., a timeslice.
36
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
Some hyperbolic shaped reflections indicate the presence of small isolated mounds of coral limestone in the wackestone. In Fig. 3b the coral limestone has disappeared and bryozoan limestone with flint layers has started to move in from the right hand side of the data cube. Wackestone still occupies major parts of this cutout of the data cube. No drill holes are situated in the area, but on the surface an area showing flint-free bryozoan limestone followed by coral limestone clearly coincides with areas in the data cube interpreted as so. 4.2. Test area 2 Test area 2 has a horizontal extension of 31 m by 31 m. Four different views of the data cube are shown in Fig. 4, all of them showing reflection data from approximately 1–11 m depth. The views reveal some of the possibilities of data presentation which are available in the Slicer program. They also indicate the presence of an area containing flint-free bryozoan limestone inside the otherwise flint-rich bryozoan limestone. Notice the very irregular shape of this flint-free limestone body. A pre-mining knowledge of shape and volume of the flint-free limestone bodies facilitates the planning of the mining operations in the quarry. Apart from the views shown in Fig. 4, various forms of slices, galleries, chair views, transparent views and others can be produced in Slicer. A drill hole located approximately in the middle of the surveyed area contains flint-free bryozoan limestone from the surface and down to a depth of 8 m. This coincides very well with the interpretation of the measured data. 5. Conclusion Interpretation of traditional 2-D radar sections as well as 3-D radar cubes allowed identification of three limestone units: bryozoan limestone containing flint layers, bryozoan limestone without flint and coral limestone. The
distribution of the different units can be mapped. In our experiments, three-dimensional visualization of GPR data greatly improve understanding of the distribution and shape of flint-free limestone units, thereby improving reserve estimations and planning of mining operations in the exploration area. Acknowledgements We wish to thank Greg Johnston, Sensors and Software for technical support and Faxe Kalk for financial support and permission to publish the results. References Folk, R.L., 1962. Spectral subdivision of limestone types. In: Ham, W.E. ŽEd.., Classification of Carbonate Rocks. Am. Ass. Petrol. Geol., Mem., 1, pp. 62–84. Hakansson, E., Thomsen, E., 1979. Distribution of types of ˚ bryozoan communities at the boundary in Denmark. In: Birkelund, T., Bromley, R. ŽEds.., Cretaceous–Tertiary Boundary Events, Symposium, I. The Maastrichtian and Danian of Denmark. University of Copenhagen, pp. 78–91. Overgaard, T., 1995. Ground penetrating radar for limestone exploration. In: Robert, N.J. ŽEd.., International Minerals and Metals Technology 1995. Sabrecrown Publishing, London, pp. 33–35. Sigurdsson, T., 1993. Ground penetrating radar for geolog˚ ical mapping. Doctoral thesis, Arhus University. Aarhus Geoscience, 3, 1995. Sigurdsson, T., 1994. Application of GPR for geological mapping, exploration of industrial mineralizations and sulphide deposits. GPR ’94, Proceedings of the 5th International Conference on Ground Penetrating Radar, Vol. 3, pp. 941–955. Thomsen, E., 1989. Kalkaflejringer fra Kridt og Danien. In: Bjørslev Nielsen, O., Sandersen, P. ŽEds.., Danmarks geologi—fra øvre kridt til i dag. Aarhus Universitet, Geologisk Institut, undervisningskompendium, pp. 1–42. Willumsen, M.E., 1995a. Early lithification in Danian azooxanthellate scleractinian lithoherms, Faxe Quarry, Denmark. Betrage Palaontol. 20, 123–131. Willumsen, M.E., 1995b. Model for dannelse af koraldominerede biogene banker i Faxe Kalkbrud. Unpublished Master Thesis, Geological Institute, University of Copenhagen.
