Interpretation and description of seismic data from Lower Cook Inlet, Alaska [unknown]

Michael A. Fisher
1976 Antarctica A Keystone in a Changing World   unpublished
72 groups of 20 aceclcroltion-cancelling hydrophones f n an ovcrlnpprrd tapered array. Each group I s 67 metres (220 ft.) lung and overlapped t o provide group centers every 33.5 metres (110 ft,). The dead section or lead in was about 305 metres (1000 ft.); the s i g n a l -d e t e c t i n g aection was 2381 metres (7810 ft .) , y i e l d i n g a distance from t h e center of the source array t o the fartheat o f f s e t group of abnut 2654 metres (8700 ft.) . Eight depth controllers and six
more » ... th transducers were used t o maintain the streamer at d nomfnal depth af 1 2 metres (40 f t . ) . Cable d r i f t was t o be monitored by an ADF transponder pasitkoned on the streamer t a i l bouy; 0 in practice, the ADF s i g n a l was blocked by equipment on the shlp unless the t a l l bouy was more than five to ten degrees off line. The data was recorded usfng DDS-888 d l g f t a l instruments which provide an instataneous floating-point, 98-channel recording c a p a b i l i t y . I S i x to seven seconds of data were recorded on I J Z inch magnetic tapes in S.E.G. "C" format (IBM short-precision floating-point format) a t a sample rate of t w o milliseconds. A 1600 bpi tape recording density was used. The seismic groups were recorded on tape I n the order shown in the folfowfng table, Hain Cable Group 1 36 37 72 ' Mini Cable Group 1 1 2 1 12 Time Break 100HZ s5gnal System Channel 1 36 49 ' 84 * Navigation was accomplished u s h g a combination b y d i s t -RPS ..positioning system operated by Navigation Service, I n c . of Ventura, California, An ea timated positional accuracy of about 15 metres (50 f t . ) was assured. In additfon t o the data from the 2700-metre streamer, seismic fnformation was also recorded from a 300-metre cable with twelve hydrophodes. This mini-cable w a s designed to provide a 3-fold stack Kith more energy in higher frequencies than the long cable; reflection8 with an 80 HZ content were recorded. Better .resolution of the ref lectors in the first 1.5 seconds of travel t h e was the objective of t h i s additfonal data. A s a~p l e rate of 1 millisecond was achieved by sampflng the mini cable twice during each multiplex sweep. In practice the low-fold of coverage, small distance t o the farthest offset hydtophone and the processing techniques left t h i s data swamped by shot noise and m u l t i p l e s and generally useless. The aeismic data were processed by Petty-Ray Geophysfcaf, Inc. of Houston, Texaa. Petty-Ray produced final-stack secttons of a quality equal to the quality of other seismic data from this area. Most problems encountered during the processtng stemmed from strong water bottom L and "peg leg" multiples that Interfered a t h accurate velocf t y analysts. Uncharacteristically fox lower Cook Tnlet the weather was superb for conductfng sefsmfc f i e l d operations. Instead of weather problems, sources of delays were crabpata, tidal-current-borne debrfa that fouled the depth controllers on the streamer, and other seismic boats operating in the area. T i d a l currents caused considerable delay in shootfng, especially when shootfng transverse l i n e s , because operations were t e d n a t s d when the angle between the ships heading m d the preplotted & l i n e direction became too large. Tha overall data qualfty I s fair t o good: h the northern part of the surveyed area ringing of lnd%vidual a v t n t~ is the dominant problem; in the southern part, longer path "peg Leg" and water b o t r m multiples are especially severe. Odd diffracted events with a rourca that appears close t o the surface occur frequently in the northeast quarter of lower Cook f n l e t . Theee events may originate from glacial erratice or irregularittee in water bottom topography. Availability The release of film and paper copies of the final sections and shot point map 'Se baing handled by: National Geophysical and Solar-Tsrrastial Data Center, Code D62, ~S I N O A A , Boulder ,CO, 80302~. Tclephone'303-499-1000 E x t . 