Land ice studies
Land ice studies Seismic studies on the Siple Coast, 1987-1988 C.R. BENTLEY, S. ANANDAKRISHNAN, and S.T. ROONEY
Geophysical and Polar Research Center University of Wisconsin Madison, Wisconsin 53706
The University of Wisconsin's field program in 1987-1988 was conducted entirely at, or out of, Downstream B camp, a location around which extensive survey work had been done previously (Bindschadler et al. 1987). Our program comprised active (explosive-charge-generated) and passive (natural-event) seismic studies, reported in this paper, and airborne and surface- based radar sounding and electrical resistivity profiling (Bentley, Blankenship, and Moline, Antarctic Journal, this issue). Active seismic. There were two principal goals of the activeseismic experiments: to discover whether an unconsolidated subglacial layer similar to that observed beneath Upstream B camp (Blankenship et al. 1987b; Rooney et al. 1987) exists beneath Downstream B and to test one prediction of the theory that the ice stream flows largely by deformation within this subglacial layer, namely that the deforming subglacial material is being deposited at the grounding line to form a "till delta" (Alley et al. 1987). Several different types of experiments were conducted. High-resolution reflection profiling was carried out along one 36-kilometer line parallel to ice flow and along four 3.6-kilometer lines transverse to ice flow (figure 1). Three-fold compressional-wave (P-wave) data were collected along all of the lines, and 3-fold shear-wave (S-wave) data were collected Seismic Lines at Downstream U flow--) presented here
aDNII
E 5 kro. Figure 1. Sketch of seismic reflection coverage at Downstream B in 1987-1988. Seismic reflection sections from the region circled are presented in figure 2, blocks a and b. (DNB denotes Downstream B. km denotes kilometer.)
along a short section of the longitudinal line. In addition, at each location where a transverse line crosses the longitudinal line, two high-resolution wide- angle experiments were performed, one along each line. All the wide-angle experiments included P waves and both vertically and horizontally polarized S waves. Finally, about 9 kilometers of 3-fold P-wave data using larger explosive charges for deeper penetration were collected. The high-resolution reflection data were collected in an attempt to image the till delta. These data are currently being processed; preliminary single-fold sections are available (Blankenship et al. in press) from one transverse line and a crossing section of the longitudinal line (figure 1). The contrast between flat-lying subglacial reflectors on the transverse line (figure 2, block a) and downstream-dipping reflectors on the longitudinal line (figure 2, block b) is what would be expected if those reflectors are foreset beds within a till delta. The high-resolution wide-angle experiments will allow us to determine the P- and S-wave velocities both in the ice and in the uppermost subglacial materials. Wave velocities in the ice are a measure of seismic anisotropy and, therefore, crystalline fabric, which reflects the strain history of the ice. Velocities in the subglacial sediments may reveal whether there is a deforming layer. The three-fold profiling with larger explosive charges indicates that Downstream B is underlain by more than 300 meters of gently deformed, probably Neogene, glacial-marine sediments (Rooney, unpublished data). Velocities within the sediments average 1.9 kilometers per second in the top 75 meters and 2.3 kilometers per second at depths between 75 and 300 meters. In addition to the reflection studies, two detailed seismic short-refraction surveys were carried out—one associated with the resistivity experiment, and one at the passive-seismic recording location. The layout and analysis were as reported earlier by Anandakrishnan et al. (1987). Wave velocities in the firn are needed to determine microearthquake fault-plane motions and locations from the passive seismic records and also yield a density-depth curve, which is important for the analysis of the resistivity data. Passive seismic. The passive seismic system (previously reported in Blankenship et al. 1987a) includes nine three-component seismometer stations ("remote units") whose outputs are relayed to a central processing station. A multi-channel event detector at the central station, newly implemented for the 1987-1988 field season, is triggered when the signal levels on selected channels exceed a specified threshold within a specified window of time. Upon receipt of the trigger, the recording system stores pre-trigger and post-trigger data from all nine stations. The multi-channel trigger ensures that all events recorded are of a magnitude sufficient to be seen at more than one station. The stations were deployed in a fan array, 7 kilometers wide and 6 kilometers deep, facing the grid southwestern margin ANTARCTIC JOURNAL
across
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Figure 2. A. Seismic reflection section along the transverse-to-flow line circle in figure 1. Our interpretation of the ice bottom, the bottom of the active(?) till layer, and one internal reflector within the active(?) till have been inked in. B. Seismic reflection section along the parallelto-flow line circled in figure 1. Our interpretations of the ice bottoms, the bottom of the active(?) till layer, and forset bedding planes have been inked in. (ms denotes milliseconds. km denotes kilometer.)
