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Studies on the morpho taxonomic variations of four Centropagidae species (Crustacea: Calanoida) with note on their abundance collected from Sonmiani Bay, northern Arabian seaPakistan Journal of Marine SciencesCentropagidae is comparatively a larger family to other families of order calanoida. The quantitative data revealed that the copepods of the genus Centropages are the dominant component in the zooplankton samples from the designated station in Sonmiani bay, a coastal lagoon at Balochistan coast, northern Arabian sea. Four epipelagic species of family Centropagidae, Centropages (Centropages dorsispinatus Thompson I.C. & Scott A., 1903, Centropages tenuiremis Thompson I.C. & Scott A., 1903, Centropages furcatus (Dana, 1849) and Centropages orsinii Giesbrecht, 1889 ) were identified. Male and female specimens were distinguished on their characteristic features of antennules, thoracic segments and fifth legs. Identified specimens were described briefly with their microscopic illustrations. Centropages dorsispinatus and Centropages tenuiremis were frequently found in zooplankton samples throughout the sampling season.
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Morphometric relationship of Perna viridis Linnaeus, (1758) from the Manora Channel, coastal waters of PakistanPakistan Journal of Marine SciencesPerna viridis (Green Mussels) is characterized by fast-growing, found in large clusters, one of the dominant bio-fouling organisms, native to West Indo Pacific region and commonly distributed along the Southeast Indian and Asian coast. They are found in the shallow sublittoral water up to a depth of 15 m and be able to tolerate an extensive range of environmental situation i.e. tolerate temperatures range of 15-32.5C. Green mussels were collected from the navigation buoy in Manora Channel and brought to the research laboratory. Samples were washed to remove the sand, vegetation, and other particles. Morphometric measurements were measured to the nearest gram. The result revealed that the shell length ranged between 0.60-7.4 cm, the width range between 0.10-2.9 cm, and the weight range between 0.037-15.549 g. In all morphometric relationships, the validation of the “b” values was significantly different from the isometric value. Significant correlation variation was observed in all variables whereas the highest correlation values were observed in the length-width (n-224, r2 = 0.966) and the lowest was observed in width-total weight (n-224, r2 = 0.85). The analysis of variance (ANOVA) revealed that all variables showed significantly different (DF=1, p<0.000).
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Estimation of population structure, growth and condition of Lates calcarifer (Bloch, 1790) in the Bay of BengalPakistan Journal of Marine SciencesThe current study illustrates the life-history traits; including length frequency distribution (LFD), length-weight and length-length relationship (LWR and LLR), condition factors (allometric, KA; Fulton’s, KF; relative condition, KR; relative weight, WR), and natural mortality (Mw) of Lates calcarifer from the Bay of Bengal, Bangladesh. A total of 103 individuals were randomly collected from January to December 2021 using traditional fishing gear including Koral net and gill net. For each individual, total length (TL), standard length (SL), and total body weight (BW) were measured with measuring scale to 0.1 cm and digital balance to 0.1 g precision, respectively. The TL size class 84.0 cm was a numerically dominant group in the population. The b value of LWR (TL vs. BW) indicated positive allometric growth (b=3.28) pattern in the Bay of Bengal. Likewise, the b value of LLR indicated also the same growth pattern. Among the four types of condition factors, KF is the best-suited tool for evaluating the well-being of L. calcarifer. Additionally, WR exhibited significant divergence from 100 (P< 0.0001), defining an imbalanced habitat. The calculated Mw was 0.23 year-1 for L. calcarifer. Therefore, these findings would be used in the future for the improved management of this species in the Bay of Bengal as well as for connecting ecosystems.
