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In this research, we investigate the propagation of lateral electromagnetic wave near the surface of sea. Interference patterns generated by the superposition of the lateral and direct waves along the sea surface (flat and rough) are shown. The field generated by a vertical magnetic dipole embedded below the sea surface (having a flat and perturbed upper surface) is shown to consist of a lateral-wave and a reflected-wave. Closed-form expressions for the lateral waves near the surface of the sea are obtained and compared with those mentioned for the reflected waves numerically for the con-sidered model.

Lateral electromagnetic waves generated by a vertical electric or magnetic dipole near the plane boundary between two different media like air and earth or air and sea have been the subject of investigation for many years beginning with the work of Sommerfeld. King [

The present study is a further contribution to [

However, these methods, involve lengthy algebra and several transformations, which are very tedious and complicated. The new technique utilized in [

Firstly the form solutions of the far-field, due to a vertical magnetic dipole in a sea (three-layered conducting media) with variable interface are expanded as an infinite series. Then with the aid of the complex image theory [

We shall adopt the following model as illustrated in

The material constants are assumed to be as follows. The dielectric constant in the air, sea, and ground are and, respectively. The magnetic permeability is taken equal to that of the free space in every layer. The

conductivity of the seawater and ground are and, respectively.

Lateral waves are the electromagnetic waves which are generated by vertical or horizontal dipoles on/or near the plane boundary between two electrically different media like air and earth or air and sea or ocean water. However, the lateral wave propagates from the antenna to the surface suffering some attenuation, then propagates in the air without attenuation over a long distance and then arrives at the receiver. Thus the communication is only through the lateral wave, since the direct and reflected waves are almost completely attenuated in the medium as in this case of flat upper surface.

To evaluate the lateral waves, we return to the Equation (29) in [

where, and its real part is positive, is the propagation constant of the medium under consideration., and is the second-kind Hankel functions of order one. Therefore, we can write as

where

According to the complex image theory [

and

where.

Using the first formula of (4) and rearranging properly

and to obtain the lateral waves. Then, we have

where

The first factor in (6) is a slowly varying part, while the second term is rapidly varying. The location of the stationary phase point is given by

The solution of (8) is

where, We can see that the approximation in (9) is valid for

and, then we get

where

In analogy to this approach we can obtain as follows

From (10) and (11), we get

The exponential in the lateral wave term all indicates that the wave travel vertically a distance from the dipole to the boundary surface in the sea, then horizontally along the boundary a distance in the air, and finally vertically in the sea to the point of observation.

In this section we shall calculate the lateral waves for secondary fields in the sea. These will represent the changes which occur in the electromagnetic field of the wave propagation in the sea due to the perturbation applied on the upper surface, where

: The secondary reflected electric field in the sea in any direction,

: The secondary lateral electric field in the sea in any direction, as in

The reflected fieldhas been calculated in the previous paper [

Then,

whereIn this research, there is always for any frequency, therefore, the condition required for (4) is met. Using the first formula of (4) and arranging properly, we get

When, there is an approximate formula for Hankel function as

.

Then, when, the first bracket in is slowly varying part while the second bracket is rapidly varying. The location of the stationary phase point in is given by:

The solution of (19) is given by

where,. We can see that the approximation in (4) is valid for. Apparently the reason of this validity is because the stationary phase point ends up being at.

Using the approach we presented in the pervious paper [

Similarly,

Then, from (21) and (22) we get

By using the following approximation, can be expressed as

In the same way,

Hence,

where,

From (25), we have found the lateral waves near the rough surface of the sea. The physical meaning of the first term in (25) indicates a series of waves that propagate upward from the source and make n round trips between the rough sea surface and the bottom, then travel along the surface on the seawater, and finally arrive at the field point in sea, where is the reflection coefficient at the sea bottom, as in

The second term represents another series of waves that propagate downward from the source first then reflected upward at the sea bottom and travel along the same path. These results also show that and are proportional with, and this means that the electric field in the sea takes the form of the Sommerfeld integral. Also, the result coincides with the result obtained by Long et al. [

The magnitudes of the electric field (the reflected and lateral waves) for the two cases (flat and rough) in sea are computed for different values of the sea thickness and different frequencies.

Case I: Flat surface of the sea: the Figures 5 to 7 show the normalized electric field of the dipole in sea versus the radial distance factor. The depth of the field point is taken equal to 5 m. To attain the numerical calculation, we ascertain that is a match for under the conditions shown in Figures 5 to 7.

We show in these figures that increasing the depth of the sea decreases the value of the electric field. Thus, the previous equations have led to those valid and useful results. These findings also indicate the importance of the derived equations in this research area that could be easily applied in treating related problems.

Plots of the variation of the electric field in the case of the lateral waves are shown in Figures 8 to 10. Also, in these Figures, the numerical calculations ascertain that

is a match for. Also, we show that as the depth of the sea increases, the magnitude of the electric field near the air-sea boundary surface proceeds parallel to the horizontal scale.

In the

Furthermore, we used the same values for the sea depths, but altered the values of the frequency to demonstrate that the absolute reflected electric field increases as we increase the frequency. This is clearly shown in Figures 5-7.

In addition to the reflected wave, we also examined the lateral wave. We used the same sea depth and the same frequencies. Accordingly, we got the previous three Figures 8, 9 and 10. They show that as the sea depth increases and as the frequency decreases, the lateral wave also decreases.

Case II: Rough surface of the sea. The perturbed lateral electric field for the rough upper surface and reflected electric field for the same mode are computed. The part of the sea under investigation takes different thickness (a = 10 m, 25 m, 50 m), the height of the source is taken to be 4 m from the ground (the bottom of the sea). We chose the sea waves represented by the roughness cosine profile with period

10 m and for frequency 10 KHz. Consequently,

Moreover, in

In this paper, we summarized our research work regarding the closed-form expressions for the far fields in the sea. The formulas describe the reflected electric field and the lateral waves near the sea surface for different frequencies 10, 20, 30 KHz. The presented results prove that the lateral waves-in the case of uniform surface are very useful in the navigation with very low frequencies as indicate in

For instance in Figures 8-10, we proved that when we fix the radial distance factor and the sea depth, the lateral waves decreased from 2, 1.5 and 1 as the frequencies decreased from 30, 20 and 10 respectively. Also, from these results we show that the reflected waves can be neglected with respect to the higher values of the lateral waves.

where is the location of the stationary phase point which is given by.

The prime and denote differentiation of with respect to and and.

.