SEISMIC VELOCITIES OF UNCONSOLIDATED SEDIMENTS IN THE VANCOUVER AREA OF BRITISH COLUMBIA

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GEOGRAPHICA 41 1 REFERE.NCES ABOAV. D.A. (1970). The arrangement of grains in a polycrystal, Metallography, 3: 383-90. BOOTS, B.M. (1975).

1 GEOGRAPHICA 41 1 REFERE.NCES ABOAV. D.A. (1970). The arrangement of grains in a polycrystal, Metallography, 3: BOOTS, B.M. (1975). Some observations on the structure of socioeconomic cellular networks, The Canadian Geographer, 19: (1977), Contact number properties in the study of cellular networks, Geographical Analysis, 9: COX, K.R. and AGNEW, J.A. (1974) Optimal and non-optimal territorial partitions: a possible approach towards conflict, Papers of the Peace Science Society (International), 23: CRAIN, I.K. (1972), Monte Carlo simulation of the random Voronoi polygons, Search, 3: (1978), The MonteCarlogenerationofrandom polygons, ComputersandGeosciences,4: DACEY, M.F. (1963), Certain properties of a polygon in a two-dimensional aggregate of polygons having randomly distributed nucleii, Warton School of Finance and Commerce, Unviersity of Pennsylvania (Mimeographed). EVANS, U.R. (1949, The laws of expanding circles ;and spheres in relation to lateral growth of surface films and the grain size of metals, Transactions ofthe Faraday Society, 41: MEIJERING, J.L. (1953), Interface area, edge length, and number of vertices in crystal aggregates with random nucleation, Philips Research Reports, 8: WEAIRE, D. (1974). Some remarks on the arrangement of grains in a polycrystal, Metallography, 7: WOLDENBERG, M.J. (1972), The average hexagon in spatial hierarchies, in R.J. Chorley (ed.), Spatial Analysis in Geomorphology (London: Methuen). [B.N. BOOTS, Wiljlrid Luurier University] SEISMIC VELOCITIES OF UNCONSOLIDATED SEDIMENTS IN THE VANCOUVER AREA OF BRITISH COLUMBIA One of the major problems involved in developing specific geologic histories for the Quaternary is that one is confronted in the field with an often bewildering array of unconsolidated sediments which are not dissimilar in appearance. The task of distinguishing among alluvium, glacial tills, glacio-marine, and marine units is often difficult, and the identification of spatially discontinuous time units of the same facies is usually a still greater problem. Of course, units which are indistinguishable in the field can usually be separated by a detailed analysis of mineralogy, grain size and shape distributions, fabric, compaction, and so on. But such techniques are very time-consuming and many local geological chronologies for the Quaternary are established only after many years of analysis in both field and laboratory. The need for more efficient techniques of characterizing sediment textural properties in the field might be met by surveys of related seismic velocity. Although the positive relationship between seismic velocity and those factors influencing the bulk density of sediment (such as composition, compaction, and water content) is widely recognized, it has not been widely exploited in Quaternary field studies; recent work on Pleistocene river-channel fills and on permafrost distribution provides several exceptions.2 The purpose of this paper is to examine the usefulness of seismic velocities in identifying surficial geological units in the Vancouver area of British Columbia. METHODS AND PROCEDURE Velocities of compressional waves were obtained in the field using a Terra Scout R-150 portable refraction seismic recorder. Means of the travel times based on forward and reverse transects were determined for each survey site; these were located in well-drained and undisturbed areas. Velocity profiles typically indicated a very low velocity in a shallow CANADIAN GEOGRAPHER, XXIV, 4,1980

2 412 THE CANADIAN GEOGRAPHER FIGURE 1. Seismic-velocity distribution for selected geological units near Vancouver, British Columbia. (0.5 m) zone of soil near the ground surface. Beneath this zone was another, usually about 2 m thick, in which velocities were somewhat higher than those above. It is this zone which is considered representative of the sediment unit for the purpose of the seismic analysis. Beneath this representative zone seismic velocities often increased to distinctly greater magnitudes, probably indicating the presence there of groundwater. All velocities were recorded during mid-summer; winter recordings could reasonably be expected to differ because of moisture and soil temperature effects. Seven geological units (from three distinct depositional environments) shown on the New

GEOGRAPHICA 413 TABLE I SIGNIFICANCE LEVELS FOR DIFFERENCE OF MEANS TEST OF SEISMIC VELOCITIES Geological units 2A 28 3 4 5 6 7 I 2A 2B 3 4 5 6 8.61 12.41 4.34 22.26 19.25 15.20 13.98 11.78 8.78 4.

