Another seismic method which has become more commonplace 

 is the REFLECTION TECHNIQUE (fig. 17. 10). This gives consid- 

 ably less information than the seismic refraction technique in 

 that it does not indicate the velocity in the sediments. You only 

 learn of the reflection time to the various horizons. This, 

 however, is very useful for some particular cases, as will be 

 shown later . 



Figure 17. II shows a record made with a sparker sound 

 source. This is just the sound of an electrical spark under water, 

 recorded from a hydrophone towed from the ship. Two gains are 

 shown: the top is a low gain; the bottom is a high gain. Several 

 of the layers in the bottom are visible and show some of the 

 complex geology at the continental margin off New York. There 

 are various types of sound sources being used for this, and I 

 expect there will be many more. About every six months a 

 new one comes along that enables us to do much more than the 

 last one. 



Figure 17. 12 is a vertical reflection profile made with 

 half-pound shots fired at 2-minute intervals across the PUERTO 

 RICO TRENCH. I would particularly like to call your attention 

 to (I) the abyssal plane part, and to (2) the many reflection 

 horizons. The rock surface comes down rather abruptly (3), 

 comes up in a pinnacle in the middle (4), down again (5), and 

 up along the side (6). You see there are about 20 reflection 

 horizons (7). There are about four kilometers of sediments 

 (8), about a half a kilometer (9). and about a kilometer and a 

 half (10), on the different parts of the trench. 



These are the types of sections that we have been getting 

 recently. Our vessel the Vema has made about 30,000 miles 

 of such geological sections which are rather startling, even 

 to us who have been startled very often by what the sea tells 

 us. 



Besides the explosive sources and the spark sources, a 

 noise source known as the thumper, which is an aluminum 

 plate caused to jump away by the eddy currents in it, an 

 oxyacetylene gas chamber, which is exploded by a spark, and, 

 more recently, a compressed air gun have been used. Each 

 one of these has a little bit more energy than the previous one 

 and allows us to see the ocean sediments in a little more detail. 



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