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A new species of Philometra reported from the ovaries of edible fish Epinephelus bleekeri (Vaillant, 1878) of the Karachi coastPakistan Journal of Marine SciencesNutritionally, fish meat is identified as healthiest meat all over the world. Nutriments that are essential for healthy diet can be easily drive from fishes. Beside fish meat there are various type of derivatives that we can obtain from them. Economically fish industry is considered as backbone for any country. Approximately, 45,000 species of helminth parasites cause pathogenicity in vertebrate hosts that are known to man. Parasite that belong to family Philometridae usually invade different body organs and cavities of various fishes. Parasite that cause pathogenicity in reproductive organs of fish ultimately cause disturbance in their release of reproductive hormones, mating behavior, difficulty in spawning and ultimately number of fry production is declined. This research was conducted on the nematode parasites of marine fish Epinephelus bleekeri (Vaillant 1878) from Karachi coast during January,2021 to December,2021. In this one year research, a new species of nematode Philometra bleekerii n.sp. was recorded from the reproductive organs of marine fish Epinephelus bleekeri (Vaillant 1878).
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Effect of freezing period on chemical composition, microbial load and some pathogenic bacteria of Nile tilapia (Oreochromis niloticus)Pakistan Journal of Marine SciencesThis study evaluated the effects of freezing periods on nutritional values, microbial loads, and some pathogenic bacteria in Nile Tilapia (Oreochromis niloticus). 8 kg. Samples were collected from El-mawourda Fish Market in Khartoum State, for four experimental treatments. The treatments were conducted on the period 0 day, 7 days, 15 days and 21 days. It is noticed that the freezing period effect on the nutritional value, microbial load and some pathogenic bacteria of Nile tilapia (Oreochromis niloticus) revealed that the lowest average nutritional assessment of Nile tilapia (Oreochromis niloticus) observed for protein 20% at the third period of freezing (21 days), and minerals were calcium, phosphorus, magnesium, iron, sodium and potassium (5.6 – 2.7%, 4.8 – 2.9%, 2.6 – 2.9%, 1.1 – 0.71%, 4.5 – 2.8%) respectively. The result of this study clarified that the freezing period has a considerable consequence (p ≤ 0.05) on the chemical composition, microbial load and same pathogenic bacteria (E. coli and Salmonella) of Nile tilapia (Oreochromis niloticus).
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Modelling current and future distribution of the invasive silver-cheeked toadfish Lagocephalus sceleratus in the Mediterranean SeaPakistan Journal of Marine SciencesThe progress in species distribution modeling has brought new insights into biological invasion management. The present study aims to model the potential current geographic distribution and future expansion of silver-cheeked toadfish Lagocephalus sceleratus in the Mediterranean Sea. Coordinates of 98 occurrence records of L. sceleratus in the Mediterranean were used, and marine climatic variables were collected from the global databases. Fifteen modeling techniques were tested, and weighted ensemble averaging of the model replicates was built. AUC values for each model ranged from 0.61 for rpart to 0.99 for rf, and TSS values varied from 0.41 for mlp to 0.95 for rf. Based on the cutoff values of TSS and AUC, the seven modelling algorithms were used for ensemble modeling. The maximum seawater temperature at minimum depth explained strong biological importance to the current adaptation, and the salinity contributed the most to the future adaptation. The ensemble forecasting of suitable habitats of L. sceleratus for current distribution modeling revealed that L. sceleratus dominantly occurred in the middle and eastern parts of coastal areas of the Mediterranean. The future distribution was extended to the western part of coastal areas of the Mediterranean, classifying the high suitability of these areas for its future distribution. The suitable bioclimatic envelope of L. sceleratus under the present study is predicted to widen because of climate change. The likely regions of invasion and the areas at risk for a potential future invasion of L. sceleratus indicate that prompt, effective practical actions by resource managers should be undertaken to mitigate its impacts and spread.
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Considerations on the low-altitude operations of the cruising-type AUV Gondou for seafloor photographic surveys.Report of Hydrographic and Oceanographic ResearchesThe Japan Coast Guard AUV Gondou is a cruising-type autonomous underwater vehicle. The Gondou is equipped with a downward-looking camera system that potentially unveils detailed seafloor feature from a different perspective than acoustic observations. As the Gondou is designed to run at relatively high-speed for wide-range seafloor mapping, the vehicle has limited maneuverability in horizontal and vertical directions. These operational characteristics of the vehicle require careful considerations in the design of mission planning and predetermination of contingency avoiding behaviors. This technical report notes our operational efforts to conduct seafloor photographic surveys with the cruising-type AUV.