3 GEOGRAPHICA 413 TABLE I SIGNIFICANCE LEVELS FOR DIFFERENCE OF MEANS TEST OF SEISMIC VELOCITIES Geological units 2A I 2A 2B * * *No significant difference at P = Westminster and Sumas map areas of the Geological Survey of Canada were examined in detail.3 Velocity measurements were obtained at points spread as evenly as road access would allow over exposures of the following units: 1. Fraser River Sediments: fluvial silty clay and organic silt. 2. Sumas Drift: glacio-fluvial gravels and sand deposited by streams flowing from wasting glacier ice; formerly mapped as the peripheral sands ( 2~) and central gravel and sand ( 2~) of an alluvial fan known as the Abbotsford Outwash. 3. Fort Langley Formation: glacio-marine silt and clay with minor sand and some gravel; formerly mapped as Cloverdale Sediments. 4. Fort Langley Formation: glacio-marine till-like mixtures of silt, clay, and gravel; formerly mapped as Whatcom Sediments. 5. Capilano Sediments: glacio-marine stony silts and clay; formerly mapped as Newton Stony Clay. 6. Vashon Drift: sandy loam lodgement till; formerly mapped as Surrey Till. 7. Sumas Drift: sandy loam lodgement and flow tills; formerly mapped as Sumas Till. RESULTS OF THE SURVEYS The results of the seismic-velocity surveys are graphically summarized in Figure 1. The significance matrix of 1 values (Table 1) indicates that there are significant differences (at P = 0.025) between the mean velocities of most pairs of geological units; only distinctions between two of the glacio-marine units and between the two tills are not statistically significant. Nevertheless, these results are clearly disappointing; the considerable overlap in the velocity distributions indicates that it would be foolhardy to base identification of a deposit onjust afew seismic measurements. In fact the results further suggest that the sample sizes adopted in the survey represent near-minirnal sizes for distinguishing among the means in several cases. This fact considerably limits the: diagnostic efficiency of a seismic analysis in this particular region. Nevertheless, where large samples (>50?) of velocity measurements are available, the trends they describe may be useful in establishing or confirming geological boundaries in the field area. For example, Figure 2 shows an 1 I-km seismic velocity transect which appears to be responsive to the type of surficial geology occurring along it. The transect from A to B indicates seismic velocities of mls in the Sumas Drift and a distinct drop to about 400 m/s just beyond the boundary with the Fraser River Sediments. This low velocity is maintained with little variation until the Fort Langley Formation is encountered. Here the seismic velocities increase abruptly to more than 1200 mls and subsequently drop across a small wedge of Vashon Drift, only to increase yet again at the boundary with the Capilano Sediments. The low velocity at B probably corresponds to apocket of Vashon Drift, although it is mapped as a peripheral exposure of Capilano Sediments; it is often extremely difficult to distinguish between these units in the field.

$414 LE GEOGRAPHE CANADIEN Sumas Drift Fraser River Sediments Fort Langley Formation \ Vashon Drift Capilano Sediments E I600 1wO 500 I 0 A 10 2.0 3.0 4.0 I 5.0 6.0 7.0 8.0 9.0 10.0 11.0 Km.$

4 414 LE GEOGRAPHE CANADIEN Sumas Drift Fraser River Sediments Fort Langley Formation \ Vashon Drift Capilano Sediments E I600 1wO 500 I 0 A I Km. B FIGURE 2. A seismic velocity transect across five geological units near Vancouver, British Columbia. A far more convincing separation of the sample distributions of seismic velocity can be made on the basis of the general environment of deposition. The units shown in Figure 1 tend to cluster in three velocity groups corresponding with the three environments of deposition: fluvial/glacio-fluvial (samples 1, 2A, and 2B), glacio-marine (samples 3, 4, and 5), and ice contact (samples 6 and 7). The basic data have been combined to provide the three velocity distributions shown in Figure 3. The fluvial/glacio-fluvial material (see Figure 3 ~ has ) a relatively low median velocity (V,J of 533 m/s, probably resulting from the intergranular shearing common in relatively dry, low density, and noncohesive sands and from the plastic rather than elastic deformation of the organics. The velocity distribution is quite peaked, is markedly skewed towards small velocities, and has a relatively small range of about 1000 m/s. The marked skewness is caused by the preponderance of very low-velocity organic sediments. The second environmental group (see Figure 3 ~), consisting of the tills, has a considerably higher median velocity of 1062 m/s. This distribution is more symmetrical than that for the fluvial environment and includes a velocity range of about 2000 m/s. It seems rather likely that the relatively fine grain size, together with glacial compaction, yield velocities in this case that are higher than those associated with sediments of the first group. The glacio-marine sediments display the highest seismic velocities (V, = 1472 m/s) and the greatest velocity range (about 3000 mls). These characteristics suggest that the glaciomarine sediments have a higher and more variable density than materials from the other two environments. Actually, the relative seismic velocities of the second and third groups shown in Figures 38 and 3c are somewhat surprising when compared with the findings of Easterbrook in northern Wa~hington;~ he found that ice-compacted tills have higher bulk-densities and lower void ratios than glacio-marine units in that area. In contrast, it seems that any compaction differences in the present case are obscured by the marked difference in grain size between tills and glacio-marine sediments: the tills average 60 per cent sand, 37 per cent silt, and 3 per cent clay, while the glacio-marine sediments consists of per cent sand, about 50 per cent silt and per cent clay.5 Furthermore, the moisture content of the glacio-marine units is likely to be higher than that in the tills during the summer months. CONCLUSION The foregoing analysis indicates that average seismic velocities can provide a means of distinguishing between most of the selected geological units. It provides also the not surprising conclusion that seismic velocities are more effective when distinguishing between sediments from different depositional environments than between units of the same depositional environment. But, in both cases, distinctions are possible only if relatively large numbers of velocity measurements are available for analysis. Offsetting this disadvantage is the fact that seismic velocity measurements are rapidly obtained (about one every 5 to 10 minutes on site).

