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a) Map interpretations based on scattered direct observations. Level 2: (Softer Constraims): Indirect o r in ferred o bservations about the Earth. Levell: (Firm Constraints): D irect observations of the Earth that you can put in your hand: a) Outcrop samples. In approaching a problem, it is wise to keep in mind a hierarchy of constraims (Fig. The problem is like that of the 'blind men and the elephant ' the interpretation depends largel y on our sample locations and the overall density of sampling. ' Nonuniqueness means that it is possible to offer more than one interpretation that agrees with all available information. Rarely is a region so well constrained that we can perceive of only one possibility for its subsurface interpretation we therefore say that most interpretations of geophysical data ar~ _'nonunique. Straints we have on a problem, the more likely we are to come to a unique solution. that result from inserting the slopes and intercept time into the inversion equations. V2), and the thickness of the upper layer (h). d) Model of seismic velocities for two layers (V. c) Equations that can be used to inven observations of slopes and T-axis intercept time (see Chapter 4). b) Two-layer model showing parameters that can be read from the observed data: slope of the direct arrival: T-axis intercept time (t 1): and slope of the critically refracted arrival. V1 t 1/(2cos8c} FIG URE 1.6 /lll'erSiOJI example a) Observed seismograms showing times of arrival (T) of direct and critically refracted waves at seismometers placed'increasing distance (X) from the source.1/(S/ope of Refracted Wave) Be = Critical Angle.however, we may learn a great deal about a region 's subsurface geology and evolution.Those other observations, in the form of geological and other geophysical data, are constraints. When that technique is combined with other observations. Alone, a geophysical technique may not tell us much about the Earth. In many respects, geophysical techniques are just another type of rock hammer we bang on the Earth and listen to what the Earth tells us about itself. to see what's there and analyze how it got to be that way.

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Geologists often use rock hammers to crack open the Earth. computed'} profile matches the observations we might consider the adjusted model as one potential interpretation of the observed data.Ĭonstraints No matter what methods are used to interpret geophysical data, we should not lose sight of the objective: We make observations of cercain properties of the Earch in order to imerpret the nawre of the Earth.Thicknesses and densities of layers are then adjusted until the predicted ('calculated ,' or calculations from the subsurface model predict the gravity profile that would result.Layers with different densities are assumed:. The forward modeling of an observed gravity profile is illustrated in Fig. We are accustomed to using inversion in math and physics courses, when we ''plug into Observed parameters are inserted into mathematical equations that yield a model of the seismic velocities and thicknesses of layers. 1.6 shows an interpretation (model) that results from the inversion of seismic refraction observations. ' What caused what we obser ved?' Inversion uses mathematical equations to calcula te a subsurface model from observed data fonvard modeling assumes a subsurface model and calculates observations that would result. Methods Both inverse and forward methods are used to interpret geophysical observations. The ''ground mo tion ' (displacement, velocuy, or accelerallon of the rela te to the passage of seismic wav~s through the Earth.

Seismic R elativelv small and rapid, up-and-down or sideways movements of Earth 's surface, mea.s ured by a seismometer. GeophysJcal techniques employed at or near E arth's surface include seismic, potemial field and hea1 flow measurements (Fig. ations in terms of properties within th ~ Earth (for example, densJty d istributions that locally change the gravitational acceleration).OVERVIEW OF GEOPHYSICAL TECHNIQUES Measuri:ments of natural or induced properties are commonly made at the surface of the Ezr:h (for example, gravi tational accelera tion).

Includes bibliographical refe rences ~nU inde.' ro srudy r/1ic structures on seism ic retlection profik:s. Whole earth geophysics: an introductory textbook for geologists and geoph ysicists I Rohcrt J. Library of Conbrre~s C :ltnl o~in g-i n - Publication O:lta North America Topography and Batnymc:try 6000 RobortJġ11111 il!llllllllllllillliilllllll IIIII Whole Earth Ge9physics An Introductory Textbook for Geologists and Geophysicists