136 * Marine Minerals: Exploring Our New Ocean Frontier 



of a 10-by 20-kilometer area could cost on the or- 

 der of $3 million. 



Resolution also continues to improve, assisted 

 by better navigation, positioning, and control meth- 

 ods. An innovation which promises to further im- 

 prove resolution is the use of chirp signals rather 

 than sound pulses. Chirp signals are oscillating sig- 

 nals in which frequency is continuously varying. 

 Using computer-generated chirp signals, it is pos- 

 sible to tailor and control emitted frequencies. In 

 contrast, pulse sources produce essentially uncon- 

 trolled frequencies, generating both useful and un- 

 needed frequencies at the same time. 



About 10 million miles of seismic profiles have 

 been run in the U.S. EEZ. Most of these data are 

 deep penetration profiles produced by companies 

 searching for oil and are therefore proprietary. The 

 Minerals Management Service within the U.S. De- 

 partment of the Interior (MMS) purchases about 

 15 percent of the data produced by industry, most 

 of the data are held for 10 years and then turned 

 over to the National Geophysical Data Center. 

 NGDC archives about 4 million miles of public 

 (mostly academic) seismic data. Much of this data 

 is for regions outside the EEZ. NGDC also archives 

 USGS data, most of which are from the EEZ (see 

 ch. 7). 



It is possible to acquire shallow-penetration seis- 

 mic information (as well as magnetic and gravity 

 data) at the same time as bathymetric data, so that 

 surface features can be related to vertical structure 

 and other characteristics of a deposit. NOAA ac- 

 knowledges that simultaneous collection of differ- 

 ent types of data could be accomplished easily 

 aboard its survey ships. Additional costs would not 

 be significant relative to the cost of operating the 

 ships, but would be significant relative to currently 

 available funds. The agency would like to collect 

 this data simultaneously if funds were available. 

 NOAA hopes to interest academia and the private 

 sector, perhaps with USGS help, to form a con- 

 sortium to coordinate and manage the gathering 

 of seismic and other data, using ships of opportu- 

 nity.*^ The offshore seismic firms serving the oil 

 and gas industry are opposed to any publicly funded 



data acquisition that could deprive them of busi- 

 ness opportunities. All but very shallow penetra- 

 tion data generally are of interest to the petroleum 

 industry and therefore could be considered compet- 

 itive with private sector service companies. 



Magnetic Methods 



Some marine sediments and rocks (as well as 

 sunken ships, pipelines, oil platforms, etc.) contain 

 iron-rich minerals with magnetic properties. Mag- 

 netic methods can detect and characterize these 

 magnetic materials and other features by measur- 

 ing differences (or anomalies) in the geomagnetic 

 field. Magnetic (and gravity) techniques are inher- 

 ently reconnaissance tools, since the data produced 

 must be compiled over fairly broad areas to detect 

 trends in the composition and structure of rock. 

 However, spatial resolution, or the ability to de- 

 tect increasingly fine detail, varies depending on 

 the design of the sensor, the spacing of survey lines, 

 and the distance of the sensor from the source of 

 anomaly. 



Satellite surveys are able to detect magnetic 

 anomalies on a global or near-global scale. Satel- 

 lite data are important for detecting global or con- 

 tinental structural trends of limited value to re- 

 source exploration. At such broad scale, mineral 

 deposits would not be detected. Airplane and ship 

 surveys record finer scale data for smaller regions 

 than satellites, enabling specific structures to be de- 

 tected. The closer the sensor to the structure be- 

 ing sensed, the better the resolution, but the time 

 required to collect the data, as well as the cost to 

 do so, increases proportionately. 



Regional magnetic surveys, usually done by air- 

 plane, can detect the regional geologic pattern, the 

 magnetic character of different rock groups, and 

 major structural features which would not be noted 

 if the survey covered only a limited area.** For ex- 

 ample, oceanic rifts, the transition between con- 

 tinental and oceanic crusts, volcanic structures, and 

 major faults have been examined at this scale. Re- 

 gional magnetic surveys also have been used ex- 

 tensively in exploring for hydrocarbons. Accurate 

 measurement of magnetic anomalies can help ge- 



*^C. Andreasen, NOAA EEZ project manager, interview by W. 

 Westermeyer at NOAA, Rockville, MD, Apr. 22, 1986. 



'*P.V. Sharma, Geophysical Methods in Geology (New York, NY: 

 Elsevier Science Publishing Co., 1976), p. 228. 



