Fathometers surveys are used primarily for determining water depths.

Basic Concept: Fathometer surveys determine water depths by repeatedly transmitting seismic energy through the water column and recording the arrival time of the reflected energy from the water bottom. The instrument calculates the water depth from these data and prints a depth value as a continuous graphic profile. Most fathometers use a narrow bandwidth 200 kHz seismic signal. They provide accurate depth information, but very little information about the subbottom. Fathometers that use a lower frequency, e.g., 20 kHz, can detect reflections from subbottom interfaces such as the bottom of an infilled scour hole.

Fathometer systems come with black-and-white chart recording systems and in-color systems. Colors are often assigned according to the different amplitudes of the reflected signals. Color step sizes as low as four db are now available, allowing quite small changes in the reflected signals to be observed. An event marker button is often available, allowing vertical line marks to be placed on the records when specific locations are selected by the operator. Sometimes the data can be downloaded to a computer, allowing digital processing to be done along with data plots.

Data Acquisition and Data Processing: Fathometer surveys are conducted while traveling in a boat at a moderate speed. Typically, the transducer is mounted on the side of the boat and placed in the water. Data recording is essentially automatic with a t recorder providing a hard copy of the data; a computer screen may also be used for the display. The data may be stored on magnetic tape for further processing and plotting. As with the CSP method, GPS can be used to position the data.

Data Interpretation: Interpretation is accomplished by viewing the plotted data. The response of specific objects may be used if these were noted on the records using a button marker.

Figure 62 shows fathometer data recorded with a 200 kHz transducer. Note that only the water depth is observed in these data. Because of the high frequency, little energy is transmitted into the bottom sediments, and, thus, no reflections are observed from within the sediments.

Figure 63 presents fathometer data using a 3.5 kHz transducer. Because of the lower frequency, some of the energy is transmitted into the sediments, and reflections are seen.

Limitations: Most of the limitations have already been described and include limited penetration into the sub-bottom sediments with high-frequency fathometers, although low-frequency fathometers can provide some data about the sediments.

Figure 62. Fathometer data recorded with 200 kHz transducer

Figure 63. Fathometer data recorded using a 3.5 kHz transducer.