Spatial Memory in Insect Navigation Collett, Matthew; Chittka, Lars; Collett, Thomas S.
CB/Current biology,
09/2013, Volume:
23, Issue:
17
Journal Article
Peer reviewed
Open access
A wide variety of insects use spatial memories in behaviours like holding a position in air or flowing water, in returning to a place of safety, and in foraging. The Hymenoptera, in particular, have ...evolved life-histories requiring reliable spatial memories to support the task of provisioning their young. Behavioural experiments, primarily on social bees and ants, reveal the mechanisms by which these memories are employed for guidance to spatial goals and suggest how the memories, and the processing streams that use them, may be organized. We discuss three types of memory-based guidance which, together, can explain a large part of observed insect spatial behaviour. Two of these, alignment image-matching and positional image-matching, are based on an insect's remembered views of its surroundings: The first uses views to keep to a familiar heading and the second to head towards a familiar place. The third type of guidance is based on a process of path integration by which an insect monitors its distance and direction from its nest through odometric and compass information. To a large degree, these guidance mechanisms appear to involve modular computational systems. We discuss the lack of evidence for cognitive maps in insects, and in particular the evidence against a map based on path integration, in which view-based and path integration memories might be combined. We suggest instead that insects have a collective of separate guidance systems, which cooperate and train each other, and together provide reliable guidance over a range of conditions.
Natural gas hydrate is ice‐like mixture of gas (mostly methane) and water that is widely found in sediments along the world's continental margins and within and beneath permafrost and glaciers in a ...near‐surface depth interval where the pressure is sufficiently high and temperature sufficiently low for gas hydrate to be stable. We categorize the myriad of geological gas hydrate deposits into five characteristic types. We then review the multiple quantitative models that have proposed to describe the genesis of these deposits and describe how each may have formed. We emphasize the importance of coupling multiphase flow (free gas and liquid water) and multicomponent reactive transport with geological history to describe the dynamical processes of gas hydrate formation and evolution in geological systems. A better insight into the kinetics of methane formation from microbial biogenesis and the processes of multiphase flow at the pore scale will advance our knowledge of how these systems form. By understanding the generation and evolution of gas hydrate through time, we will better decipher the role of gas hydrate in the carbon cycle, its potential to contribute to climate change and geohazards, and how to design optimal strategies for gas production from hydrate reservoirs.
Plain Language Summary
Scientific drillings and geophysical investigations have revealed various occurrences of gas hydrate under the seafloor and within and beneath permafrost. We summarize their key features and categorize their occurrences into five major types. We then review the different quantitative models that have been developed to explain their formation in the field and link different models to field observations. We identify the key advances achieved, the major remaining challenges, and the efforts required to further understand the formation of gas hydrate in geological systems. This knowledge can help us learn the role of gas hydrate in natural environments.
Key Points
Geological gas hydrate deposits can be categorized into five major types and tied to six different formation mechanisms
Free gas flow and capillary pressure play significant roles in forming many concentrated hydrate deposits
A better understanding of microbial methanogenesis will illuminate how methane hydrate deposits are formed in geological systems
Memory use in insect visual navigation Collett, Thomas S; Collett, Matthew
Nature reviews. Neuroscience,
200207, 2002-Jul, 2002-7-00, 20020701, Volume:
3, Issue:
7
Journal Article
Peer reviewed
The navigational strategies that are used by foraging ants and bees to reach a goal are similar to those of birds and mammals. Species from all these groups use path integration and memories of ...visual landmarks to navigate through familiar terrain. Insects have far fewer neural resources than vertebrates, so data from insects might be useful in revealing the essential components of efficient navigation. Recent work on ants and bees has uncovered a major role for associative links between long-term memories. We emphasize the roles of these associations in the reliable recognition of visual landmarks and the reliable performance of learnt routes. It is unknown whether such associations also provide insects with a map-like representation of familiar terrain. We suggest, however, that landmarks act primarily as signposts that tell insects what particular action they need to perform, rather than telling them where they are.
