Chloride contamination in construction aggregates due to periodic saline water intrusion: a case study in the Kaluganga River Estuary, Sri Lanka

The Kaluganga River Estuary is one of the main sources of construction sand in Sri Lanka. Salt water intrusion along this estuary due to extensive sand mining has increased over the years. Thus, the focus of the current research is to understand the relationship between river sand mining, salt water intrusion, and the resultant effects on construction sand. Two surveys were conducted along the Kaluganga Estuary along an 11 km stretch from the river mouth at predetermined intervals to measure depth water quality profiles, and to collect sediment samples. These surveys were carried out during maximum spring tide; first in a dry period and then in a wet period, to understand hydrographic effects on the quality of river sands. Sand samples were analysed for absolute chloride content and grain size distribution. Results showed significant salt water intrusion during the dry period, averaging 2,307 μS cm^?1 in surface waters throughout the surveyed 11 km stretch along with 3,818 μS cm^?1 (average) in bottom waters up to 5.6 km upstream from the river mouth causing above normal chloride content in the bottom sandy sediments. The high chloride content in bottom sands was recorded up to 5.5 km from the river mouth making them unsuitable for construction purposes. However, during wet period, salt water intrusion levels in the bottom waters were insignificant (average 61 μS cm^?1) and the chloride content in bottom sediments was very low. This study highlighted the requirement for regulations on river estuary sandmining for construction purposes.     

Flushing of dense, hypoxic water from a cavity of the Swan River estuary, Western Australia

Flushing of dense water from cavities of the upper reaches of the Swan River estuary in Western Australia was investigated using measured salinity and dissolved oxygen profiles and a two-dimensional, laterally averaged hydrodynamic model (TISAT). Seasonal flushing of dense, hypoxic bottom waters from a relatively deep site took place over ∼3 days at the onset of winter in 1994. Model simulations of the purging of this dense water did not correspond closely with changes in the densimetric Froude number. Purging, expressed as depth of the halocline as a fraction of the total cavity depth, occurred when the simulated mean horizontal velocity at 2 m depth (top of cavity) changed from negative to strongly positive, indicating arrest of upstream flow and continuous downstream flow. This corresponded to freshwater discharge of about 50 m³ s⁻¹. Oxygen depletion of bottom waters was closely related to stratification. Oxygen dynamics at the onset of winter river flow was analysed using an exponential decay model, assuning that there was no net inflow or outflow across the halocline and thus no vertical transport of oxygen during a period of strong stratification. The rate constant for oxygen decay at Ron Courtney Island (RCI) was estimated to be 0.232 d⁻¹ for this period. Bottom waters at RCI declined to less than 1 mg 1⁻¹ prior to complete flushing through increased river flows. This study provided in sights to how freshwater flows may be allocated to maintain suitable oxygen levels in the bottom waters of estuarine cavities.     

基于泥沙中碳含量的变化表征黄河调水调沙入海泥沙的扩散范围

主要通过黄河口悬浮物中有机碳和无机碳的含量,表征黄河调水调沙入海的泥沙在黄河口外近海的扩散范围。结果表明:黄河口入海前泥沙中的颗粒无机碳(PIC),颗粒有机碳(POC)具有稳定性,分别为(1.75×0.28)%和(0.51×0.08)%;在悬浮物含量约为30 mg/L时,黄河口临近海区悬浮物中的PIC、POC含量分别出现突跃性降低和升高趋势,叶绿素分析表明,POC含量的升高是由于浮游植物对颗粒有机碳贡献的结果,同时正构烷烃轻重比值ΣC₂₀-/ΣC₂₀+和碳优势指数(CPI)值也分别增加和降低;X衍射发现悬浮物浓度低于30 mg/L时,黄河泥沙在悬浮物中所占比例迅速下降,从而造成悬浮物中PIC突跃性下降;因此,黄河口近海悬浮物含量30 mg/L可以被认为是陆源和海源颗粒物对黄河口近海区悬浮物贡献的有效分界线。调水调沙入海的泥沙在黄河口外近海表层主要扩散到河口以南海域,而底层扩散范围主要集中在河口以北和河口以南海域,这些泥沙最终可能停留在莱州湾中部。     