of the ice stream (the "Snake") (figure 3). A number of microearthquakes possibly associated with shear fracture or crevassing, most of them shallow, were detected during approximately 500 hours of operation. Some events were located within the boundaries of the seismic array; the remainder were between the array and the Snake, or within the Snake. We are currently determining source locations and source parameters of all events. This research was supported by National Science Foundation grant DPP 86-14011. This is contribution number 501a of the University of Wisconsin at Madison, Geophysical and Polar Research Center.
ft I
DOWNSTREAM B PASSIVE SEISMIC ARRAY •1
ICE FLOW
.7
•4 7k
References Alley, R.B., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1987. Till beneath ice stream B. 3. Till deformation: Evidence and implications. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(B9), 8921-8929. Anandakrishnan, S., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press. Density-depth profile determined by seismic refraction studies: Ice stream B, West Antarctica. Annals of Glaciology. Bindschadler, R.A., S.N. Stephenson, D.R. MacAyeal, and S. Shabtate. 1987. Ice dynamics at the mouth of ice stream B, Antarctica. Journal of Geophysical Research, 92(139), 8885-8894. Blankenship, D.D., S.T. Rooney, R.B. Alley, and C.R. Bentley. In press. Seismic evidence for a thin basal layer at a second location on ice stream B. Annals of Glaciolo gy . Blankenship, D.D., S. Anandakrishnan, J.L. Kempf, and C.R. Bentley. 1987a. Microearthquakes under and alongside ice stream B, detected by a new passive seismic array. Annals of Glaciology, 9, 30-34. Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1987b. Till beneath ice stream B. 1. Properties derived from seismic travel times. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(B9), 8903-8911. Rooney, ST., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1987. Till beneath ice stream B. 2. Structure and continuity. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8913-8920.
1988 REVIEW
HOME UNIT
• 5 k >1
9
E-
.6
:::^I>
SHEAR MARGIN CAMP
(25 km)
(IISNAKE") -1 km
[6 Remote unit] Figure 3. Layout of the passive seismic array. The numbered diamonds are the nine seismometer stations. (km denotes kilometers.)
55
Geophysical and Polar Research Center University of Wisconsin Madison, Wisconsin 53706
The University of Wisconsin's field program in 1987-1988 was conducted entirely at, or out of, Downstream B camp, a location around which extensive survey work had been done previously (Bindschadler et al. 1987). Our program comprised active (explosive-charge-generated) and passive (natural-event) seismic studies, reported in this paper, and airborne and surface- based radar sounding and electrical resistivity profiling (Bentley, Blankenship, and Moline, Antarctic Journal, this issue). Active seismic. There were two principal goals of the activeseismic experiments: to discover whether an unconsolidated subglacial layer similar to that observed beneath Upstream B camp (Blankenship et al. 1987b; Rooney et al. 1987) exists beneath Downstream B and to test one prediction of the theory that the ice stream flows largely by deformation within this subglacial layer, namely that the deforming subglacial material is being deposited at the grounding line to form a "till delta" (Alley et al. 1987). Several different types of experiments were conducted. High-resolution reflection profiling was carried out along one 36-kilometer line parallel to ice flow and along four 3.6-kilometer lines transverse to ice flow (figure 1). Three-fold compressional-wave (P-wave) data were collected along all of the lines, and 3-fold shear-wave (S-wave) data were collected Seismic Lines at Downstream U flow--) presented here
aDNII
E 5 kro. Figure 1. Sketch of seismic reflection coverage at Downstream B in 1987-1988. Seismic reflection sections from the region circled are presented in figure 2, blocks a and b. (DNB denotes Downstream B. km denotes kilometer.)