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Accuracy verification of a vessel-mounted laser scanner and its application to hydrographic surveys.Report of Hydrographic and Oceanographic ResearchesA technology has been developed to measure topographies and features on land in three dimensions using a vessel-mounted laser scanner while performing multibeam echo sounders. As a first step to examining whether this technology can be used in hydrographic survey for nautical charting, we tried to measure the coastline using a laser scanner. In this trial measurement, we devised procedures of bias value measurement and accuracy confirmation and verified the horizontal position accuracy of the point cloud data measured by the laser scanner. Accuracy verification showed that there was almost no difference between the position of point cloud data measured by the laser scanner in calm sea and the coastline measured by the kinematic GNSS, so we concluded it can be used to draw the coastline on nautical charts. On the other hand, the measurement accuracy was inferior in rough seas. In addition, measurement by laser scanner enables the precise measurement of the coastline such as wave-dissipating blocks and is expected to be applied to vertical clearance measurements under bridges and position measurement of buoys and fishing implements.
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New analytical system for monitoring ocean conditions especially the sea surface temperature, with the aid of the remote sensing products of Himawari.Report of Hydrographic and Oceanographic ResearchesWe have used the remote sensing products for sea surface temperature （SST） provided by the artificial satellites to monitor SST and to perceive ocean currents around Japan, as well as to publish the Quick Bulletin of Ocean Conditions. In September 2022, we changed the method from using the SST observed by NOAA and MetOp to using the SST observed by Himawari-8. We had directly received the signals of SST data observed by NOAA and MetOp using an analytical system with parabolic antenna. We replaced that with the new system witch receives SST data observed by Himawari-8 provided by the Japan Aerospace Exploration Agency （JAXA） via an internet site. The replacement could reduce cost and workload for maintenance. In this report, we describe the outline of our new system for monitoring SST and the characteristic of the new SST images. The new system receives the SST products observed by Himawari-8 from JAXA. The observation interval of Himawari-8 is every 10 minutes. The high frequency observation could reduce data blanks in a one-day composite image caused by cloud coverages in comparison with the previous products. The daily average value was selected for composite SST images in order to avoid improbable SST patterns resulting from the comparison with the daily maximum and minimum values. To assure the accuracy of the new SST images, we compared the new product against SST datasets based on the result of direct observation in several cases. The anomaly between new images and the CTD dataset of the Chiba light beacon in the Tokyo bay is approximately ±0.6℃ among the 10－28℃ in SST. In other case, the anomaly compared with the SST datasets of Kuroshio-Bokujo buoys off the coast of Shikoku is approximately －1.1℃ and mostly accorded at 17℃ and 29℃ of buoy-observed temperature, respectively. When comparing the water temperature data of NOAA and Metop with the water temperature data of Himawari-8, it was found that Himawari-8 was approximately 0.7 ℃ lower at water temperature 28℃, and approximately 0.4℃ at water temperature 5℃.
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Initial study on the visualization of physical structures in the ocean utilizing the water column imaging of multibeam echo sounders.Report of Hydrographic and Oceanographic ResearchesModern multibeam echo sounders have come to be capable of recording entire underwater echo profiles. The multibeam water column imaging discovers the spatial and temporal distribution of acoustic scatterers in water mass, potentially providing essential information to elucidate dynamic processes occurring inside the ocean. This technical report considers a processing method to highlight characteristic acoustic patterns that result from oceanographic phenomena and applies the technique to data obtained by the typical Mills-cross echo sounders. The result shows distinctive mid-water acoustic anomalies of laminar and periodical patterns. The laminar pattern shows abrupt vertical changes in the echo intensity, which might correlate with the seawater profile obtained by XCTD. The periodical pattern exhibits a train of intertwined waves resembling shear instability like the Kelvin-Helmholtz billow. The result suggests the ability of multibeam echo sounders to visualize pycnocline in the water column, which would further advance physical oceanographic study in terms of analyzing the 3-dimensional spatial distribution of midwater acoustic scatterers.
