ELECTROMAGNETIC INVESTIGATION OF THE SEA FLOOR

Geophysics ◽  
1970 ◽  
Vol 35 (3) ◽  
pp. 476-489 ◽  
Author(s):  
J. H. Coggon ◽  
H. F. Morrison
Keyword(s):  
Mineral Exploration ◽  
Skin Depth ◽  
Electromagnetic Response ◽  
Horizontal Magnetic Field ◽  
Electromagnetic System ◽  
Sea Floor ◽  
Sea Bottom ◽  
Sea Bed ◽  
Receiver System ◽  
Field Strengths

Numerical evaluation of integral expressions for the fields about a vertical magnetic dipole in the sea allows analysis of the electromagnetic response over wide ranges of sea induction number and sea floor conductivity. Our analysis indicates that a marine electromagnetic system for measurement of bottom conductivity variations could readily be designed, with such applications as oceanographic and geologic studies, and mineral exploration. For a source‐receiver system on a homogeneous sea bottom, it is found that: (i) when the ratio k=(sea‐bed conductivity)/(seawater conductivity) is greater than about 0.03, both horizontal and vertical magnetic fields are useful for measurement of bottom conductivity at sea induction numbers less than 30 [induction number =√2 (horizontal transmitter‐receiver separation/skin depth)]. A separation of 30 m and frequencies in the range 300–3500 hz appear suitable for investigation of the upper few meters of unconsolidated bottom sediments. (ii) When the ratio k is less than 0.03, sea induction numbers from 10 to a few hundred are required for detection of seabed conductivity variations. In this case, the horizontal magnetic field, resulting from energy transmission mainly through the seafloor, is the suitable field to use. Electromagnetic sounding of indurated rocks may thus call for frequencies of 100 to 20,000 hz at a separation of 200 m. Field strengths vary strongly with relative sea depth D/R (D=sea depth, R=horizontal source‐receiver separation) when D/R is small; but sensitivity to bottom conductivity is little affected by D/R. Elevation of source and receiver above a seafloor less conductive than seawater reduces field strengths and sensitivity to seabed properties.

On the electromagnetic response of a resistive cylinder buried in a conductive host

Geophysics ◽  
2014 ◽  
Vol 79 (6) ◽  
pp. E341-E351 ◽  
Author(s):  
Andrei Swidinsky
Keyword(s):  
Electric Dipole ◽  
Mineral Exploration ◽  
Skin Depth ◽  
Hydrocarbon Exploration ◽  
Electromagnetic Response ◽  
Dipole Source ◽  
Additional Information ◽  
Dipole System ◽  
Single Dipole ◽  
Current Systems

The frequency-domain electromagnetic response of a confined conductor buried in a resistive host has received much attention, particularly in the context of mineral exploration. In contrast, the problem of the electromagnetic response of a confined resistor buried in a conductive host has been less thoroughly studied. However, resistive targets are important in geotechnical and hydrologic studies, archaeological prospecting, and, more recently, offshore hydrocarbon exploration. I analytically address the problem of the electromagnetic response of a completely resistive cylindrical cavity buried in a conductive host in the presence of a simplified 2D electric dipole source. In contrast to the confined conductor, which channels and induces current systems, the confined resistor deflects current and produces additional eddy current systems in the conductive host. I apply this theory to model the response of a grounded electric dipole-dipole system operating over a range of frequencies from 0 Hz to 10 kHz, in the presence of a horizontal 5-m radius insulating cylinder located 1-m beneath the surface of a uniform earth. This represents a common hazard encountered during mining and civil engineering operations. Results show that such an insulating cavity increases the recorded electric field amplitude and phase delay at all transmitted frequencies. These observations suggest that a broadband electromagnetic prospecting system may provide additional information about the location and extent of a void, over and above a standard dipole-dipole resistivity survey. When the host skin depth is much larger than all other length scales, the response can be approximated by an equivalent single dipole unless the cylinder’s radius is much larger than its distance from the transmitter. This result provids a useful rule of thumb to determine the acceptable range over which a resistive target can be modeled by a distribution of dipoles.