Application of GPR for 3-D visualization of geological and structural variation in a limestone formation Thrainn Sigurdsson a , Torben Overgaard
b,)
a
b
TS Geokonsult, P.O. Box 44, S-443 21 Lerum, Sweden Faxe Kalk, P.O. Box 2183, DK-1017 Copenhagen, Denmark Received 13 February 1997; accepted 12 March 1998
Abstract Ground-penetrating radar ŽGPR. offers a simple and rapid means of providing valuable information for mapping of geological and structural variations in limestone. Data from closely spaced radar sections were gathered in two areas in Faxe Kalk’s limestone quarry on the island of Zealand, Denmark. A ‘pulseEKKO 100’ GPR system with 100 MHz antennas was used for data gathering. In this paper we describe the steps required to obtain three-dimensional Ž3-D. data. Data were collected in continuous mode with readings every 20 cm. The 3-D cubes were prepared for visualization by using several processing routines. Various data presentations, including chair views, and multiple and single slices, were generated from the 3-D cube to enhance the outline and distribution of flint-free limestone in the surveyed areas. Animation of 3-D data was found to be a very powerful way of visualizing the geological structures in the surveyed areas. The limestone formation in the quarry is of middle Danian age Žapprox. 63 million years old. and is divided into two main deposition facies—coral limestone and bryozoan limestone. The coral limestone appears as reef structures in the more abundant bryozoan limestone. In the bryozoan limestone frequent flint layers appear often delineating mound-like structures. Coral limestone and areas of bryozoan limestone without flint are quarried selectively and used for high quality products. Division of the limestone formation into areas of coral reefs and bryozoan mounds and separation of flint-free areas is made possible through distinctive reflection patterns and differing penetration depths between the rock units. The 3-D visualization allows estimation of potential volumes of limestone for selective exploitation. q 1998 Elsevier Science B.V. All rights reserved. Keywords: ground-penetrating radar; 3-D visualization; limestone; industrial mineral exploration; Denmark
1. Introduction Faxe Kalk is an industrial mineral exploration company dealing with products based on calcium carbonate. One of their limestone quarries is situated in the southernmost part of the island of Zealand, Denmark. Earlier groundpenetrating ŽGPR. works on the limestone in
)
Corresponding author.
this quarry ŽSigurdsson, 1993, 1994. have shown that distinction between some of the different limestone types in the quarry is possible. The implementation of GPR as an every day tool for exploration of flint-free limestone facies in the limestone quarry has been described by Overgaard Ž1995. . The geometry of the various limestone facies is fairly complicated making threedimensional Ž3-D. visualization a perfect tool for planning of selective mining of flint-free limestone types. An experimental 3-D GPR sur-
0926-9851r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 9 8 5 1 Ž 9 8 . 0 0 0 1 5 - 9
30
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
vey was therefore conducted in two promising areas in the bottom of the quarry. 1.1. Geological description Extensive bryozoan bioherms as well as local coral mounds developed during the Early and Middle Danian Ž Lower Palaeocene. in a narrow epicontinental seaway that followed the Thornquist–Sorgenfrei lineament ŽHakansson and ˚ Thomsen, 1979; Thomsen, 1989. Ž Fig. 1. . The stratigraphically lowest parts in Faxe Kalk’s limestone quarry are dominated by bryozoan bioherms, followed by an interval dominated by coral mounds Ž Willumsen, 1995a.. The bryozoan limestone and the coral limestone show a wide range of sub-facies and transitional facies as well as variations in diagenesis. The bryozoan limestone facies varies from mudstone through wackestone and packstone to grainstone going from completely un-cemented to well-cemented types ŽClassification according to Folk, 1962... Nodular or continuous layers of flint are very abundant in major parts of the bryozoan limestone. The flint is predominantly derived from silica fungi which were dissolved and for
unknown reasons concentrated in specific levels in the bryozoan limestone. The coral limestone can be described as a rudstone consisting of azooxanthellate coral-colonies. Degree of cementation and amount of matrix varies strongly in the coral limestone. Flint is not found in the coral limestone, but locally a weak silicification can be seen ŽFig. 1.. 1.2. Industrial implications All the limestone types are very pure typically consisting of more than 98% carbonates Žalmost purely calcite. when flint layers and flint nodules in the bryozoan limestone are excluded. From an industrial point of view, a division in three main types of limestone is required: Ž 1. coral limestone; Ž 2. bryozoan limestone with abundant flint layers; and Ž 3. bryozoan limestone with more than 1.5 m between flint layers Žhereafter named flint-free bryozoan limestone.. This division is based on the different products which can be manufactured from these limestone types, and the fact that bryozoan limestone with more than approximately 1.5 m between flint layers can be selectively
Fig. 1. Ža. Structural divisions in the Danish area. Žb. Facies distribution in the Danish area in the upper Middle Danian. ŽBoth figures are from Willumsen, 1995b.
