Deltaic Sands GPR

Detailed lithofacies mapping of heterogeneous sand and gravel deposit using GPR

Fig. 1. Typical 400-MHz GPR profile penetrating to 5 m depth across a terraced mound in Taylor Valley, Antarctica. Yellow lines outline GPR facies identified from GPR events. See Fig. 5 for a map of this and crossing GPR transects.*

Fig. 2. Interpretation of GPR profile in Fig.1 showing GPR events and GPR facies. Legend in Fig 3. Facies explained in Fig. 4. Based on identical appearance, the GPR facies are interpreted to represent sedimentary facies. This is because contrasts in relative permittivity are caused by differences in grain size frequency distribution, grain shape, porosity, and mineralogy, all of which change with sedimentation process. These fundamental factors cause changes in water and ice content which represent a significant part of the interfacial permittivity contrast. The facies associations and sequences in this mound indicate that the mound is composed of deltaic sands over alluvial gravelly sand. *

Fig. 3.  Legend for GPR interpetations. Radar profiles show both reflection and diffraction horizons, and individual diffractions, all of which are referred to as events. Reflection horizons emanate from one or more continuous interfaces. Diffraction horizons emanate from discontinuous interfaces, such as an aligned series of boulders. Hyperbolically-shaped horizons indicate either point objects or linear features crossed obliquely. We draw reflection horizons on GPR profiles at the leading edge of the radar pulse.

GPR bounding surfaces bound GPR facies and GPR stratigraphic units. Bounding surfaces are drawn at GPR discontinuities that are determined from angular discordances between GPR reflection terminations. We use the conventional termination patterns of onlap and downlap, above the discontinuity, and toplap, truncation, and apparent truncation below the discontinuity.*

Fig. 4. GPR facies codes for stratified sediments consists of facies symbology, code letters, name, and interpretation. All aspects of GPR reflection patterns, that are related to sedimentary facies, are used to define GPR facies. GPR facies are named using the descriptive terms used in seismic stratigraphy and several other terms that describe patterns distinctive to GPR. A few GPR reflection patterns that are caused only by non-sedimentary phenomena (e.g., ringing, multiples, data processing, reflections from man-made objects) are not considered in GPR facies definition.

Interpretation of most reflection events in terms of sedimentary depositional environment depends on their 3-D geometry. Deposit external form and, in many cases, internal GPR events depend on orientation of the radar profile relative to depositional dip of the sediments. Therefore, it is preferable to determine 2-D GPR facies for a deposit along several transects in different directions. Intersecting profiles that permit evaluation and reconstruction in 3-D offer the best approach to correctly assign depositional facies. If GPR profiles in multiple directions for a deposit are available, 3-D GPR reflection facies can be assigned using names that follow the interpretation of the sedimentary depositional environment (e.g., deltaic, braided stream, or lacustrine GPR facies).

GPR commonly only images larger scales of strata in stratified sediments, such as a set of large-scale inclined strata (decimeters to meters). Therefore, it is vital to consider data collected at multiple frequencies. *

Fig. 5. Topographic map of mound with stratigraphy, GPR transects (black dots on blue lines), and contour lines in meters above sea level. GPR like Fig.1 indicate that mound is composed of deltaic sands several gravelly sand alluvial deposits.*

* Prentice, M. L., Arcone, S. A., Horsman, J. L., and Delaney, A. J., in prep, Stratigraphic architecture beneath terraces in Taylor Valley, Antarctica based on GPR: Implications for the paleolakes and Ross-Sea Ice-Sheet since the Last Glacial Maximum: Quaternary Science Reviews.