Quantitative dam break analysis on a reservoir earth dam

Mathematical simulations on dam break or failure using BOSS DAMBRK hydrodynamic flood routing dam break model were carried out to determine the extent of flooding downstream, flood travel times, flood water velocities and impacts on downstream affected residences, properties and environmental sensitive areas due to floodwaters released by failure of the dam structure. Computer simulations for one of the worse case scenarios on dam failure using BOSS DAMBRK software accounted for dam failure, storage effects, floodplains, over bank flow and flood wave attenuation. The simulated results reviewed a maximum flow velocity of 2.40 m/s with a discharge of approximately 242 mз /s occurred at 1.00 km downstream. The maximum discharge increased from 244 m³/s (flow velocity = 1.74 m/s occurred at 8^th. km) to 263 m³/s (flow velocity = 1.37 m/s occurred at 12^th. km); about a 39% drop in flow velocity over a distance of 4.00 km downstream. If the entire dam gives way instantly, some spots stretching from 0.00 km (at dam site) to approximately 3.40 km downstream of the dam may be categorized as “danger zone”, while downstream hazard and economic loss beyond 3.40 km downstream can be classified as “low” or “minimal” zones.     

Results of Scripps long range profiles in the Pacific

During 1976 the first installment of a long range seismic profile was conducted in the North Pacific to a range of 600 km using shots to two tons in size. The line was shot to a closely-spaced array of Scripps ocean bottom seismographs and was parallel to magnetic anomaly 32 at an age of approximately 70 · 10⁶ yr. The line extended between the Clarion and Molokai Fracture Zones and did not cross any major topographic features. Linearized and extremal travel-time inversions were conducted to provide bounds on the compressional velocity as a function of depth. The velocity does not exceed 8.4 km s⁻¹ to a depth of 60 km at which point the data no longer provide any resolution. The constraints on the acceptable models were improved by using array processing methods to measure phase velocity and synthetic seismogram techniques to model phase and amplitude information. The oceanic crust is composed of a series of gradients with no first order discontinuities. The “Moho” is smeared out over a depth of 1.5–2.0 km even though “wide-angle reflections” from the Moho, the phase P_MP, are clearly seen in the data. The upper lithosphere is characterized by a general tendency for the velocity to decrease with depth and the tendency is occasionally overwhelmed (at about 27 and 52 km depth) by rapid velocity changes perhaps associated with phase or compositional changes.     

Synopsis of strontium isotope variations in groundwater at Äspö, southern Sweden

Strontium isotope ratios are used to identify end-member ground-water compositions at Äspö in southeastern Sweden where the Hard Rock Laboratory (HRL) has been constructed to evaluate the suitability of crystalline rock for the geologic disposal of nuclear waste. The Hard Rock Laboratory is a decline (tunnel) constructed in 1.8 Ga-old granitic rock that forms islands in an archipelago along the Swedish coast. Ground-water samples were obtained for isotopic analyses from boreholes drilled from the surface and from side boreholes drilled within the HRL. Infiltration at Äspö occurs primarily through fractures zones in the granitic bedrock beneath thin soils throughout the area. Because of extremely low Sr concentrations, rain and snow are not important contributors to the Sr isotope budget of the ground-water system. At shallow levels, water percolating downward along fractures and fracture zones acquires a δ⁸⁷Sr between +9.5 and +10.0‰ and maintains this value downward while Sr concentrations increase by two orders of magnitude. Ground-water samples from both boreholes and from in the HRL show the effects of mixing with saline waters containing as much as 59 mg/L Sr and δ⁸⁷Sr values as large as +13.9‰. Baltic Sea water is a potential component of the groundwater system with δ⁸⁷Sr values only slightly larger than modern marine values (+0.3‰) but with much lower concentrations (1.5 mg/L) than ocean water (8 mg/L). However, because of large Sr concentration differences between the saline groundwater (59 mg/L) and Baltic Sea water (1.5 mg/L), δ⁸⁷Sr values are not particularly sensitive indicators of sea-water intrusion even though their δ⁸⁷Sr values differ substantially.     