Estimation of sea‐floor wave velocities and density from pressure and particle velocity by AVO analysis

Geophysics ◽  
1995 ◽  
Vol 60 (5) ◽  
pp. 1575-1578 ◽  
Author(s):  
Lasse Amundsen ◽  
Arne Reitan
Keyword(s):  
Numerical Study ◽  
Offshore Structures ◽  
P Wave ◽  
Rock Properties ◽  
Sea Floor ◽  
S Waves ◽  
Interface Waves ◽  
Wave Velocities ◽  
Sea Bottom ◽  
Sea Bed

Sea‐bottom properties play an important role in fields as diverse as underwater acoustics, earthquake and geotechnical engineering, and marine geophysics. Water‐column acousticians study shear and interface waves in the nearbottom sediments with the aim of inferring sea‐bed geoacoustic parameters for predicting reflection and absorption of waves at the sea floor. On the other hand, geotechnical engineers working on design and siting of offshore structures focus on these waves to characterize soil and rock properties. In the field of geophysics, sea‐bottom parameters are of interest for several reasons. In conventional marine acquisition, these parameters determine the partitioning of the incident P‐wave energy from the source into transmitted P‐waves and mode‐converted S‐waves (Tatham and Goolsbee, 1984; Kim and Seriff, 1992). The sea‐floor P‐ and S‐wave velocities and density are also necessary inputs for decomposing multicomponent sea‐floor data into P‐ and S‐waves (Amundsen and Reitan, 1995a and b), as well as in the numerical study of wave propagation phenomena.

MOORING FLOATING OIL RIGS - NORTHWEST AUSTRALIA

1970 ◽  
Vol 10 (1) ◽  
pp. 100
Author(s):  
S. Stroud
Keyword(s):  
Continental Shelf ◽  
Water Depth ◽  
Drill String ◽  
Weight System ◽  
Oil Exploration ◽  
Sea Floor ◽  
Sea Bottom ◽  
Sea Bed ◽  
Floating Type ◽  
Northwest Australia

B.O.C. of Australia Limited has drilled five wells on the continental shelf of Northwest Australia using floating type oil exploration vessels. To date three different methods of anchoring have been used in the Group's operations depending on the type of sea bottom encountered and the progress of technology.On coral and shoal areas south of Timor good anchoring ground has been located. Both drilling exploration vessels S.S. "Glomar Tasman" and the "Investigator," a drilling barge, have been anchored in this area using all chain anchor systems.Three wells have been drilled in an area approximately 100 miles northeast of Barrow Island where surveys revealed smooth limestone sea beds of such hardness that conventional ships anchors could not penetrate the surface to obtain a firm hold. Anchor pilings to hold the "Glomar Tasman" have been successfully installed on the sea floor at two drill sites using a small vessel, M.V. "Nyhavns Rose," of 400 tons. These piles were drilled into the sea bed using a drill string and a power swivel at the surface, and utilizing a counter weight system to allow for vessel heave. At the first locality, in 180 feet of water, divers were used to manually connect the heavy pendant wires to the anchor piles. At the second location a diverless method of installing drilled in anchor piles with chain mooring pendants attached was designed and successfully used in a water depth of 265 feet.

Seasonal deposition of phytodetritus to the deep-sea floor

1986 ◽  
Vol 88 ◽  
pp. 265-279 ◽  
Author(s):  
A. L. Rice ◽  
D. S. M. Billett ◽  
J. Fry ◽  
A. W. G. John ◽  
R. S. Lampitt ◽  
...  
Keyword(s):  
Deep Sea ◽  
Benthic Communities ◽  
Time Lapse ◽  
Sediment Traps ◽  
Temporal Variations ◽  
Sea Floor ◽  
Porcupine Seabight ◽  
The Past ◽  
Sea Bed ◽  
Seasonal Deposition

SynopsisEvidence has accumulated over the past twenty years to suggest that the deep-sea environment is not as constant as was at one time thought, but exhibits temporal variations related to the seasonally in the overlying surface waters. Recent results from deep-moored sediment traps suggest that this coupling is mediated through the sedimentation of organic material, while observations in the Porcupine Seabight indicate that in this region, at least, there is a major and rapid seasonal deposition of aggregated phytodetritus to the sea-floor at slope and abyssal depths.This paper summarises the results of the Porcupine Seabight studies over the past five years or so, using time-lapse sea-bed photography and microscopic, microbiological and chemical analyses of samples of phytodetritus and of the underlying sediment. The data are to some extent equivocal, but they suggest that the seasonal deposition is a regular and dramatic phenomenon and that the material undergoes relatively little degradation during its passage through the water column. The mechanisms leading to the aggregation of the phytodetritus have not been identified, and it is not yet known whether the phenomenon is geographically widespread nor whether it is of significance to the deep-living mid-water and benthic communities.

