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Mapping the internal structure of sand dunes with GPR: A case history from the Jafurah sand sea of eastern Saudi Arabia ADEMOLA Q. ADETUNJI, ABDULLATIF AL-SHUHAIL, and GABOR KORVIN, King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia
T
he study of the geometry and internal structure of modern aeolian deposits (sand dunes) can give a better understanding of sandstone hydrocarbon reservoirs like the A and B members of Unayzah Formation in Saudi Arabia. These members have been shown to contain depositional facies like aeolian dunes, aeolian sand sheets, and interdunes (Melvin and Heine, 2004). Fortunately, ground-penetrating radar (GPR) allows noninvasive data acquisition for the study of the stratigraphy and internal sedimentary structures of unconsolidated deposits. Under ideal conditions, GPR has a vertical resolution of the order of a few centimeters and depth of penetration in the range of tens of meters. GPR has potential for describing the stratigraphy and internal geometry in specific depositional facies in the same way as seismic does for larger-scale features. Interpretation of a correctly processed radar reflection profile can provide detailed information about the internal structure and sedimentary architecture of a deposit. Similarities between the kinematic properties of seismic and GPR data (assuming low losses) suggest that processing and interpretation techniques used for seismic data may be applied to GPR data sets. In eastern Saudi Arabia, outcropping geological strata are overlain by aeolian sands, fluviatile sand, and gravel. There are four main dune fields in Saudi Arabia: the Jafurah, the great Nafud, Ad-Dahna, and Rub Al-Khali. The Jafurah sand sea (Figure 1) is between latitudes 24° and 27°N and longitudes 30° and 49°E in the coastal lowlands along the Arabian Gulf. The aeolian dunes in Jafurah are relatively small, primarily barchans with associated barchanoid ridges and parabolic dune fields. Such features are the result of the constant wind direction and high velocity. Two sites within Jafurah were selected for this study. Location 1, a barchan sand dune at N26° 05' 54.2" and E50° 06' 34.7" (around Half Moon Bay) has no Tertiary outcrop. The area consists dominantly of surficial deposits of siliciclastic aeolian sands that migrate as dune systems across the coastal sabkha plains. The dune’s height is about 7.5 m with slip face angle of 32° and is bordered by the Arabian Gulf on one side and sabkha on the other. Its surface was dry with patches of vegetation. An area without vegetation was selected for the study. The data at this location were collected between 31 January and 2 February 2008, about 45 days after the last rain. Location 2, another barchan sand dune, is at N26° 24' 34.6" and E49° 56' 43.7", about 3 km from the Dammam International Airport Road. The surrounding area is covered by sand dunes, interdune sand sheets on one side and exposed Tertiary limestone on the other. The dune itself is about 6.5 m high with slip face angle of 32°. Its surface was dry with patches of vegetation. Data were collected between 3 and 6 February 2008, about 48 days after the last rain. 1446
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Figure 1. A map of Saudi Arabia showing the Jafurah sand sea and locations 1 and 2 (modified from Al-Shuhail, 2006).
Data collection All GPR data were collected with a Geophysical Survey Systems SIR-3000 control unit and a monostatic antenna with central frequency of 400 MHz using continuous time mode. The velocity of EM waves in the sand dune was estimated by measuring the two-way traveltime from the dune surface to metal cans buried at known depths. An average velocity of 0.11 m/ns was found to best fit the diffraction hyperbolas from the cans. Field parameters included a trace length of 100 ns, 512 samples per trace, a five-point gain function, and a band-pass filter of 100–800 MHz. We determined a 6-m depth of penetration would be appropriate since we were interested in the features that could be as small as 2–8 cm.
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Figure 2. 3D cube from location 1 showing the recorded internal structure further away from the sea.
Figure 3. 3D cube from location 2 showing the base of the internal structure.
Data processing and analysis In GPR studies, attenuation of EM wave energy over a relatively short vertical distance represents one of the biggest challenges. In order to improve the signal-to-noise ratio of the low-amplitude reflected energy, several standard processing steps were applied. Time-zero correction, band-pass fil-
tering, background noise removal, and migration all focused on improving the SNR. Once the optimal SNR had been reached, the data were displayed for interpretation. The profiles from location 2 were collected on a surface with a slope of about 5.7°, and each was topographically corrected before depth conversion. The GPR data are presented using real amNovember 2008
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Figure 4. Some basic terminology used in describing radar packages and their boundaries (modified from Gawthorpe et al., 1993).
Figure 5. Radar profile from location 1 recorded very close to the sea and perpendicular to the slip face. Vertical resolution is 8 cm, and the dipping angle within radar packages is shown.
