Study on Sound-Speed Dispersion in A Sandy Sediment at Frequency Ranges of 0.5–3 kHz and 90–170 kHz

In order to study the properties of sound-speed dispersion in a sandy sediment, the sound speed was measured both at high frequency （90-170 kHz） and low frequency （0.5-3 kHz） in laboratory environments. At high frequency, a sampling measurement was conducted with boiled and uncooked sand samples collected from the bottom of a large water tank. The sound speed was directly obtained through transmission measurement using single source and single hydrophone. At low frequency, an in situ measurement was conducted in the water tank, where the sandy sediment had been homogeneously paved at the bottom for a long time. The sound speed was indirectly inverted according to the traveling time of signals received by three buried hydrophones in the sandy sediment and the geometry in experiment. The results show that the mean sound speed is approximate 1710-1713 m/s with a weak positive gradient in the sand sample after being boiled （as a method to eliminate bubbles as much as possible） at high frequency, which agrees well with the predictions of Biot theory, the effective density fluid model （EDFM） and Buckingham＇s theory. However, the sound speed in the uncooked sandy sediment obviously decreases （about 80%） both at high frequency and low frequency due to plenty of bubbles in existence. And the sound-speed dispersion performs a weak negative gradient at high frequency. Finally, a water-unsaturated Biot model is presented for trying to explain the decrease of sound speed in the sandy sediment with plenty of bubbles.

Grain-size distribution characteristics of red sandy sediments in Dongjiang River valley,southern Nanling Mountains,during the MIS2 stage

Layer LJ3 of Linjiang stratigraphic section in Dongjiang River valley in the south of the Nanling Mountains is a set of red sandy sediments.Measured by thermoluminescence(TL) dating,it was found to be formed in MIS2-9,500 ± 800 yr to 19,600 ± 1,800 yr B.P.After analy-sis of the grain sizes of the 16 samples(LJ3-100 to LJ3-85) in this layer,it was discovered that(1) The contents of each grain group in dif-ferent samples are similar.(2) The values of Md,Mz,,Sk,and Kg vary from LJ3-100 to LJ3-85 in a narrow range.(3) The segments of each sample in the accumulative curves extend parallel with similar slopes.All the three aspects reveal the Aeolian characteristics of Layer LJ3.Therefore,it is thought that Layer LJ3 consists of red sandy sediments formed in MIS2 in the south of Nanling Mountain,which reflects the arid climate at that time.

Origin and depositional model of deep-water lacustrine sandstone deposits in the 7th and 6th members of the Yanchang Formation(Late Triassic),Binchang area,Ordos Basin,China

Sandstones attributed to different lacustrine sediment gravity flows are present in the 7th and 6th members of the Yanchang Formation in the Ordos Basin, China. These differences in their origins led to different sandstone distributions which control the scale and connectivity of oil and gas reservoirs. Numerous cores and outcrops were analysed to understand the origins of these sandstones. The main origin of these sandstones was analysed by statistical methods, and well logging data were used to study their vertical and horizontal distributions. Results show that the sandstones in the study area accu- mulated via sandy debris flows, turbidity currents and slumping, and sandy debris flows predominate. The sand- stone associated with a single event is characteristically small in scale and exhibits poor lateral continuity. How- ever, as a result of multiple events that stacked gravity flow-related sandstones atop one another, sandstones are extensive overall, as illustrated in the cross section and isopach maps. Finally, a depositional model was developed in which sandy debris flows predominated and various other types of small-scale gravity flows occurred frequently, resulting in extensive deposition of sand bodies across a large area.