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Length-weight relationship and condition factor in Sarda orientalis (Temminck & Schlegel 1844) (Family-Scombridae) fish from Karachi fish harbour.Pakistan Journal of Marine SciencesSarda orientalis (Family-Scombridae) of assorted length (cm) and weight (gm) were obtained from Karachi fish harbor during January to December (2019) for measurement of length-weight relationship (LWR’s) and Fulton‘s condition factor (K). The range of length (57-60.8) and weight (2123-2641) were estimated. Total mean length (cm) (gm) (59.14±0.94) and weight (gm) (2336.32±114.21) were measured. LWR was estimated as W = 0.08166134 × L 2.5149 (log W = -1.088 + 2.5149log L). The highest mean of condition factor (K) (1.15±0.02) were measured in post-monsoon season. Assorted (ANOVA) analysis indicates that there is a significant correlation between weight, length, and season (p0.05).
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Tank-Based Nursery Production of the Sea Cucumber Holothuria scabra in Various Seaweed Feed RegimensThe Philippine Journal of FisheriesThis study explored the feasibility of Laurencia papillosa, Sargassum spp, and Gracilaria bailinae crude extracts as feeds for the juvenile sea cucumber Holothuria scabra in a tank-based system. Post-metamorphic juveniles (~1 mm) were harvested from larval tanks and were reared until the late juvenile stage in indoor tank systems at 300 ind./tank stocking density for 45 days. Growth and survival rates of H. scabra juveniles were monitored to assess the performance of each seaweed treatment. L. papillosa consistently yielded the best growth (SGR: 7.63-11.32%.d-1) and highest survival rates (54.6–56.8%) of H. scabra juveniles. The performance of L. papillosa did not differ from that of Sargassum spp., while G. bailinae yielded poor growth (SGR of 5.01–9.38%.d-1) and low survivorship of juveniles (8.77–19.77%). Growth and survival rates were similar between fresh and dried seaweeds (p>0.05), suggesting dried seaweeds' applicability as feeds for juvenile H. scabra. Moreover, increased feed rations resulted in better growth of juveniles, but survival rates among feed rations (4.5, 6.0, 7.5, and 9.0 L.d-1) did not differ significantly (p>0.05). The monthly mean water temperature ranged from 26.6°C to 28.8°C, salinity of 33.2–35.1 ppt, dissolved oxygen of 6.1–8.3 mg/L, and pH of 8.1–8.3. High yields of sea cucumber juveniles were observed during the dry season, while low survival rates (<20%) were observed during wet months. The success of this study could set forth the application of tank-based nursery systems for H. scabra, especially during the season of high rainfall and extreme weather conditions that heavily affects cage rearing in the open-sea setting.
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Population Parameters of Shortfin scad Decapterus macrosoma (Bleeker, 1851) in Antique, PhilippinesThe Philippine Journal of FisheriesGrowth, mortality, exploitation rate, and recruitment of Decapterus macrosoma in the waters off Antique province, Philippines, were investigated based on the length-frequency data collected from April 2019 until March 2020. Using the FISAT-II software, D. macrosoma growth parameters were computed as follows: L∞ = 26.18 cm TL, K = 1.00 yr-1, ø' = 2.836 yr-1, and an estimated life span of 3 years. Mortality values were estimated as: total mortality (Z) = 4.66 yr-1; natural mortality (M) = 1.88 yr-1; and fishing mortality (F) = 2.78 yr-1. The exploitation rate (E) was computed as 0.60 while Emax was 0.42 yr-1. The length at first capture Lc/L50 was estimated at 11.96 cm TL. There were two recruitment events in one year: July and December. D. macrosoma in Antique was growing isometrically and in good condition with the relative condition factor (Kn) values ranging from 0.99 to 3.39. The computed exploitation rate suggested that D. macrosoma faces a high level of exploitation in Antique waters. These results offer valuable information on the utilization, conservation, and management of D. macrosoma in the East Sulu Sea area.