Figure 6. Picture of a trench (depth of 0.8 m) along the profile in Figure 5 showing the cross-strata. Some shrubs were also found in this area. 1448
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plitudes; red indicates positive and blue negative amplitudes. 3D visualization has proven useful in looking at how subsurface features change at various depths and different directions (Figures 2 and 3). Principles of radar stratigraphy are consistent with seismic stratigraphy. As in seismics, terminations of reflections are used to identify radar surfaces (Figure 4), which in turn define genetically related packages referred to as radar packages. Also, these radar surfaces geologically represent nondepositional or erosional hiatuses in a sedimentary sequence. Radar packages are sets of reflections with distinctive configurations, continuity, frequency, amplitude, velocity characteristics, and external form. However, in deposits like sand dunes, radar-reflecting surfaces are indicative of bounding surfaces, and intervening radar packages are therefore formed from reflections generated by the stratifications within the sediments. The radar surfaces in this study were identified by reflection geometries which include truncation (reflector cut by a geologic feature), downlap (underlying surface dip < overlying strata), onlap (underlying surface dip > overlying strata), and toplap (termination of inclined reflectors against an overlying lower-angle surface). Location 1 The data analysis for location 1 was on a profile with a length of 20 m collected close to the sea (Figure 5). Using the reflection characteristics of the profile, we were able to compute the crossstrata dipping angle to be 30°. The profile demonstrates that we were able to resolve cross-strata with thicknesses of 8 cm (Figure 5), which is in agreement with the targeted resolution. Crossstrata and layers containing dark grains were observed in the trenches dug along the profile with dip angles similar to the current repose angle of the dune slip face (Figure 6). The dark layers occurred at varying vertical distances within this dune. We also encountered plant roots in the trench along the profile. Some vertical events that start at about 0.8 m could be attributed to these roots. Four radar packages (A, B, C, and D) defined by three radar surfaces (bounding surfaces) were identified on the profile (Figure 7). The radar surfaces were identified by onlap and toplap reflection geometries. The radar packages consist of high-amplitude dipping reflections with foresets dipping at 30° in the direction of wind transport. Package A consists of both low- and high-amplitude reflections. The cross-strata have a minimum vertical resolution of about 8 cm
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Figure 7. The radar profile in Figure 5 interpreted to show the radar surfaces (bounding surfaces) identified by reflection geometry and four radar packages (A, B, C, and D).
Figure 8. A profile from location 2 showing the vertical resolution (10 cm) and the dipping angles within radar packages.
and a lateral extent of about 14 m. This level of resolution was observed in all packages except D, which is not well covered by our survey line. The measured thickness of package B was 1.5 m. Location 2 The profile analyzed at location 2 has a length of 25 m (Fig1450
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ure 8). Using the characteristics of the profile, we computed cross-strata dipping angles of 30–32°. This profile shows that we resolved sand beds with thickness of 10 cm which is in fair agreement with our targeted resolution. Again, dark layers dipping at angles similar to the current repose angle of this dune were observed in the trench (depth of 1 m) dug along a 2D profile. Most dark layers have a vertical spacing of
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Figure 9. Picture of a trench (depth of 1 m) along the profile in Figure 8 showing some dark-colored layers and cross-strata with thickness of 10 cm.