Toxic metal contamination in the lateral lakes of the Coeur d'Alene River valley, Idaho

 The 11 lateral lakes of Coeur d'Alene River valley in northern Idaho have received heavy metal contamination from over a century of upstream mining. The lateral lakes lie within the flood plain of the Coeur d'Alene River, and in their bottom sediments is preserved a stratigraphic record of the upstream mining operations. To characterize the contaminated sediments in the lateral lakes, sampling techniques, including the Livingston piston corer and the Huttenen freeze box, have been developed by Quaternary geologists to preserve the vertical stratigraphy in the samples. From 26 cm to over 55 cm of undisturbed tailing sediments, commonly with “varve-like” features, have been found in each of the lateral lakes, with maximum concentrations by weight of lead at 3.8%, zinc at 3.4%, arsenic at 340 mg/kg, cadmium at 120 mg/kg and mercury at 7 mg/kg. The contamination in the lakes appears to be restricted to the shallow subsurface and heavy metal concentrations generally drop to background levels within a meter of depth. Received: 22 May 1998 · Accepted: 21 September 1998     

Land-based sources of pollution along the Izmit Bay and their effect on the coastal waters

The degree and the contribution of each point source to the pollution were determined in the Izmit Bay during the period 1999–2000. During 8 campaigns, samples from 11 points in the channels and water samples from 5 points in the coastal sea were collected for chemical analysis. The important pollutant parameters taken into account are inflow of total organic carbon (TOC), total suspended solids (TSS), total phosphorus (TP), total nitrogen (TN), nitrate, ortho-phosphate, ammonia and total polycyclic aromatic hydrocarbons (t-PAHs) in the discharge channels, and TOC, TSS, nitrate, ortho-phosphate, chlorophyll-a, temperature, dissolved oxygen (DO), and salinity in the coastal stations of the Bay. It should be pointed out that the industrial wastewaters entering the bay are partially treated but domestic wastes are discharged directly into the surface waters without any treatment. Of the pollution parameters measured in the channels, the highest concentrations, except TP, were observed in the Dil River and in the Eastern Channel. Concentrations of TOC, TSS, TN, TP, ammonia, nitrate and o-phosphate were found at concentrations of 231, 290, 152, 3.8, 16, 79, and 3.07 mg/L, respectively. Annual inflows of TOC were 21,301, 580, and 775 t/year and for TSS were 26,742, 585, and 1505 t/year in the western, central and eastern parts, respectively. The results show that the water quality of the bay has been deteriorated and 80% of the pollution was caused by Dil River for all parameters measured.     

Long-term improvements in water quality due to sewage abatement in the lower Hudson River

Long-trend trends in dissolved oxygen (DO) and total coliform bacteria concentrations are used to evaluate the impact of almost 60 yr of sewage abatement and treatment in the lower Hudson River near New York City. Although some water pollution control plants have been in operation in the region since the 1930s, the most significant abatement of untreated sewage in the lower Hudson River has occurred since the late 1970s, when most of the existing plants were upgraded to secondary treatment, and additional plants were constructed. From at least 1922 through the early 1960s, average summer DO percent saturation varied between 35% and 50% in surface waters and 25% and 40% in bottom waters. Beginning in the late 1970s, DO concentrations generally increased through the 1980s and especially into the 1990s, coinciding with the upgrading of the 7.4 m³ s^?1 (170 million gallons per day, mgd) North River plant to secondary treatment in the spring of 1991. Average summer percent saturation in the early 1990s exceeded 80% in surface waters and 60% in bottom waters. In addition, summer DO minima have increased from less than 1.5 mg 1^?1 in the early 1970s to greater than 3.0 mg 1^?1 in the 1990s, and the duration of hypoxia during summer months has been reduced. Total coliforms also display strong declining trends from the 1970s into the 1990s, with declines attributed to plant upgrades, construction of two New York City plants (North River and Red Hook) in the mid 1980s, and improved operation of the sewer system.     

The salinity of Djibouti''s aquifer

Fresh water supplied to Djibouti town is essentially groundwater located inthe fractured Gulf and Somali basalt aquifers. About 30 wells (3 to 6 km inland from the sea) are exploited and provide water of rather poor quality (TDS between 1000 mg 1⁻¹ and 2800 mg 1⁻¹). A sea water interface has been recognized locally some 3.8 km from the sea at 35 m below sea level. However, a well at Hidka Gisiyed, some 11 km from the sea, also contains water with high salinity (TDS=14,000 mg 1⁻¹) at depth. The over-exploitation of the aquifer and the high pumping rate are contributing to an increase in the salinity due to the intrusion of sea water, as shown by the chemical results. The Hidka Gisiyed saline water is another possible source of saline water at depth.The isotopic results from part of the Djibouti aquifer have shown that current recharge from local rain or surface runoff is occurring. An understanding of factors influencing the evolution of the salinity will allow the better management of the aquifer.     