During the Indian National Gas Hydrate Program Expedition 01 (NGHP‐01), one of the richest marine gas hydrate accumulations was discovered at Site NGHP‐01‐10 in the Krishna‐Godavari Basin. The ...occurrence of concentrated gas hydrate at this site is primarily controlled by the presence of fractures. Assuming the resistivity of gas hydrate–bearing sediments is isotropic, the conventional Archie analysis using the logging while drilling resistivity log yields gas hydrate saturations greater than 50% (as high as ∼80%) of the pore space for the depth interval between ∼25 and ∼160 m below seafloor. On the other hand, gas hydrate saturations estimated from pressure cores from nearby wells were less than ∼26% of the pore space. Although intrasite variability may contribute to the difference, the primary cause of the saturation difference is attributed to the anisotropic nature of the reservoir due to gas hydrate in high‐angle fractures. Archie's law can be used to estimate gas hydrate saturations in anisotropic reservoir, with additional information such as elastic velocities to constrain Archie cementation parameters m and the saturation exponent n. Theory indicates that m and n depend on the direction of the measurement relative to fracture orientation, as well as depending on gas hydrate saturation. By using higher values of m and n in the resistivity analysis for fractured reservoirs, the difference between saturation estimates is significantly reduced, although a sizable difference remains. To better understand the nature of fractured reservoirs, wireline P and S wave velocities were also incorporated into the analysis.
High-quality logging-while-drilling (LWD) downhole logs were acquired in seven wells drilled during the Gulf of Mexico Gas Hydrate Joint Industry Project Leg II in the spring of 2009. Well logs ...obtained in one of the wells, the Green Canyon Block 955 H well (GC955-H), indicate that a 27.4-m thick zone at the depth of 428 m below sea floor (mbsf; 1404 feet below sea floor (fbsf)) contains gas hydrate within sand with average gas hydrate saturations estimated at 60% from the compressional-wave (P-wave) velocity and 65% (locally more than 80%) from resistivity logs if the gas hydrate is assumed to be uniformly distributed in this mostly sand-rich section. Similar analysis, however, of log data from a shallow clay-rich interval between 183 and 366 mbsf (600 and 1200 fbsf) yielded average gas hydrate saturations of about 20% from the resistivity log (locally 50−60%) and negligible amounts of gas hydrate from the P-wave velocity logs. Differences in saturations estimated between resistivity and P-wave velocities within the upper clay-rich interval are caused by the nature of the gas hydrate occurrences. In the case of the shallow clay-rich interval, gas hydrate fills vertical (or high angle) fractures in rather than filling pore space in sands. In this study, isotropic and anisotropic resistivity and velocity models are used to analyze the occurrence of gas hydrate within both the clay-rich and sand dominated gas-hydrate-bearing reservoirs in the GC955-H well.
► Estimates saturation of hydrate in sand reservoirs using isotropic models. ► Estimates saturation of hydrate in fractured reservoirs using anisotropic models. ► Presents a cross plot to identify fractured reservoirs. ► Isotropic analysis of the resistivity significantly overestimates saturations.
In 2006, the U.S. Geological Survey (USGS) completed detailed analysis and interpretation of available 2-D and 3-D seismic data and proposed a viable method for identifying sub-permafrost gas hydrate ...prospects within the gas hydrate stability zone in the Milne Point area of northern Alaska. To validate the predictions of the USGS and to acquire critical reservoir data needed to develop a long-term production testing program, a well was drilled at the Mount Elbert prospect in February, 2007. Numerous well log data and cores were acquired to estimate in-situ gas hydrate saturations and reservoir properties.
Gas hydrate saturations were estimated from various well logs such as nuclear magnetic resonance (NMR), P- and S-wave velocity, and electrical resistivity logs along with pore-water salinity. Gas hydrate saturations from the NMR log agree well with those estimated from P- and S-wave velocity data. Because of the low salinity of the connate water and the low formation temperature, the resistivity of connate water is comparable to that of shale. Therefore, the effect of clay should be accounted for to accurately estimate gas hydrate saturations from the resistivity data. Two highly gas hydrate-saturated intervals are identified – an upper ∼43 ft zone with an average gas hydrate saturation of 54% and a lower ∼53 ft zone with an average gas hydrate saturation of 50%; both zones reach a maximum of about 75% saturation.