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Method Standardization and Guidance Value Determination for Regulation on Formaldehyde in Round Scad (Decapterus spp.).The Philippine Journal of FisheriesThere are reports of traded Galunggong (Fil.), Decapterus spp., adulterated with formaldehyde (FA) to lengthen shelf life. FA is a noxious substance with negative repercussions to general consumer health. This warrants the determination of a guidance value due to a lack of a local regulatory value specific to Galunggong amidst rising Filipino public concern in the wake of importations complicated by the natural occurrence of FA in the marine fish post-mortem. Comparisons were made on FA levels of fish treated with different conditions exposure and with colorimetric measurements using 405 nm and 412 nm via Nash’s method optimized for a fish matrix with no significant difference in measurement to at most 30 ppm FA in an aqueous matrix. There is also no significant difference in FA content of samples measured immediately from those frozen overnight. Therefore, in line with the regulatory mandates of the Bureau of Fisheries and Aquatic Resources (DA-BFAR) of the Department of Agriculture (Republic of the Philippines), a guidance value of 46.24 μg FA/g of Galunggong or 46.24 ppm is suggested, which is obtained from FA measurements of fish subjected to minimal anthropogenic treatments in different conditions of adulteration. Samples from different Metro Manila wet markets serve as the baseline. The guidance value is suggested to be used as a reference for regulatory purposes and can be improved with a more widespread sampling of fish from landing to selling.
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Status of coral reefs, butterflyfishes, and benthic macro-invertebrates in Araceli and Dumaran, Palawan, PhilippinesThe Palawan ScientistAs a major fishing ground in Palawan, the reefs in the municipal waters of Araceli and Dumaran are continuously facing anthropogenic and climate-related threats. Hence, to provide information about the reef conditions, surveys were undertaken in three sites of each municipality as the basis for management. Data collection used the C30 method where a 75 m x 25 m sampling area was established at the upper reef slope (2-5 m deep) of each site. Substrates were photo-documented at predetermined random positions and the photos were processed using Coral Point Count with excel extension software (CPCe) to determine the percent substrate categories. Identification and counting of butterflyfishes and benthic macro-invertebrates were also undertaken. The hard-coral cover (HCC) ranged between 27.10 and 53.88% (fair to very good) for Araceli and 22.66 and 48.62% (fair to good) for Dumaran. The number of species and density of butterflyfishes largely varied across reefs. The benthic macro-invertebrates only included the blue Linckia starfish and giant clams. The current reef condition calls for urgent management actions.
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Molluscans species diversity and protein expression analysis in Lunella coronate spp of family Turbinidae from the Manora Channel Karachi, Pakistan.Pakistan Journal of Marine SciencesManora rocky shore characterize as a shore having moderate to strong wave exposure, zonation pattern characterize by intertidal habit, different communities of invertebrates and algae bands dominated by molluscan diversity and associated fauna. The current study contribute the species diversity of molluscs from the Manora rocky ledge, coast of Pakistan. A total of 11 species were recorded pertaining to 9 families and 11 genera including family Turbinidae: Turbo bruneus, Lunella coronata, family Cerithiidae: Cerithium caeruleum, family Babyloniidae: Babylonia spirata, family Neritidae: Nerita albicilla, family: Chilodontaidae Euchelus asper, family Veneridae: Circenita callipyga, family Arcidae: Barbatia obliquata. Family Pectinidae, Mimachlamys townsendi, family Chamidae: Chama sp, family Littorinidae and Sub family Trachycardiinae V. assimile. In addition, Protein expression analysis of L. cornata was also observed through Polyacrylamide gel electrophoretic. L. coronata was previously reported from the coastal water of Pakistan with some morphological variation in the shell shape and the results reveled that electrophoretic patterns of proteins in L. cornata spp 1 and L. cornata spp2 differentiate from each as seen in morphological characters.
