EVALUATION OF DUPLICATE SAMPLES REGIONAL SEDIMENT SURVEYS , BRITISH COLUMBIA
INTRODUCTION The ability to discriminate real trends related to geological and geochemical causes from those that result from spurious factors such as sampling and analytical errors is of paramount importance in the suI:cess of geochemical data interpretation. Since the estimate of reproducibility (precision) allows us t o quantify the amount of variation due t o sampling and laboratory analysis it is an integral part of the evaluetion of geochemical data and should be conducted prior to carrying out any detailed interpreation. As part of our continuing study of rapid but thorough evaluation procedures for multi-element stream-sediment data (for example, Matysek. et a/., 1981) we designed a systematic computer-oriented method of evaluating the quality of geochemical survey data based on field-site duplicates. It irlcorporates a bias test, an analysis of variance technique, and the Thompson and Howarth (1976) approach to qua'ytifying precision. This detailed procedure utilizes the type and quality of data incorporated in various regional geochemical programs undertaken by the British Columbia Ministry of Energy, Mines and Petloleum Resources (the Ministry), but can be adapted easily for other programs, GENERAL METHODOLOGY steps: In brief, our general approach t o evaluation of the quality of geochemical data sets involves the following (1) Extraction of a t least 50 independent, field-site duplicate pairs from a geochemical data set f cmr use in subsequent data analysis. (2) Determination of the degree of systematic bias between duplicate pairs based upon the number of cases in which the first observation is greater than the duplicate. (3) Evaluation of the duplicated geochemical data set in terms of metal variability a t the regional, between-sample site and a t the local, within-sample site, sampling, and analvtical levels by a two-factor analysis of variance technique. (4) Quantification of the within-sample site variability by estimating precision utilizing the Thsmpson and Howarth (19761 method. D A T A Data obtained from the three most recent regional geochemical surveys undertaken Iby the Ministry (1981 I were evaluated using this method. The base data consist o f analyses of stream sediments and water samples collected a t an average density of one sample per 17.3 square kilometres over NTS map-sheet areas 92H, 921, and 92J. Silt samples were field dried and the minus 80-mesh (177 microns) fraction retained for subsequent analyses. The samples were analyzed for zinc, copper, lead, nickel, cobalt, silver, manganese, iron, molybdenum, arsenic, and antimony by the atomic absorption …
A PRELIMINARY EVALUATION OF CATEGORICAL FIELD OBSERVATIONS FOR REGIONAL STREAM SEDIMENT SAMPLES ( 82 F ,
INTRODUCTION We are in the process of examining regional stream geochemical data obtained during i3 joint Federal-Provincial Uranium Reconnaissance Program of NTS map-areas 82 F and K released in 1978. The purposes of our study are to evaluate the usefulness of individual variables coded in the course of these surveys, to develop a statistical procedure for extracting useful information from the data, and to utilize the data base as an effective means of defining problems of geological interest that warrant further inv,estigation. Our work i s developmental in nature and is confined to data for NTS map-area 82F. Initial work (Sinclair and Fletcher, 1980) considered only numeric (concentration) data and emphasized a systematic approach to i t s evaluation using standard statistical procedures. However, in addition to the quantitative data the Uranium Reconnaissance Program files contain field observations describing the drainage sediments and collection site for each sample. For example, in addition to rock type, coded comments are recorded to describe the presence of contamination, the nature of the bank material, water and sediment colour, texture, the presence of organics or precipitates, and such physiographic features as landscape maturity, drainage pattern, and stream class. Although similar data are often collected as part of drainage surveys, they are so seldom utilized in any systematic fashion that a recent review (Meyer, et a/., 1979) suggested limiting field observation to those of proven significance. Our ongoing studies attempt to establish which field observations are significant and to assess their influence on metal content of the drainage sediments. Two complementary procedures are being utilized: (1) Contingency tables and the chi-square test, and (2) Duncan's multiple range test. CONTINGENCY TABLES Two-way contingency tables represent an ordered arrangement of counts of 'intersection' of pairs of variables. A simple example is illustrated in Table 1 for rock type versus stream velocity for 1 318 stream sediment samples from NTS map-area 82F. The count of 245 a t the intersection of the GRNT-35 and MOD columns means that 245 sediments from the area are characterized by both these features. With such a table one can quickly, if subjectively, evaluate the distribution of one variable in relation to another. For example, in Table 1 we can look at the distribution of samples in velocity categories for individual rock types (that is, distribution of counts along rows) and compare the distribution for one rock type with the distribution for another. …
Exploration geochemistry—Distribution of elements and recognition of anomalies
The exploration geochemist faces a serious problem in reconciling theoretical statistics with the empirical distribution of elements in materials in the earth's crust. The results of computer-simulation experiments being conducted by the Exploration Geochemistry Group at the University of New Brunswick illustrate some of the problems of the relation between frequency distributions and spatial distributions of elements and raise some interesting questions about sampling patterns and physical size of samples. Most importantly, the simulation experiments and empirical examples support the contention that attempts to make an assumption of normality more efficient by performing log transformations on positively skewed data may defeat the purpose of statistical analysis for exploration work. New developments in computer-data presentation and interpretation, including population sorting and classification, illustrate the increasing use of the computer in exploration geochemistry.