Gas hydrate was discovered in the Krishna–Godavari (KG) Basin during the India National Gas Hydrate Program (NGHP) Expedition 1 at Site NGHP-01-10 within a fractured clay-dominated sedimentary ...system. Logging-while-drilling (LWD), coring, and wire-line logging confirmed gas hydrate dominantly in fractures at four borehole sites spanning a 500
m transect. Three-dimensional (3D) seismic data were subsequently used to image the fractured system and explain the occurrence of gas hydrate associated with the fractures. A system of two fault-sets was identified, part of a typical passive margin tectonic setting. The LWD-derived fracture network at Hole NGHP-01-10A is to some extent seen in the seismic data and was mapped using seismic coherency attributes. The fractured system around Site NGHP-01-10 extends over a triangular-shaped area of ∼2.5 km
2 defined using seismic attributes of the seafloor reflection, as well as “seismic sweetness” at the base of the gas hydrate occurrence zone. The triangular shaped area is also showing a polygonal (nearly hexagonal) fault pattern, distinct from other more rectangular fault patterns observed in the study area. The occurrence of gas hydrate at Site NGHP-01-10 is the result of a specific combination of tectonic fault orientations and the abundance of free gas migration from a deeper gas source. The triangular-shaped area of enriched gas hydrate occurrence is bound by two faults acting as migration conduits. Additionally, the fault-associated sediment deformation provides a possible migration pathway for the free gas from the deeper gas source into the gas hydrate stability zone. It is proposed that there are additional locations in the KG Basin with possible gas hydrate accumulation of similar tectonic conditions, and one such location was identified from the 3D seismic data ˜6 km NW of Site NGHP-01-10.
A global inventory of data from gas hydrate drilling expeditions is used to develop relationships between the base of structure I gas hydrate stability, top of gas hydrate occurrence, sulfate‐methane ...transition depth, pressure (water depth), and geothermal gradients. The motivation of this study is to provide first‐order estimates of the top of gas hydrate occurrence and associated thickness of the gas hydrate occurrence zone for climate‐change scenarios, global carbon budget analyses, or gas hydrate resource assessments. Results from publically available drilling campaigns (21 expeditions and 52 drill sites) off Cascadia, Blake Ridge, India, Korea, South China Sea, Japan, Chile, Peru, Costa Rica, Gulf of Mexico, and Borneo reveal a first‐order linear relationship between the depth to the top and base of gas hydrate occurrence. The reason for these nearly linear relationships is believed to be the strong pressure and temperature dependence of methane solubility in the absence of large difference in thermal gradients between the various sites assessed. In addition, a statistically robust relationship was defined between the thickness of the gas hydrate occurrence zone and the base of gas hydrate stability (in meters below seafloor). The relationship developed is able to predict the depth of the top of gas hydrate occurrence zone using observed depths of the base of gas hydrate stability within less than 50 m at most locations examined in this study. No clear correlation of the depth to the top and base of gas hydrate occurrences with geothermal gradient and sulfate‐methane transition depth was identified.
Key Points
An inventory of 21 gas hydrate drilling campaigns is presented
The top of gas hydrate occurrence and depth of gas hydrate stability are defined from drilling with uncertainties depending on proxy used
Statistical relationships are developed to predict thickness of gas hydrate occurrence zone from depths of gas hydrate stability
Desert ants returning from a foraging trip to their nest navigate both by path integration and by visual landmarks. In path integration, ants compute their net distance and direction from the nest ...throughout their outward and return journeys, and so can always return directly home from their current location. As the path-integration vector is updated over the entire journey, we call it a global vector. On a familiar route, when ants can steer by visual landmarks, they adopt a fixed and often circuitous path consisting of several separate segments that point in different directions,,. Here we show that, as in honeybees, such multisegment journeys are composed partly of stored local movement vectors, which are associated with landmarks and are recalled at the appropriate place. We also show that a local vector learnt at one value of the global vector can be recalled at many values, and that expression of the global vector is temporarily inhibited while the local vector is used. These results indicate that the global vector is ignored during navigation through familiar, cluttered territory, but that it re-emerges to take the ant home once the insect leaves the clutter and other guidance strategies cease to operate.
PDF