Occurrence and distribution of fluoride in the groundwater of the Tamiraparani River basin,South India: a geostatistical modeling approach

Fluoride in drinking water has both beneficial and detrimental effects on public health, and a narrow range between .6 and 1.5 mg/L is optimal for consumption. However, natural groundwater sources exceed these guidelines affecting the entire population. This study aims to assess the distribution and controlling factors of fluoride concentration in the Tamiraparani River basin, South India. A total of 124 groundwater samples were analyzed for their fluoride content and other hydrogeochemical parameters. The fluoride concentration in the study area varied from .01 to 1.67 mg/L, and the highest concentrations were measured in the northern and central parts of the study area, which is underlain by charnockites and hornblende biotite gneiss. The sampling indicated (as per the Bureau of Indian Standards) that 53.9% of the area has fluoride concentrations below levels that are protective of teeth from dental caries (<.6 mg/L). .1% of the area is considered to be at risk of dental fluorosis, and the remaining 46% of the area is considered to have fluoride levels at desirable to permissible limit in groundwater. The groundwater in the study area belongs to Ca–Mg–Cl–SO₄ and Ca–Mg–HCO₃ types. A positive correlation between fluoride and TDS, Na⁺, K⁺ and HCO₃ ^? indicates its geogenic origin, and positive loading between pH and fluoride shows that alkaline environment enhances the dissolution of fluoride-bearing minerals into the groundwater. An empirical Bayesian kriging model was applied to interpolate the fluoride concentration in the study area. This geostatistical model is found to be better than other kriging methods, and it yielded an average standard error of .332 and root-mean-square standardized value of .986.     

Historical deposition and fluxes of mercury in Narraguinnep Reservoir,southwestern Colorado,USA

Narraguinnep Reservoir has been identified as containing fish with elevated Hg concentrations and has been posted with an advisory recommending against consumption of fish. There are presently no point sources of significant Hg contamination to this reservoir or its supply waters. To evaluate potential historical Hg sources and deposition of Hg to Narraguinnep Reservoir, the authors measured Hg concentrations in sediment cores collected from this reservoir. The cores were dated by the ¹³⁷Cs method and these dates were further refined by relating water supply basin hydrological records with core sedimentology. Rates of historical Hg flux were calculated (ng/cm²/a) based on the Hg concentrations in the cores, sediment bulk densities, and sedimentation rates. The flux of Hg found in Narraguinnep Reservoir increased by approximately a factor of 2 after about 1970. The 3 most likely sources of Hg to Narraguinnep Reservoir are surrounding bedrocks, upstream inactive Au–Ag mines, and several coal-fired electric power plants in the Four Corners region. Patterns of Hg flux do not support dominant Hg derivation from surrounding bedrocks or upstream mining sources. There are 14 coal-fired power plants within 320 km of Narraguinnep Reservoir that produce over 80 × 10⁶ MWH of power and about 1640 kg-Hg/a are released through stack emissions, contributing significant Hg to the surrounding environment. Two of the largest power plants, located within 80 km of the reservoir, emit about 950 kg-Hg/a. Spatial and temporal patterns of Hg fluxes for sediment cores collected from Narraguinnep Reservoir suggest that the most likely source of Hg to this reservoir is from atmospheric emissions from the coal-fired electric power plants, the largest of which began operation in this region in the late-1960s and early 1970s.     

Heavy metals removal from synthetic wastewater by a novel nano-size composite adsorbent

The effects of varying operating conditions on metals removal from aqueous solution using a novel nano-size composite adsorbent are reported in this paper. Characterization of the composite adsorbent material showed successful production of carbon nanotubes on granular activated carbon using 1 % nickel as catalyst. In the laboratory adsorption experiment, initial mixed metals concentration of 2.0 mg/L Cu²⁺, 1.5 mg/L Pb²⁺ and 0.8 mg/L Ni²⁺ were synthesized based on metals concentration from samples collected from a semiconductor industry effluent. The effects of operation conditions on metals removal using composite adsorbent were investigated. Experimental conditions resulting in optimal metals adsorption were observed at pH 5, 1 g/L dosage and 60 min contact time. It was noted that the percentage of metals removal at the equilibrium condition varied for each metal, with lead recording 99 %, copper 61 % and nickel 20 %, giving metal affinity trend of Pb²⁺ > Cu²⁺ > Ni²⁺ on the adsorbent. Langmuir’s adsorption isotherm model gave a higher R² value of 0.93, 0.89 and 0.986 for copper, nickel and lead, respectively, over that of Freundlich model during the adsorption process of the three metals in matrix solution.     