along a short section of the longitudinal line. In addition, at each location where a transverse line crosses the longitudinal line, two high-resolution wide- angle experiments were performed, one along each line. All the wide-angle experiments included P waves and both vertically and horizontally polarized S waves. Finally, about 9 kilometers of 3-fold P-wave data using larger explosive charges for deeper penetration were collected. The high-resolution reflection data were collected in an attempt to image the till delta. These data are currently being processed; preliminary single-fold sections are available (Blankenship et al. in press) from one transverse line and a crossing section of the longitudinal line (figure 1). The contrast between flat-lying subglacial reflectors on the transverse line (figure 2, block a) and downstream-dipping reflectors on the longitudinal line (figure 2, block b) is what would be expected if those reflectors are foreset beds within a till delta. The high-resolution wide-angle experiments will allow us to determine the P- and S-wave velocities both in the ice and in the uppermost subglacial materials. Wave velocities in the ice are a measure of seismic anisotropy and, therefore, crystalline fabric, which reflects the strain history of the ice. Velocities in the subglacial sediments may reveal whether there is a deforming layer. The three-fold profiling with larger explosive charges indicates that Downstream B is underlain by more than 300 meters of gently deformed, probably Neogene, glacial-marine sediments (Rooney, unpublished data). Velocities within the sediments average 1.9 kilometers per second in the top 75 meters and 2.3 kilometers per second at depths between 75 and 300 meters. In addition to the reflection studies, two detailed seismic short-refraction surveys were carried out—one associated with the resistivity experiment, and one at the passive-seismic recording location. The layout and analysis were as reported earlier by Anandakrishnan et al. (1987). Wave velocities in the firn are needed to determine microearthquake fault-plane motions and locations from the passive seismic records and also yield a density-depth curve, which is important for the analysis of the resistivity data. Passive seismic. The passive seismic system (previously reported in Blankenship et al. 1987a) includes nine three-component seismometer stations ("remote units") whose outputs are relayed to a central processing station. A multi-channel event detector at the central station, newly implemented for the 1987-1988 field season, is triggered when the signal levels on selected channels exceed a specified threshold within a specified window of time. Upon receipt of the trigger, the recording system stores pre-trigger and post-trigger data from all nine stations. The multi-channel trigger ensures that all events recorded are of a magnitude sufficient to be seen at more than one station. The stations were deployed in a fan array, 7 kilometers wide and 6 kilometers deep, facing the grid southwestern margin ANTARCTIC JOURNAL
across
flow
LI
fl ow )
I
I
I
lot)
1 ti 1- layer
hot
to,,
)h p pi op
IrI p
"f ______
) )
Wo I.
!
-II
rp
p)"*111 )
*P
s
16 "1 bt t PDOP rJ v.4
.I
r.)4hr
.8k,,,.
I .8 km.
Figure 2. A. Seismic reflection section along the transverse-to-flow line circle in figure 1. Our interpretation of the ice bottom, the bottom of the active(?) till layer, and one internal reflector within the active(?) till have been inked in. B. Seismic reflection section along the parallelto-flow line circled in figure 1. Our interpretations of the ice bottoms, the bottom of the active(?) till layer, and forset bedding planes have been inked in. (ms denotes milliseconds. km denotes kilometer.)
of the ice stream (the "Snake") (figure 3). A number of microearthquakes possibly associated with shear fracture or crevassing, most of them shallow, were detected during approximately 500 hours of operation. Some events were located within the boundaries of the seismic array; the remainder were between the array and the Snake, or within the Snake. We are currently determining source locations and source parameters of all events. This research was supported by National Science Foundation grant DPP 86-14011. This is contribution number 501a of the University of Wisconsin at Madison, Geophysical and Polar Research Center.
ft I
DOWNSTREAM B PASSIVE SEISMIC ARRAY •1
ICE FLOW
.7
•4 7k
References Alley, R.B., D.D. Blankenship, C.R. Bentley, and S.T. Rooney. 1987. Till beneath ice stream B. 3. Till deformation: Evidence and implications. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(B9), 8921-8929. Anandakrishnan, S., D.D. Blankenship, R.B. Alley, and C.R. Bentley. In press. Density-depth profile determined by seismic refraction studies: Ice stream B, West Antarctica. Annals of Glaciology. Bindschadler, R.A., S.N. Stephenson, D.R. MacAyeal, and S. Shabtate. 1987. Ice dynamics at the mouth of ice stream B, Antarctica. Journal of Geophysical Research, 92(139), 8885-8894. Blankenship, D.D., S.T. Rooney, R.B. Alley, and C.R. Bentley. In press. Seismic evidence for a thin basal layer at a second location on ice stream B. Annals of Glaciolo gy . Blankenship, D.D., S. Anandakrishnan, J.L. Kempf, and C.R. Bentley. 1987a. Microearthquakes under and alongside ice stream B, detected by a new passive seismic array. Annals of Glaciology, 9, 30-34. Blankenship, D.D., C.R. Bentley, S.T. Rooney, and R.B. Alley. 1987b. Till beneath ice stream B. 1. Properties derived from seismic travel times. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(B9), 8903-8911. Rooney, ST., D.D. Blankenship, R.B. Alley, and C.R. Bentley. 1987. Till beneath ice stream B. 2. Structure and continuity. Journal of Geophysical Research (Chapman Conference on Fast Glacier Flow issue), 92(139), 8913-8920.
1988 REVIEW
HOME UNIT
• 5 k >1
9
E-
.6
:::^I>
SHEAR MARGIN CAMP
(25 km)
(IISNAKE") -1 km
[6 Remote unit] Figure 3. Layout of the passive seismic array. The numbered diamonds are the nine seismometer stations. (km denotes kilometers.)
55