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
quarried without enhancing the silica content in the otherwise pure limestone. Coral limestone is mainly used for making burnt lime, which among a variety of products, can be used in the production of PCC ŽPrecipitated Calcium Carbonate.. The flint-free bryozoan limestone is used as an industrial limestone for making different kinds of fillers and for making burnt lime. The bryozoan limestone containing abundant flint layers can, because of the enhanced silica content Žaround 3%. , only be used as an agricultural limestone.
2. Methodology Subsurface profiling with GPR uses electromagnetic ŽEM. wave energy reflection for detecting material variations. The reflection is caused by contrasts in the dielectric constant of different materials. The velocity and attenuation of the radar wave are derived from the electromagnetic wave equation allowing formulation of the conditions for the wave propagation.
31
2.1. Field methods A ‘pulseEKKO 100’ GPR system with 100MHz antennas was used for data gathering. The data were collected in step mode using an odometer wheel to trigger readings every 20 cm in a grid with 1-m spacing between the individual measuring lines. Data were collected in two separate areas in the southern part of the quarry. In test area 1, the surface geology showed presence of an area containing flint-free bryozoan limestone followed by coral limestone. In test area 2, a drill hole indicated presence of 8 m of flint-free bryozoan limestone. 2.2. 3-D Õisualization and data processing The 3-D visualizations of GPR data were performed in a program called Slicer, from Spyglass. Gathered GPR data were pre-processed and converted into a special format to fit into the Slicer program. The following processing
Fig. 2. Two-dimensional view of line 6 from test area 1. In the right hand side of the radar section the complicated reflection pattern seen in the coral limestone is displayed. In the bottom left hand side and central part of the radar section flint-free bryozoan limestone is indicated by few and weak reflections. The strong and continuous reflections seen on the top left hand side of the radar section correspond to flint-rich bryozoan limestone.
32
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
routine was applied to the collected data, as follows: location of timezero; dewow correction; timezero points in data file shifted to the same point; fill in of gaps in collected data; data decimated in time direction; data resampled in time direction; data chopped to the same length; data converted from 16 bit to 8 bit data.
In Slicer the GPR data can be displayed as a 3-D data cube which can be rotated, squeezed, stretched, sliced and coloured. This can greatly simplify the interpretation of large volumes of data. To control the ‘natural’ variation between reflectors from different interfaces ŽSigurdsson, 1993., the raw data are usually processed in a relative simple way. The processing of data was made with the Spreading and Exponential Compensation Ž SEC. gain in combination with low-
Fig. 3. Three-dimensional views and geological interpretations of the data cube from test area 1. Both views of the data cube show cutouts of the reflection patterns from approximately 1 m to 11 m depth. Fig. 2a shows the whole data cube from line 0 to line 45. In the geological interpretation of the cube, all the three main types of limestone found in the quarry can be seen. Fig. 2b shows a cutout of the data cube from line 35 to line 45. The geological interpretation shows that only two of the main limestone types are present in this cutout of the data cube.
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
33
Fig. 3 Žcontinued..
pass filtering. The SEC gain will compensate for the amplitude fall off with reflection depth. The down the trace average operation performs the signal averaging by replacing the amplitude by the average over a range centred about that point. The processed GPR sections allow division of the measuring results into areas with: Ž a. relative high amplitude attenuation, weak and limited numbers of reflections, Žb. irregular reflection pattern and Žc. relatively regular and frequent reflection pattern.
ences between the layers. The thickness of the flint layers as well as the inclination of the layer will also influence the strength of the reflection. Reflection and refraction of EM waves are described by Snell’s law and Fresnel’s equation is well-known. The angle of incidence and reflection is: sin u i s sin u t
and for a given angle of incidence and angle of refraction: sin u i sin u t
3. Theoretical considerations The electromagnetic wave reflection from interfaces between flint layers and the bryozoan limestone is dependent on the dielectric differ-
Ž ui s ut .
s
kt ki
where k is the magnitude of the wave vector for layer n s i and n s t, respectively. The amplitude reflection coefficient, r, is given as a characteristic impedance of two media. The connection between the reflection coefficient and the
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
34
dielectric constant is for perfect dielectric materials defined by Fresnel as follows: 1
sin u i sin u t
k2
s
s
yj vm 2 Ž s 2 q j ve 2 .