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Benthic assemblages for ecological evaluation of Lake Borollus, Mediterranean Sea, Egypt.Pakistan Journal of Marine SciencesLake Borollus is one of the largest and oldest Egyptian lakes, whereas it is located in the far north of the Delta and mediates between the two branches of Rosetta and Damietta. It is connected to the Mediterranean Sea through Boghaz El Borollus and to the Nile through the Bermbal Canal, which was established in 1926 to feed the lake with abundant quantities of Nile water and Nile fish. Central coordinates: latitude 31°30' N and longitude 30°49' E with an area has decreased from 160,000 acres to 108,000 acres. The present work provides an ecological study on the regional and temporal distribution of benthic communities in particular along the wetlands lakes waters. Indicated that, the effective sediment and water variables which significantly correlated with the distribution and abundance of the identified benthos assemblages of vegetation and animal’s groups include sediments structure, water quality (pH, S%, DO, BOD, temperature. Overall, 10 taxa and 27 species of macroinvertebrates were identified. They were exclusively associated with submerged vascular plants Potamogeton pectinatus while only 10 were alive forms and the rest were represented by calcareous shells of molluscs. The benthic flora and fauna were estimated in the Lake Botollus, Egypt is subjected to the influx of sewage effluents, agriculture drainages and industrial wastes. Also considerable amounts of drain water are constantly discharged along the southern drains. Quantitative hauls were performed at the different habitats during winter and summer, 2017 covering 12 sampling sites and 9 drains discharged directly to the lake which bottom sampling was collected by grab sampler were selected to represent the covering the different environments. The dredged area per each station was 225 cm2 per haul. Results illustrated that the distribution of benthos in the estimated area is greatly affected by the degree of pollution as well as the prevailing hydrological conditions. The aquatic submerged vascular plants Potamogeton pectinatus appeared as the most common species in the lake in addition the bottom animals appeared more- sensitive to pollution than submerged vascular; thus, their lowest counts were recorded at the most sites and the fauna was mainly represented by the Nematoda, Oligochaeta, Polychaeta, Insects larvae, Ostracoda, Amphipoda, Barnacles, and juvenile larvae of Mollusca. Sediments in some areas consist of organic materials (sludge) is composed of waste sewage and agricultural and industrial and others of sandy silt (silt), calcareous shells are mainly of molluscs’ and therefore it can determine some environments prevailing in the lake. Results indicated for examining configuration qualitative benthic macro during the study period were recorded 27 species, including 9 species of living benthic macro organisms include one type of aquatic submerged vascular plants of species Potamogeton pectinatus. Although higher values of taxonomic richness, the Shannon and index (H`) and evenness (J`) were obtained with the water Potamogeton pectinatus habitat as well as Ostracoda Sclerochilus contortus were most abundant.
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Description of the first to fourth zoeal stages of Macrobrachium equidens (Dana, 1852) (Crustacea: Decapoda: Palaemonidae).Pakistan Journal of Marine SciencesAn ovigerous female of Macrobrachium equidens (Dana, 1852) collected from near shore of Sandspit back waters (Karachi, Pakistan) (Long. 66° 54’ 24”E Lat. 24° 50’ 24”N) on 19, October, 2009, and kept under the laboratory conditions. Larvae were hatched out after 13 days. Zoea I to zoea III stages passed within 6 days at room temperature 31°C, water temperature 29°C, in filtered seawater of a salinity of 37 ‰ and pH 7.9. The Zoea IV of the same species was collected from planktonic sample of Manora Channel, Karachi (Long. 66° 59’ E Lat. 24° 48’ N) on 1995. The larvae of this species are described and illustrated along with comparisons to those described previously.
