Addendum to Report High Resolution Seismic Reflection
Survey at Mud Mountain Dam near Enumclaw, Washington
Richard D. Miller
Joe M. Anderson
David R. Laflen
Brett C. Bennett
Choon B. Park
Summary
The shallow seismic reflection program at Mud Mountain Dam focuses
on defining and delineating stratigraphic and structural features within the
Pleistocene sequence that overlies igneous bedrock. The portion of the project
contained in the first report consisted of a series of acquisition/feasibility
tests followed by two CDP profiles along gravel logging roads. The goal of the
first CDP profiles was to extrapolate/extend sediments identified in sidehill
outcrop into the embayment to establish some understanding of horizontal continuity.
The second part of the seismic program (the portion reported here) included
a series of source tests and a 300+ shotpoint CDP profile along the main access
road to the Mud Mountain Dam overlook. This second survey line was designed
to allow extrapolation and enhancement of interpretations from the first set
of survey lines immediately north of the main dam road. Improvements in acquisition
and processing techniques and equipment on the second trip allowed a much improved
image of the subsurface and confidence in interpretation of the entire sedimentary
sequence beneath the main access road. Interpretations of the CDP profile collected
during March of 1996 are consistent with drill holes 38, 23, 24, and 34. The
bedrock reflection possesses almost 40 msec or about 100 ft of variation across
this profile. An apparent bedrock high is interpreted beneath and slightly east
of the logging road bridge. The configuration and topography of the bedrock
surface suggests borehole 23 must have a TD very near the bedrock surface. The
bedrock is deepest at the west end of the line beyond borehole 34. The Mud Mountain
Complex (MMC) is stratigraphically the next higher unit. Several reflection
events can be interpreted within this interval that are consistent with logs
from on-line boreholes which show discrete changes in material. Reflections
from the MMC and bedrock surface can be interpreted with a high degree of confidence
and are both very consistent with logs. The division between the MMC and Vashon
Lake Bed (VLB) sequences and Hayden Creek Till (HCT) is acoustically distinct
and can be confidently interpreted 2 everywhere except beneath the bridge where
the fold drops and the lithology appears to change dramatically. It is difficult
to determine with confidence whether the MMC possesses a relative elevation
high near the center of the line due to missing data, hence two possible interpretations
are provided. The interpretation suggesting a local thickening of the MMC near
the center of the line is consistent with off-line borehole data. Above the
MMC there appears to be a very complex set of erosional and depositional sequences
that are made up of VLB, HCT, and Vashon Outwash (VO). The most likely interpretation
of this sequence puts a complete relatively undisturbed section of bedrock,
MMC, VLB, and VO at borehole 34 with dramatic cut-and-fill features possessing
little or no distinct horizons above the deltaic layer. A very well-defined
erosional channel cut-and-fill within the VLB is interpreted at boreholes 38
and 23. The high amplitude nature of the reflection with the VLB is likely related
to saturation and possibly the pooling of water at that interface. Between boreholes
38 and 23 a mound of HCT can be interpreted which is consistent with borehole
24, previous seismic lines, and hillside outcrop maps. The distinct character,
reflection orientation, and shadowing effects of this area on deeper reflections
is strong evidence to support a distinct horizontal change in material within
this area. The borehole data was used to help identify the possible source of
this acoustically unique area within the context of the entire seismic profile
line. Preliminary processed sections possess distinct high angle apparent reflection
arrivals with apparent dip to the west that are likely indicators of the western
boundary of the HCT mound. The geologic interpretation of this HCT mound was
only possible with the inclusion of borehole logs and interpretations. Intermediate
processed seismic sections enhanced the identification of secondary channels
and approximate amounts of fill material redeposited into these apparent deep
cuts into the VLB sequences. Without digital filtering and spectral balancing
the channel feature between stations 1070 and 1110 is quite pronounced, possessing
well-defined sides and top. The unique acoustic characteristics of this feature
as displayed on the intermediate processed section is not consistent with the
final processing flow that was designed to enhance reflections from deeper more
consistent and competent layers. A much less well-defined channel is discernable
between stations 1220 and 1280 on this same intermediate processed section.
It is reasonable to suggest the strong reflection altered by these channels
is the top of the VLB sequence. The high amplitude reflection truncated by the
western edge of the western channel is consistent with the missing lower outwash
layer as interpreted in borehole 34. It is not unreasonable to suggest the section
as sampled by well 34 might be unaltered while the section encountered in boreholes
38 and 23 have the upper portion of the VLB altered by erosional fill.
Full Paper KGS-97-65.PDF 4.68MB
Orignal Mud Mountain Dam Abstract KGS-95-8.PDF 4.90MB