2 1
k1 yj vm 1 Ž s 1 q j ve 1 .
2
if the conductivity is zero and the magnetic permeability insignificant varying the equation can be written as: sin u 1 sin u 2
s
k2
s
k1
e2
1 2
e1
Finally the power reflection coefficient, R is: Rs
(e (e
( q (e
1
y e2
1
2
The indices 1 and 2 refer to the layers on both sides of the reflecting interface. The bryozoan limestone is usually strongly bedded with thin layers of flint Ž 0.05–1.00 m thick. alternating with thicker layers of limestone Ž0.2–10.0 m thick.. The flintrlimestone contacts act as excellent reflectors depicting smaller and larger mound shapes in the limestone. The reflection pattern is therefore characterized by medium to strong, mainly continuous reflections caused by these flintrlimestone contacts. Flint nodules can often be seen as short discontinuous reflections. The strength of the reflectors depends mainly on the thickness and the dip of the flint layers. Smaller flint nodules, thin flint layers and strongly dipping flint layers will give comparatively faint reflections. Larger areas in the bryozoan limestone without flint layers show practically no reflections, and can therefore easily be detected. The penetration depth in the bryozoan limestone is usually between 7 and 11 m. The coral limestone seldom has a visible internal bedding, but consists of rather porous rock with differing degrees of cementation and matrix contents. The reflection pattern is generally characterized by strong and numerous dis-
continuous reflections often dominated by overlapping hyperbolas. These are probably caused by the variations seen in cementation and matrix content of the coral limestone leading to areas with higher air or water content and thereby acting as point reflectors. Steep bedding with slopes ranging from 448 to 648 have, though, been observed in some coral mounds. The layering arises from repetitive shifts between matrix-rich and matrix-poor coral limestone ŽWillumsen, 1995a.. The penetration depth in the coral limestone is usually better than in the bryozoan limestone, normally more than 10 m and in some instances, up to 18 m. This is probably caused by a high degree of cementation leading to a lower content of fines in the coral limestone than in the bryozoan limestone.
4. Results and interpretation The two-dimensional Ž2-D. radar section in Fig. 2, shows the GPR data from line 6 in test area 1. The radar section has a length of 70 m and a depth of 400 ns corresponding to 14 m using a velocity of 0.07 mrns. This section displays the typical reflection patterns of the main limestone types and therefore explains the general principle behind the interpretation of the data cubes shown in Fig. 3. The coral limestone is indicated by the complicated reflection pattern seen on the right hand side of the radar section. The central part of the radar section contains relatively few and weak reflections, which indicate the presence of flint-free bryozoan limestone. In the top left hand side of the radar section bryozoan limestone with numerous flint layers are indicated by strong and continuous reflections. 4.1. Test area 1 The horizontal extension of this test area is 45 m by 70 m. The 3-D data cube can be divided into three main limestone domains, sim-
T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
ilar to the interpretation of the two-dimensional radar section in Fig. 2. Two views of the data cube and their interpretations can be seen in Fig. 3. The views show data from approximately 1 m to 11 m depth. In Fig. 3a, a reef of coral limestone is seen in the right hand corner of the cube. Alternating
35
matrix-rich and matrix-poor layers causes the dipping, semi-continuous reflectors seen in parts of the interpreted coral reef. The top section of the cube is dominated by bryozoan limestone with abundant flint layers, and the bottom and central parts of the cube consist of a flint-free bryozoan limestone type called wackestone.