Mineral Exploration of the Sea bed by Towed Sea bed Spectrometers

1983 ◽  
pp. 437-449 ◽  
Author(s):  
B.W. THOMAS ◽  
C.G. CLAYTON ◽  
V.V.C. RANASINGHE ◽  
I.M. BLAIR
Keyword(s):  
Mineral Exploration ◽  
Sea Bed

scholarly journals The Formation and Composition of the Gas Content of Sea Ice

1979 ◽  
Vol 22 (86) ◽  
pp. 67-81 ◽  
Author(s):  
V. L. Tsurikov
Keyword(s):  
Sea Ice ◽  
Sea Water ◽  
Major Effect ◽  
Relative Volume ◽  
Gas Content ◽  
Dominant Factor ◽  
Sea Floor ◽  
Sea Bottom ◽  
Initial Freezing ◽  
Air Pockets

Abstract The different factors contributing to the formation of the gas porosity of sea ice are: (Ia) gases captured during the formation of the initial ice cover, (Ib) gases released from solution during the initial freezing of sea-water, (Ic) the inclusion of gases rising from the sea bottom, (2a) the substitution of gas for brine drained from the ice during times of melting, (2b) the release of gas from the brine within the ice during the course of partial freezing, and (2c) the formation of voids filled with water vapour during the course of internal melting. An analysis is made of each of these processes and it is concluded that processes Ib, 2a, and 2C are important. Process Ic may also be a major effect but it is difficult to evaluate until the rate of gas release from the sea floor is better known. The migration of air pockets into the ice from the overlying snow is shown to be a possible but not a significant effect. Available data on the composition of gas in sea ice are reviewed and it is shown to be significantly different from air. Possible causes for these differences are discussed. The porosity of sea ice, i.e. the total relative volume of its gas plus its brine inclusions, is one of the factors strongly affecting its strength, as has been shown by Tsurikov (1947) and by Weeks and Assur (1968). In seas with high salinities the effect of the presence of brine within the ice will usually be the dominant factor. However on water bodies with low salinities the effect of the gas included within the ice may be greater than the effect of the brine. Despite its significance there have not been any attempts at a quantitative analysis of the entrapment of gas in sea ice. This paper is an attempt at such a study.

Possibility of Evaluating a Depth of Defects as a Hole in a Composite Dielectric Sample by the Parameters of Electric Response to Pulse Mechanical Excitation

2018 ◽  
Vol 781 ◽  
pp. 155-158 ◽  
Author(s):  
Petr Khorsov ◽  
Anatoly Surzhikov ◽  
Vladimir Surzhikov ◽  
Roman Laas
Keyword(s):  
Mathematical Model ◽  
Electromagnetic Response ◽  
Electric Response ◽  
One Dimensional ◽  
Response Parameter ◽  
Dielectric Sample ◽  
Receiver System ◽  
Mechanoelectrical Transformations ◽  
Spurious Component ◽  
The One

The applicability of the method mechanoelectrical transformations (MET) to determine the depth of the macrodefects location in the sample on parameters of the electromagnetic response is evaluated. As the response parameter it was used the phase characteristic of the signal analytical representation.On the one-dimensional mathematical model it was shown the possibility to detect phase response changes when reflected from defect acoustic wave is mixed with the signal spurious component generated by a distributed MET sources. Experimental verification of mathematical model on a sample of concrete was conducted. It has been shown that the sensitivity of the method MET to evaluation of the macrodefect locate depth depends on the wavelength of the excitation pulse and the area of the macrodefect border closest to the emitter-receiver system.