5 GEOGRAPHICA 415 FIGURE 3. Seismic-velocity distributions for sediments from three depositional environments near Vancouver. British Columbia. NOTES AND REFERENCES 1 See, for example, H. Brandt, A study of the speed of sound in porous media, Journal ofapplied Mechanics, 22 (1955). pp , and Factors affecting compressional wave velocity in unconsolidated marine sediments, Journal ofrhe Acousrics Society ofamerica, 32 (1960), pp ; M.R.J. Wyllie, A.R. Gregory, and L.W. Gardner, Elastic wave velocities in heterogeneous and porous media, Geophysics, 23 (1956), pp , and Experimental investigation of factorsaffecting elastic wave velocities in porous media, Geophysics, 23 (1958), pp ; J.W. Nafe and C.L. Drake, Variation with depth in shallow and deep water marine sediments of porosity, density, and velocities ofcompressional and shear waves, Geophysics, 22 (1957), pp ; B.O. Hardin and R. Richart,

6 416 THE CANADIAN GEOGRAPHER Elastic wave velocities in granular soils, Journal of Soil Mechanics (Foundations Division), 89 (1963), pp ; W. Brutsaertand N.J. Luthin, The velocities of sound in soils near the surface as a function of the moisture content, Journal of Geophysical Research, 69 (1964). pp ; G.P. Eaton and J.S. Watkins, The use of seismic refraction and gravity methods in hydro-geologic investigations, in L. W. Morley (ed), Mining and Groundwater Geophysics (Geological Survey of Canada, Economic Geology Report 26, 1967), pp ; and A.A. Briones and G. Vehara, Soil elastic constants: I. Calculations from sound velocities, Soil Science Society ofamerica Journal, 41 (1977), pp J.S. Watkins and A.M. Spieker, Seismic refraction survey of Pleistocene drainage channels in the lower Great Miami River Valley, UnitedStates GeologicalSurvey Professional Paper, 605-B (1971), pp. 1-17; and W.J. Scott and J.A. Hunter, Applications of geophysical techniques in permafrost regions, Canadian JournalofEarth Sciences, 14 (1979), pp , and references therein. 3 The standard reference maps and geological reports for this area are by J.E. Armstrong, Surficial geology of Vancouver area, British Columbia, Geological Survey of Canada, Paper and Map (1956); Surficial geology of New Westminster map-area, British Columbia, Geological Survey of Canada, Paper 57-5 and Map (1957); and Surficial geology of Sumas map-area, British Columbia, Geological Survey of Canada, Paper 55-9 and Map 92~/1(1960). Although Armstrong has updated the geological maps for the area they had not been published at the time of writing. Because they will soon be available (as Maps 1484~ and 1485~ ofthe New Westminster and Mission map-areas respectively), however, the revised nomenclature has been adopted in this study. 4 D.J. Easterbrook, Void ratios and bulk densities as means of identifying Pleistocene tills, Geological Society ofamerica Bulletin, 75 (1964), pp These data are taken from the published Armstrong papers noted in 3. [EDWARD J. HICKIN, Simon Fraser University] THE TREATMENT OF COMPARATIVE ADVANTAGE IN ECONOMIC GEOGRAPHY TEXTBOOKS The principle of comparative advantage provides considerable insight into our understanding of two fundamental questions in the field of economic geography. These are, first, why a region specializes in the production of specific goods and, second, why a specific movement of goods takes place between particular regions. These two questions are closely related for, as Isard (1956: 53) noted, (1) location cannot be explained unless at the same time trade is accounted for and (2) trade cannot be explained without the simultaneous determination of locations. Given the importance of the questions to which the comparative advantage concept is relevant, it might be expected that it would have received a careful and thorough treatment in introductory textbooks in economic geography. Surprisingly, this is not the case, as I will attempt to show in this review. First, however, a brief theoretical exposition should prove helpful. Probably the best way to illustrate what is meant by comparative advantage is to consider a simple example of the type used by Ricardo. Suppose that there are only two countries in the world, x and Y, and that they produce only two goods, wheat and cloth. Assume that labour is the only production factor used in producing the goods. Assume also that the wheat and cloth can be transported readily between the countries at zero transport costs, but that labour cannot move from one country to the other. Despite the lack of realism in these assumptions, an examination of two cases involving the hypothetical countries can provide considerable insight into the principles underlying specialization and trade. In the first case suppose that, for each man-day of labour, country x can produce more wheat but less cloth than country Y. x is said to have an absolute advantage over Y in the

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