The deep interior of the Earth remains a major frontier, since most of the subsurfac.... ocean explor

The deep interior of the Earth remains a major frontier, since most of the subsurface exploration so far undertaken has been limited to the Earth's uppermost crust. The methods used in exploring the Earth's subsurface (as well as its surface in many cases) are of two general types: direct and indirect. Indirect methods involve geochemical analysis and measurement and geophysical surveys of such phenomena as reflectivity, gravity, magnetism, seismic waves, and heat flow. and other techniques of direct exploration are of somewhat less significance in the scientific study of the deep Earth, because of high cost and the limited depth attainable. As a consequence, scientific investigators have to rely heavily on measurements, particularly of seismic waves associated with earthquakes, to secure information about the Earth's interior and its dynamic processes. Scientific investigation of the ocean, which includes the study of the physical and chemical properties of seawater, all fo! rms of marine life, and the geologic and geophysical features of the ocean floor, has also proved beneficial. Exploration of the oceanic depths also has resulted in the discovery of previously unknown forms of marine life and made it possible to determine the distribution of diverse fish populations. Scientific study of the ocean involves the collection of data on ocean currents (surface and bottom), seawater temperature and salinity, marine life, and the topography of the ocean floor. Remote sensing has become an increasingly important means of exploring the ocean because it can provide a sweeping view of the ocean.