10 cm (Figure 9). Several other dark surfaces on the data are separated by more than 10 cm; however, most thin sand beds in the trench could not be resolved on the displayed profile because they are beneath our resolution range. The height of the slip face was computed to be 3 m using the measured repose angle and the length of the slip face. This also agrees with the value obtained using differential GPS measurements and the value obtained from the data after topographic correction. Figure 10 shows the correlation between the measured height of the dune above the exposed Tertiary limestone surface, and thickness of the GPR profile above the reflection generated by the limestone surface. The depth of penetration achieved by this survey decreased as we approached the slip face of the sand dune due to the proximity to the underlying limestone, which reflected most of the GPR wave. At the beginning of the profile, 25 m from the slip face, we could image up to 5.5 m. Also, a depth of about 3.2 m was achieved around the slip face of the dune; this is the depth under the limestone surface where diffractions were observed. These diffractions are still present in our data because we migrated using only the velocity measured for the sand dune. Four radar packages (A, B, C, and D) defined by three radar surfaces (bounding surfaces) can be identified on this profile (Figure 11). The radar surfaces were identified by toplap, downlap, and truncation reflection geometries. Package A is the oldest. Package D, the youngest, includes the current slip surface of the dune. Package A comprises low-to-medium amplitude reflectors and a dipping reflection pattern. The low-amplitude reflectors display a somewhat chaotic pattern of reflections with the dip angles ranging from 23 to 30°. These structures can be interpreted as cross-bedded aeolian strata. The cross-strata often have a lateral extent of less than
15 m, and the best vertical resolution of the strata is 10 cm. This package is bounded at the base by the underlying bedrock (super surface) and at the top by a second-order bounding surface. Package B consists of a series of high-amplitude continuous reflections displaying sigmoidal beds. The dip angle is 32° on the average, and the cross-strata have a lateral extent of 8–10 m. The best vertical resolution is also 10 cm. This package is bounded at the top and the base by a second-order bounding surface (radar surface) with thickness of about 2.5 m. It downlaps on the bedrock (super surface) and the reflections inside it can be categorized as third-order bounding surfaces. Package C, mainly medium-amplitude reflections, is very similar to package B except for its lower amplitude. Package D consists of high-amplitude dipping reflections with foresets dipping in the direction of wind transport at 32° repose angle. The best vertical resolution of the crossstrata is about 8 cm. The super surface (first-order bounding surface) indicated in the radar reflection profile (Figure 11) is the reflection generated by the underlying limestone surface which forms the flat interdune. The series of hyperbolas (Figure 11) are diffractions from beneath this limestone surface and could indicate karsting or remnants of previous mining activities before the dune migrated to its present location. We noticed some limestone mining activities around this dune at the time we collected the data. Conclusions Sand dunes have proven to be remarkably good radar targets, producing clear images of many bounding surfaces and other notable reflectors such as the bedrock. Radargrams (radar images of the subsurface) allow a unique, high-resolution insight into the stratigraphy and internal sedimentary structure of the dunes, consistent with growth under the prevailing winds in these regions. The method also produced clear images of slip surfaces which are related to the manner of dune growth and migration. The principles of radar stratigraphy were successfully used to interpret the acquired reflection profiles and define the bedforms within the studied dunes. Second-order and thirdorder radar surfaces (bounding surfaces) were identified in both dunes, and a super surface was identified at location 2. We were able to accurately map cross-strata thickness (8–10 cm), dip angles (30–32°), packages of cross-strata with thickness of 1.5–2.5 m, and lateral extent of 8–15 m. The radar packages were identified based on the character and geometry of reflections; they are interpreted to be products of primary sedimentary structures within these dunes. These packages can be used to formulate models of formation and migration of these dunes. They can also help in characterizing the architecture of hydrocarbon sandstone reservoirs of similar aeolian origin. Suggested reading. “Geology of the Arabian Peninsula” by Powers et al. (United States Geological Survey Professional Paper 560D, 1966). An Overview of the Fundamentals of Sequence November 2008
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Figure 10. Correlation of the reflection from the limestone surface at location 2 on the GPR profile and the picture taken on the field with a scale.
Figure 11. The radar profile from location 2 showing the radar surfaces (bounding surfaces) identified by reflection geometry, the super surface (reflection from the limestone surface), the hyperbolas generated by diffractions from the underlying limestone, and four radar packages (A, B, C, and D).
Stratigraphy and Key Definitions by Van Wagoner et al. (SEPM Special Publication, 1988). “Ground-penetrating radar for high resolution mapping of soil and rock stratigraphy” by Davis and Annan (Geophysical Prospecting, 1989). Ground Penetrating Radar: Application to Sandbody Geometry and Heterogeneity Studies by Gawthorpe et al. (Geological Society of London Special Publication, 1993). “Similarity analysis: A new tool to summarize seismic attributes information” by Michelena et al. (TLE, 1998). “Quaternary Evolution of Dawhat Zulum (Half Moon Bay) Region Eastern Province, Saudi Arabia” by Weijermars (GeoArabia, 1999). “Internal structure of mixed-sand-andgravel beach deposits revealed using ground-penetrating radar” by Neal et al. (Sedimentology, 2002). “Sequence stratigraphy of
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an eolian gas sand: Layering in the Permian Unayzah—A reservoir at south Ghawar, Eastern Saudi Arabia” by Melvin and Heine (Geo-2004 Abstracts). “Mapping the surface of a shallow groundwater system using GPR: A case study in eastern Saudi Arabia” by Al-Shuhail (TLE, 2006). Acknowledgments: The authors thank King Fahd University of Petroleum and Minerals in Dhahran, Saudi Arabia for its support. Help and advice of Dave Cantrell, Saudi Aramco, are gratefully acknowledged. Corresponding author:
[email protected] 