Impact—Response Behavior of Offshore Pipelines

1982 ◽  
Vol 104 (4) ◽  
pp. 325-329 ◽  
Author(s):  
P. G. Bergan ◽  
E. Mollestad
Keyword(s):  
Plastic Material ◽  
Material Behavior ◽  
Hydrodynamic Drag ◽  
Offshore Pipelines ◽  
Sea Floor ◽  
Drag Forces ◽  
Sea Bottom ◽  
Nonlinear Dynamic Equations ◽  
Numerical Formulation ◽  
Inertia Forces

A method for analyzing the dynamic behavior of marine pipelines subjected to impact loads or sudden forced movements is outlined. Inertia forces (also from hydrodynamic mass), hydrodynamic drag forces as well as friction and lift effects for a pipe at the sea bottom are accounted for. An extensive nonlinear formulation is used for the pipe itself; it includes large displacements and elasto-plastic material behavior. Aspects of the numerical formulation of the problem and the solution of the nonlinear dynamic equations are discussed. The examples show computed dynamic response for pipelines lying on the sea floor and for a pipe section freely submerged in water when subjected to various force and displacement histories.

Suprabenthic crustacean fauna of a denseAmpeliscacommunity from the English Channel

1996 ◽  
Vol 76 (4) ◽  
pp. 909-929 ◽  
Author(s):  
Jean-Claude Dauvin ◽  
Souaad Zouhiri
Keyword(s):  
Atlantic Ocean ◽  
Water Column ◽  
Vertical Migration ◽  
Fine Sand ◽  
Abundant Species ◽  
Sexually Dimorphic ◽  
English Channel ◽  
Free Swimming ◽  
Sea Bottom ◽  
Sea Bed

Ninety-six species (97, 677individuals) were collected over the course of 6 h in five suprabenthic sledge hauls from a very denseAmpeliscafine sand community from the Bay of Morlaix (western English Channel). All the species migrated into the water column at night (98% of the specimens collected in the suprabenthos were found in the night hauls). The 23 most abundant species collected were classified into five groups based on their height within the water column, but two groups predominated: the upper suprabenthic species, abundant at 0–80–145 m above the sea-bed; and the lower suprabenthic species which were abundant only near the sea bottom (-0–1–0–75 m high). Three different patterns of nocturnal vertical migration were distinguished based on the timing of maximum swimming activity: at dusk; at the beginning of the night; or later in the night. Sexually dimorphic patterns of free-swimming behaviour was observed inAmpeliscaand some other species of Amphipoda (Bathyporeia teniupes, Metaphoxusfultoni), and Cumacea (Bodotria pulchella, Pseudocuma longicornis), with many more males than females migrating into the water column at night. Finally, the density of suprabenthic crustaceans in nocturnal hauls was amongst the highest reported from infralittoral or circalittoral suprabenthic studies on other parts of the Atlantic Ocean sampled during spring.

Removal of water‐layer multiples from multicomponent sea‐bottom data

Geophysics ◽  
1999 ◽  
Vol 64 (3) ◽  
pp. 838-851 ◽  
Author(s):  
Are Osen ◽  
Lasse Amundsen ◽  
Arne Reitan
Keyword(s):  
Vertical Velocity ◽  
Water Layer ◽  
Real Data ◽  
Physical Principle ◽  
Sea Floor ◽  
Numerical Tests ◽  
Sea Bottom ◽  
Marine Source ◽  
Suppression Technique ◽  
Fundamental Physical

A method for suppressing water‐layer multiples in multicomponent sea‐floor measurements is presented. The multiple suppression technique utilizes the concept of wavefield separation into upgoing and downgoing modes just below the sea floor for eliminating the sea‐floor ghost, the sea‐surface ghost, and the accompanying water‐layer reverberations. The theory applies to each of the recorded components: pressure, vertical velocity, and horizontal velocities. The fundamental physical principle for the multiple suppression technique rests on identifying these multiples as downgoing waves just below the sea floor, while the primaries of interest arriving from the subsurface are upgoing waves. White presented this realization for the pressure component three decades ago; hence, the theory for the velocity field is an extension of the theory. In this paper, the theory is derived for an experiment with a marine source in the water layer above a locally flat, elastic sea floor with known elastic parameters. The method is otherwise multidimensional and operates on a shot‐to‐shot basis; hence, it is computationally fast. Aside from this, we show that this demultiple method removes the strongest multiples in sea‐floor data without knowledge of the source wavelet. Synthetic and real data examples are provided to illustrate the application of the algorithms to the pressure, in‐line velocity, and vertical velocity components. The numerical tests show that strong multiples have been attenuated on the pressure and the velocity recordings, producing promising results.

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