Wavelength Depth Research Articles

Coupling effects of two electric dipoles on an interface

Numerical modeling of thin‐wire antennas is interesting to people working on antenna design and to people working on the processing and interpretation of radar data. They have different demands regarding the output of the model code. Our primary interest lies with improving image resolution through processing of ground‐penetrating radar data. For this goal the effects of two antennas placed close together on the Earth's surface are studied. To study the antenna to antenna coupling and antenna to ground coupling, a model for linear dipoles is used. The model uses a thin‐wire approximation for the transmit and receive antennas. The antennas are placed just above the air‐Earth interface. The Earth is modeled as a horizontal layer stack. Using this model, we study the coupling effects between two parallel antennas, either in each other's E‐plane or H‐plane. For ordinary antenna distances, the coupling between two parallel antennas when they are resistively loaded is on the order of −70 dB, which is not negligible for GPR applications. The influence of the lower half‐space on antenna behavior is strong but limited to a fraction of the wavelength in depth.

Photometric Investigations of Precipitable Water and Optical Depth Wavelength Exponents in an Urban Area

Abstract A six-channel Volz sunphotometer was used in the St. Louis urban area during Project METROMEX 1976 to monitor aerosol loading and atmospheric precipitable water. A weighted least-square fit of photometric observations to spatially and temporarily proximate radiosonde measured precipitable water vapor has enabled determination of the exponent (1/1.993) in the precipitable water equation indicating compliance with the square-root absorption law of band models. Wavelength exponents calculated from least-square fits of water vapor transmission band optical depths to wavelengths have been found to be represented by α = 1.71 ± 0.54 for a set of 70 observations. Modal values of the wavelength exponent are centered at 1.60 and 1.90. Good agreement has been found between photometrically estimated aerosol distributions and in situ aircraft measured aerosol distribution daily trends.

Acoustic variability due to layered finestructure in the Arctic

Acoustic transmission measurements, in the frequency range 10–60 kHz, have been carried out in the presence of layered, thermal fine structure under arctic ice. The term “layered fine structure” refers to relatively stable, slowly changing thermal layers in the ocean that have a vertical scale of about a meter to several meters. Their horizontal extent may be 10–100 times larger. In these measurements, acoustic transmission is strongly affected by the presence, near the transmitter, of a layer or layers with a sound speed maximum. In the literature these warm layers are sometimes called “antiwaveguide” or “barrier” layers. Three types of space-time variability have been found in the received intensity versus depth records: (1) A small random variability observed at relatively steep transmission angles, (2) a frequency-independent variability which can be related to convergence/divergence of rays in the refraction field of the layer, and (3) a frequency-dependent variability which seems to arise from the interference between direct rays and those sharply refracted by the layer. A spectral analysis of the interference region confirms that the spatial wavelength in depth is inversely proportional to the acoustic frequency. [Work supported by Arctic Submarine Laboratory of the Naval Ocean Systems Center, San Diego, CA.]

Digital acquisition and interactive processing of ultrasonic echoes.

The requirements for an interactive digital signal processing system for ultrasonic pulse-echo signals are discussed. A system based on an Interdata Model 80 mini-computer and micro-processor interface is described. The system is capable of acquiring ultrasonic data at a sampling rate of 6 MHz. Ultrasonic B-mode data may be acquired in Line Mode, when echo waveform data and transducer position and orientation are stored, or in Section Mode when the data is converted directly into picture form in memory in the same way that a standard echogram is formed on the screen of an oscilloscope. In each case the data for single complete high resolution echogram may be acquired in less than 15 sec. It is shown that the 6 MHz sampling rate is sufficient to faithfully preserve the echo waveshape of a 2 MHz system independently of the relation to the phase of the sampling. Also shown is a cross-sectional echogram of the pregnant uterus, and its digital representation with a raster density of 80 × 100 and 160 × 200 picture elements. The computer is programmed with an interactive program to allow ultrasonic signals to be acquired, stored, processed and examined with the convenience of a desk calculator. Sample operations are illustrated including data interpolation, spectrum analysis, filtering and complex signal deconvolution. The ability of deconvolution techniques to resolve targets separated by less than one wavelength in depth is demonstrated. Possibilities of further processing techniques are outlined.