秋季黄河pCO2控制因素及水-气界面通量

根据2006年11月1~10日,秋季黄河平水期二氧化碳分压(pCO₂)的现场实测数据及相关同步观测资料,对黄河表层水pCO₂的分布及其影响因素进行了研究。结果表明:水体pCO₂在80~166Pa,平均值110Pa,在世界主要河流中属中等偏下水平;空间分布存在较大的不均匀性,中游高于上游和下游。浮游植物的光合作用对pCO₂有一定的影响但强度较弱,即使在叶绿素最高值3.58μg/L的包头站pCO₂仍达到91Pa。黄河水体有机物含量较低且继承了陆源有机物难降解的特性,干流和库区EpCO₂/AOU的比值为0.14和0.20,远低于生物好氧呼吸作用控制水体pCO₂的理论下限0.62,因此,生物好氧呼吸作用对水体pCO₂的贡献不大。悬浮物(TSS)含量为3.77~1308mg/L,溶解无机碳(DIC)含量为3.03~4.14mmol/L,普遍高于世界其它河流且最大值均出现在潼关站;同时水体pCO₂与TSS、PIC、DIC含量具有极好的正相关性。因此黄河流域强烈的机械侵蚀和化学风化作用形成的碳酸盐体系是控制水体pCO₂的主要因素。利用Wanninkhof提出的淡水水-气交换系数的通量模式估算,黄河水域水-气界面CO₂交换速率约为0.229μmol/m2·s,秋季可向大气释放CO₂14.5亿moL,相当于8250km2草原或是112km2森林一年的固碳量。黄河CO₂释放通量与渥太华河相近,但要远小于亚马逊河。     

Traveling screens as sampling gear for vertical distribution studies

A sampling technique has been developed for increasing the information gathered during routine monitoring of impingement at water intake structures. Samples from impingement catches on traveling screens were taken from the sluiceway of the Brunswick Steam Electric Plant near Southport, North Carolina during the screen wash process so as to divide the catch into vertical catch components. Each component represented 1.2 m of the water column. Results showed differences in day and night vertical distributions of impinged organisms according to the spatial preferences of each species. Impingement during daytime was lower than during night. Impingement of surface-oriented species during daytime occurred at mid-depth, midwater species near the bottom, and bottom species were seldom impinged. During the night surface-oriented species were impinged at the surface, midwater species at mid-depth, and bottom species near the bottom. Residue (animals and debris) which remained within the screen wash system from collection of the previous sample, and those organisms which became impinged on the screens during retrieval of the sample, were used to calculate the rate of “continuous impingement” on the traveling screens. These rates were compared to the impingement catch in order to determine biases in the vertical catch components. Comparison of these rates indicated that impingement estimates determined by routine monitoring methods may under-estimate true impingement rates for certain species. We have concluded that this sampling technique for monitoring impingement at intake structures will increase knowledge of the local biologic system while minimizing the cost of obtaining the information. The technique will also aid in pinpointing specific impingement problems which may be corrected through modifications to the design of the intake structure.     

Secondary turbidity maximum in a partially mixed microtidal estuary

Data from a two-year period of monthly slackwater surveys reveal that in addition to the classical estuary turbidity maximum (ETM), another peak of bottom total suspended sediment (TSS) concentration, or a so-called secondary turbidity maximum (STM), often exists in the middle part of the York River estuary, Virginia. This STM, observed in most (but not all) of the slackwater surveys, moves back and forth in the region of about 20 to 40 km from the York River mouth where the mud percentage of bottom sediment is very high. The distribution of the potential energy anomaly, which was calculated using salinity data, indicates that the STM usually resides in the transition zone between the upstream well mixed and the downstream more stratified water columns. An analysis using the conservation equation of suspended sediment concentration in the water column reveals that four processes may contribute to the formation of the STM: convergence of bottom residual flow, tidal asymmetry, inhibition of turbulent diffusion by stratification, and bottom resuspension. The along-channel variations of the strength of bottom residual flow, the effect of tidal asymmetry, and the stratification patterns are probably due to the geometric features of the York River estuary.     