6542. This office wfll have information about ordering and reproduction costs. Obsexver'~ logs for t h l s susvey are _ appended to thfs report. Preliminary Interpretation of the Data w There are both economic and scienriffc incentives for exploring lower Cook fnlet. The possibility of duplfcating the petroleum reserves of upper Cook Inlet and the fever technological obstacles t o petrol-I eum resource development, relative to the Gulf of Alaska, make the ares seonodcally interesting. The setting of lower Cook Inlet Invites active scientific investigation of the geologic history of t h i s area. A preliminary interpretation of the ssiamic data follows; stratigraphic ties with the geology rimming the' 1nlet must await fur thex work. A a i l~p l i f i e d schematic representation of the major reflecting horfeona observed in the seismic data is shown fn a northwest-southeapr cross sect5on compiled from the data north of Cape Douglas (fig. 4). Strata above the A-horizon onfap to the north and nokthwest. Thcre i a good seismic evidence that the A-horizon s a p resents an erosional Figure 4. Siarplified relations between mefor horizons in a northwest-southeast cross section. The A-and B-hoxizon~ may be returned by erosional surfaces; t h a Crhorizon forma the acovatic basement. surface: it appears to truncate deeper reflectors, and angular discordance bf reflectore is evident across the A-horizon ( fig. 4) . , The B-horizon apparently truncates deeper reflectors so it too may be an e r o s i o h l surface. The acoustic basement, labeled C-horizon in figure 4, allows no information t o return from deeper reflectors, so the geologic nature of that surface and underlying rock unita remains unknown. The strata j u s t above the C-horizon appear t o l a p southeastward onto northeas t-sloping topography f omed by the C-horizon. Prelimfnary ties to on~hore geology yield the following tentatfve age assignments: the A-herfzen might be near the base of the younger Tertiarg; the 3-horizon could be basal ~e r t k a r~ or a Cretaceous s r o s i o n a~ aurface; and t h e C-horizon might be from a Jurassic or older interface. Further study is necessary to date the offshore stratigraphy adequately. Generalized contour maps of two-way seismic traveltime have been produced for the shallow A-horizon (f f g , 5 ) and the deep C-horizon ( f i g . 6 ) . These map^ d s o show axes of anticlfnes interpreted from t h e data, The water-transit time {between .04 and .25 aeconds) has not been removed from the traveltimes, so the contours ate referenced t o sea level. Seismic data above the A-horizon f ndicate that the reflectors onlap northwestward to the outcrop of the A-horizon a t the bottom of the I n l e t waters. The outcrop line ( f i g . 5 ) fellows the water bottom, therefore it is not an isochron. Near that outcrop l i n e t h e reflectors above the A-harizon onlap an anticlinal structure. Further to the southeast, near line 757, the same reflectors are involved in a n t i c l i n a l folding. Time transgression m y thus be indicated for the deposition of the reflectors or the formation of the atructures or both. There are t o distivgufsh the order of occurrence, but the folds seem t o have formed sometime during thc'buriel of the A-horizon. The snticlinal folds mentioned above appear t o r trike t o t h e northeast wherever determination of the trend can be based on two s e l s d c l i n e s . The nor theas t-trending s truc ttaral graf n cbnf oms t o the s trlkea of structural axes reported from upper Cook Inlet by Kirschner and Lyon 1 (1973), from the Iniskin Peninsula by Petterman and Hartsock (19661, and from the Kamishak H i l l s by L. B. &goon and W. 2 , . Adkison (oral cornnitation). Consequently, where no control Lxists f o r the trend of structures interpreted from the sefsmlc data, a northeaec trend has been assumed. Some antfclines are bseeched on one flank by h-igh-angle reverse faults. Along line 752 these faults are mostly confined t o the flank that faces the deeper past of the basin. Similar faulted anticlines in upper Cook Inlet: are described by Kfrschner and Lyan (1973). Thus upper Cook I n l e t map be useful as a structural analog for lower Cook I n l e t . The wavelengths of the folds appear to average about 8 to 12 kflometxes. Line 752 shows that t h~ wavelengths of the anticlines appear t o Increase aa the isediment in the basin thickens (rtlatfvc pediment thickness is assumad to bc roughly indicated by the t i m e t o * < Al;:;&-,.bL>& *
doi:10.3133/ofr76561 fatcat:c4kisg53s5ha5irwokbsqvvvhe