Fig. 4. Three-dimensional views of the data cube from test area 2. All the views show cutouts of the reflection patterns from approximately 1 m to 12 m depth. Flint-free bryozoan limestone is seen as areas in the cube with few and weak reflections. Fig. 3a shows the whole data cube as seen from line 31. In Fig. 3b, the front right hand corner has been cut out of the cube. Fig. 3c shows the data cube from line 14 to line 0, and Fig. 3d shows a horizontal cut of the data cube, i.e., a timeslice.
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T. Sigurdsson, T. OÕergaardr Journal of Applied Geophysics 40 (1998) 29–36
Some hyperbolic shaped reflections indicate the presence of small isolated mounds of coral limestone in the wackestone. In Fig. 3b the coral limestone has disappeared and bryozoan limestone with flint layers has started to move in from the right hand side of the data cube. Wackestone still occupies major parts of this cutout of the data cube. No drill holes are situated in the area, but on the surface an area showing flint-free bryozoan limestone followed by coral limestone clearly coincides with areas in the data cube interpreted as so. 4.2. Test area 2 Test area 2 has a horizontal extension of 31 m by 31 m. Four different views of the data cube are shown in Fig. 4, all of them showing reflection data from approximately 1–11 m depth. The views reveal some of the possibilities of data presentation which are available in the Slicer program. They also indicate the presence of an area containing flint-free bryozoan limestone inside the otherwise flint-rich bryozoan limestone. Notice the very irregular shape of this flint-free limestone body. A pre-mining knowledge of shape and volume of the flint-free limestone bodies facilitates the planning of the mining operations in the quarry. Apart from the views shown in Fig. 4, various forms of slices, galleries, chair views, transparent views and others can be produced in Slicer. A drill hole located approximately in the middle of the surveyed area contains flint-free bryozoan limestone from the surface and down to a depth of 8 m. This coincides very well with the interpretation of the measured data. 5. Conclusion Interpretation of traditional 2-D radar sections as well as 3-D radar cubes allowed identification of three limestone units: bryozoan limestone containing flint layers, bryozoan limestone without flint and coral limestone. The
distribution of the different units can be mapped. In our experiments, three-dimensional visualization of GPR data greatly improve understanding of the distribution and shape of flint-free limestone units, thereby improving reserve estimations and planning of mining operations in the exploration area. Acknowledgements We wish to thank Greg Johnston, Sensors and Software for technical support and Faxe Kalk for financial support and permission to publish the results. References Folk, R.L., 1962. Spectral subdivision of limestone types. In: Ham, W.E. ŽEd.., Classification of Carbonate Rocks. Am. Ass. Petrol. Geol., Mem., 1, pp. 62–84. Hakansson, E., Thomsen, E., 1979. Distribution of types of ˚ bryozoan communities at the boundary in Denmark. In: Birkelund, T., Bromley, R. ŽEds.., Cretaceous–Tertiary Boundary Events, Symposium, I. The Maastrichtian and Danian of Denmark. University of Copenhagen, pp. 78–91. Overgaard, T., 1995. Ground penetrating radar for limestone exploration. In: Robert, N.J. ŽEd.., International Minerals and Metals Technology 1995. Sabrecrown Publishing, London, pp. 33–35. Sigurdsson, T., 1993. Ground penetrating radar for geolog˚ ical mapping. Doctoral thesis, Arhus University. Aarhus Geoscience, 3, 1995. Sigurdsson, T., 1994. Application of GPR for geological mapping, exploration of industrial mineralizations and sulphide deposits. GPR ’94, Proceedings of the 5th International Conference on Ground Penetrating Radar, Vol. 3, pp. 941–955. Thomsen, E., 1989. Kalkaflejringer fra Kridt og Danien. In: Bjørslev Nielsen, O., Sandersen, P. ŽEds.., Danmarks geologi—fra øvre kridt til i dag. Aarhus Universitet, Geologisk Institut, undervisningskompendium, pp. 1–42. Willumsen, M.E., 1995a. Early lithification in Danian azooxanthellate scleractinian lithoherms, Faxe Quarry, Denmark. Betrage Palaontol. 20, 123–131. Willumsen, M.E., 1995b. Model for dannelse af koraldominerede biogene banker i Faxe Kalkbrud. Unpublished Master Thesis, Geological Institute, University of Copenhagen.