GRE READING PASSAGE OF THE WEEK

The term “remote sensing” refers to the techniques of measurement and interpretation of phenomena from a distance. Prior to the mid-1960’s the interpretation of film images was the primary means for remote sensing of the Earth’s geologic features. With the development of the optomechanical scanner, scientists began to construct digital multispectral images using data beyond the sensitivity range of visible light photography. These images are constructed by mechanically aligning pictorial representations of such phenomena as the reflection of light waves outside the visible spectrum, the refraction of radio waves, and the daily changes in temperature in areas on the Earth’s surface. Digital multispectral imaging has now become the basic tool in geologic remote sensing from satellites.

The advantage of digital over photographic imaging is evident: the resulting numerical data are precisely known, and digital data are not subject to the vagaries of difficult-to-control chemical processing. With digital processing, it is possible to combine a large number of spectral images. The acquisition of the first multispectral digital data set from the multispectral scanner (MSS) aboard the satellite Landsat in 1972 consequently attracted the attention of the entire geologic community. Landsat MSS data are now being applied to a variety of geologic problems that are difficult to solve by conventional methods alone. These include specific problems in mineral and energy resource exploration and the charting of glaciers and shallow seas.

A more fundamental application of remote sensing is to augment conventional methods for geologic mapping of large areas. Regional maps present compositional, structural, and chronological information for reconstructing geologic evolution. Such reconstructions have important practical applications because the conditions under which rock units and other structural features are formed influence the occurrence of ore and petroleum deposits and affect the thickness and integrity of the geologic media in which the deposits are found.

Geologic maps incorporate a large, varied body of specific field and laboratory measurements, but the maps must be interpretative because field measurements are always limited by rock exposure, accessibility and labor resources. With remote-sensing techniques it is possible to obtain much geologic information more efficiently than it can be obtained on the ground. These techniques also facilitate overall interpretation. Since detailed geologic mapping is generally conducted in small areas, the continuity of regional features that have intermittent and variable expressions is often not recognized, but in the comprehensive views of Landsat images these continuities are apparent.

However, some critical information cannot be obtained through remote sensing, and several characteristics of the Landsat MSS impose limitations on the acquisition of diagnostic data. Some of these limitations can be overcome by designing satellite systems specifically for geologic purposes; but, to be most effective, remote-sensing data must still be combined with data from field surveys and laboratory tests, the techniques of the earlier twentieth century.

1) By using the word “interpretative” in line 40, the author is indicating which of the following?
(A) Some maps are based more on data from aerial photography than on data from field operations.
(B) Some maps are based almost exclusively on laboratory measurements.
(C) Some maps are based on incomplete data from field observations.
(D) Some maps show only large geologic features.
(E) Some maps can be three-dimensional.

2) With which of the following statements about geologic mapping would the author be most likely to agree?
(A) Geologic mapping is basically an art and not a science.
(B) Geologic mapping has not changed significantly since the early 1960’s.
(C) Geologic mapping will have limited practical applications until remote-sensing systems are perfected.
(D) A developmental milestone in geologic mapping was reached in 1972.
(E) Without the present variety of remote-sensing techniques, geologic mapping could not be done.

3) According to the passage, measurements of which of the following can be provided by the optomechanical scanner but not by visible-light photography?
(A) The amount of visible light reflected from oceans
(B) The density of foliage in remote areas on the Earth’s surface
(C) Daily temperature changes of areas on the Earth’s surface
(D) The degree of radioactivity emitted by exposed rocks on the Earth’s surface
(E) Atmospheric conditions over large landmasses

4) It can be inferred from the passage that a major disadvantage of photographic imaging in geologic mapping is that such photography
(A) cannot be used at night
(B) cannot focus on the details of a geologic area
(C) must be chemically processed
(D) is always enhanced by digital reconstruction
(E) cannot reflect changes over extended periods of time

5) It can be inferred from the passage that Landsat images differ from conventional geologic maps in that Landsat images
(A) reveal the exact size of petroleum deposits and ore deposits
(B) indicate the continuity of features that might not otherwise be interpreted as continuous
(C) predict the movements of glaciers
(D) provide highly accurate data about the occurrence of mineral deposits
(E) reveal the integrity of the media in which petroleum deposits and ore deposits are found

6) The passage provides information about each of the following topics EXCEPT:
(A) the principal method of geologic remote sensing prior to the mid-1960’s
(B) some of the phenomena measured by digital multi-spectral images in remote sensing
(C) some of the practical uses of regional geologic maps
(D) the kinds of problems that are difficult to solve solely through conventional methods of geologic mapping
(E) the specific limitations of the Landsat multi-spectral scanner

7) The passage suggests which of the following about the “conventional methods” mentioned in line 29?
(A) They consist primarily of field surveys and laboratory measurements.
(B) They are not useful in providing information necessary for reconstructing geologic evolution.
(C) They have rarely been used by geologists since 1972.
(D) They are used primarily to gather compositional information about geologic features.
(E) They are limited primarily because of difficulties involved in interpreting film images.