Determination of design criteria for UASB reactors as a wastewater pretreatment system in tropical small communities

A pilot scale study was set up to investigate the principle design parameters of up flow anaerobic sludge blanket (UASB) reactors for treating wastewater of small communities in the tropical regions of Iran. A steel pipe with a diameter of 600 mm and a height of 3.6 m was used as the reactor in which a digestion and a 3-phase separator element had a volume of 0.848 and 0.17 m³ respectively. During this study, which lasted for 203 days, two distinct phases were carried out according to the ambient temperature. The temperature of the wastewater entering the reactor was naturally ranged from 22 to 26 °C and no heat exchanger was used. The hydraulic retention times including 2, 4, 6, 8, and 10 hours with various loading rates of 0.95 to 5.70 kg COD/m³/day for colder period and from 1.35 to 6.40 kg COD/m³/day for warmer period were examined. On the basis of the results the optimal hydraulic retention time for warmer period with a 2.20 kg COD/m³/day organic loading rate was 6 hours which BOD5, COD and TSS removal efficiency were 71, 63 and 65 percent respectively. During the colder period the removal ratio of BOD5, COD and TSS with an optimal hydraulic retention time of 8 hours and organic loading rate of 1.22 kg COD/m³/day were 54, 46 and 53 percent respectively.     

Sulfate transport in a Coastal Plain confining unit, New Jersey, USA

 A transient 1-D, two-pathway non-equilibrium deterministic advective dispersion model was used to examine the distribution of chloride (43–100 mg/L) and sulfate (57–894 mg/L) concentrations in the 35-m-thick section of the Lower confining unit, Atlantic Coastal Plain, New Jersey, USA. The model was used to constrain hypotheses about how pore-water chemistry changed over time. Explanations of the solute concentrations were explored by inverse and direct methods given a few known constraints, including concentrations of pore-water constituents from 12 core samples, reported simulated flow rates, and estimated hydrogeologic properties. The hypothesis that is best supported by the model results is that the distribution of chloride and sulfate concentrations in the confining unit reflect the history of the aquifer system since it was filled with seawater at the last eustatic high, about 84×10³yr BP. The model simulates fresh-water flushing of the seawater-permeated silts at a steady upward pore-water flow velocity of 8.8×10^–6 m/d, with a dispersion coefficient of 9.2×10^–7 m²/d, a dimensionless partition expression for chloride, β_Cl=0.981, and a dimensionless exchange coefficient, ω_Cl=0.31×10^–2. Sulfate concentrations were simulated over the flow path using flow and dispersion values calculated for chloride transport plus a retardation term. Parameters for sulfate transport include retardation coefficient=4.51, β_SO4=0.994, and ω_SO4=0.31×10^–2. Sensitivity analysis indicates that the model is most sensitive to flow velocity, and that fresh-water flushing of the confining unit is best simulated by having seawater concentration levels at the inflow boundary of the confining unit exponentially decrease with a concentration half-life rate of 825 yr. Received, January 1997 / Revised, April 1998, October 1998, January 1999 / Accepted, January 1999     

Crustal structure of the western Arafura Sea from ocean bottom seismograph data

Refraction data taken from ocean bottom seismograph recordings in the western Arafura Sea indicate a continental‐type structure for the region. This structure is characterised by a thin column (2 km) of sediments, with velocities ranging from about to 2 to 4 km s^‐1, overlying an essentially two layer crust. The compressional wave velocities in the upper and lower crust are 5.97 and 6.52 km s^‐1, respectively, with the boundary between the layers at a depth of 11 km. Very weak mantle‐refracted arrivals with a velocity of about 8.0 km s^‐1 were recorded. Large‐amplitude, later arrivals, beginning at distances near 100 and 150 km, have been interpreted to be part of the retrograde branches from the 8.0 and 7.33 km s^‐1 layers, respectively. Model studies indicate that a small positive velocity gradient is required between 17 and 30 km, and that the Moho is at a depth of 34 km. A third set of large amplitude, later arrivals starting at a distance near 250 km has been interpreted as most probably multiple refraction‐reflection arrivals from the 5.97 and 6.52 km s^‐1 layers. Correlation of this structure with the stratigraphic logs from exploratory oil wells in the Arafura Sea using layer velocities indicates that rocks younger than Jurassic appear to thin towards the east.