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Morphometric relationships and meristic characteristics of Megalaspis cordyla (Linnaeus, 1758) from the Bay of Bengal.Pakistan Journal of Marine SciencesMegalaspis cordyla is one of the Carangid fish species commonly found in marine areas of the Sundarbans and the Bay of Bengal in Bangladesh. This study described the morphometric relationships including length-weight and length-length relations (LWRs and LLRs) and meristic characteristics of M. cordyla. A total of 100 individuals were collected from commercial fishers’ catch in the Bay of Bengal, Bangladesh during January-December 2021. For each individual, a total of fourteen various lengths were measured by measuring board with 0.01 cm and body weight (BW) was taken by digital balance with 0.01 g accuracy. Fin-ray numbers from all fins as well as scutes were counted by a magnifying glass. Total length (TL) varied from 17.0-36.5 cm and the total body weight (BW) ranged from 75.0-416.6 g. All LWRs were significantly related (p < 0.001), with r2 values ≥ 0.910. Based on r2 value (0.979), LWR by BW vs. TL (W= a × Lb) was the best fitted model among 14 equations. Besides, the LLRs were also significant with r2 values ≥ 0.908. According to r2 value (0.995), LLR by TL vs. FL (TL = a + b× FL) was the best fitted model among 14 equations. The fin formula of M. cordyla is: first dorsal, D1 I+VII; second dorsal, D2 I/14-18+6-8 finlets; pectoral, P1 20-26; pelvic, P2 I/5; anal, A II+I/12-16 +5-7 finlets. This study provided knowledge on morphometric and meristic features that would be very useful for species identification and stock assessment of M. cordyla in the Bay of Bengal and other marine ecosystems.
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Least-Squares Reverse Time Migration in Pseudodepth Domain and Its ApplicationThe Cartesian-based least squares reverse time migration (LSRTM) aims to obtain a relatively high-resolution amplitude preserving imaging by calculating and solving the Hessian matrix. In addition, conjugate gradient algorithm is proved to be an efficient iterative method, which makes the traditional LSRTM feasible in practical data processing applications. Each iteration process of LSRTM includes two main parts; The first part is the forward simulation of wavefield, and the second part is the back propagation of wavefield. However, the calculation of these two parts takes a lot of time, and oversampling effects will occur in the calculation process. The thesis develops a smooth understanding of the LSRTM scheme in pseudodepth domain and gives information about this type of wavefield extrapolation method to achieve amplitude-preserved image with low computational costs in terms of memory and time. The underground media faced by seismic exploration usually include low velocity bodies, high steep structures and pore fracture units. When simulating the seismic wavefield in these media, in order to ensure the simulation accuracy and computational stability by finite difference methods, the grid spacing needs to be very small, which leads to the oversampling problem of the traditional finite difference forward simulation method in Cartesian coordinate system. In order to overcome this problem, the pseudo depth domain algorithm is applied to the least-squares reverse time migration to improve its computational efficiency. The problem of stabilizing this Pseudodepth wavefield arises from the introduction of the mapping function and velocity and also the vertical axis operator that converts the finite difference solution partially from time into frequency domains. Stability and convergence analysis suggests that the spatial derivatives of Riemannian axis should be approximated by a mixed Fourier pseudo-spectral and ordinary finite-difference schemes methods using a special Gaussian-like impulse function to generate the vector-matrix of the complex source term within the finite-difference operator, in addition to the mapping velocity, which is a differential form of the initial input velocity model, manifestly controls the CFL conditions of the associated Riemannian-finite difference operator. Numerical and synthetic examples indicated that this approach is more stable and efficient in extrapolating a smooth Riemannian wavefield while maintaining Claerbout’s principle for locating subsurface reflectors also choosing an appropriate sampling rate for the new vertical axis is related inversely by the maximum frequency of the impulse wavelet and directly with minimum velocity value in the given model. The LSRTM wavefield extrapolation usually uses the two-dimensional constant density acoustic wave equation, by considering the change of velocity field distribution, and converts it to the corresponding pseudodepth domain, so as to solve the oversampling problem in the true depth domain. For each point in the Cartesian coordinate system, there is a corresponding point in the pseudo depth domain. Therefore, we can interpolate and remodel the reconstructed finite difference modelled wavefield in the new coordinate system through the Cartesian-to-pseudodepth mapping function. Regardless of the applied finite difference algorithm and boundary conditions, wavefield extrapolation in pseudodepth domain can ensure high accuracy and efficiency. Through the test of synthetic and actual data, compared with the traditional LSRTM results, pseudodepth domain LSRTM shows great potentiality in amplitude preserving imaging. On the other hand, pseudodepth domain LSRTM has great advantages in computational efficiency and ensures computational accuracy.
