Title

Quantitative Analysis of Unconsolidated Coarse Fluvial Sediments from the Boise Hydrogeophysical Research Site: Statistical Analysis of Core and Porosity Data

Publication Date

10-2003

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Geology

Department

Geosciences

Major Advisor

Warren Barrash

Abstract

The qualitative and quantitative analysis of the distribution of hydrologic parameters such as porosity and permeability increases our ability to understand and model ground water flow and contaminant transport in heterogeneous hydrogeologic systems. Core and porosity logs from the Boise Hydrogeophysical Research Site (BHRS), a research well field emplaced in a shallow, unconsolidated, cobble-dominated alluvial aquifer, were analyzed and their statistical characteristics were employed to determine the textural variation within this type of deposit as a whole. The spatial variations of these characteristics within these deposits exhibit strong controls over permeability distributions of the deposit.

Core from 12 of the 18 wells at the BHRS were analyzed in detail and divided into sample intervals. Each of these sample intervals underwent a grain-size analysis that was used to determine physical characteristics of the core material and to derive a grain- size based lithotype for each sample. After some simple corrections for artifacts of the drilling process and elimination of intervals consisting of drilling slough, the analyzed core consisted of 953 sample intervals that were classified into one of five lithotypes. Three of these lithotypes contain a large proportion of matrix material, but represent a small proportion of the deposit. These three lithotypes are: sand to fine pebble, floating gravel, and bimodal. The majority of the samples were dominated by cobbles and had gravel to cobble-sized clast-supported framework with interstitial matrix; these two lithotypes are mixed gravel and cobble dominated. The grain-size analysis based lithotypes only take into account the solid-volume fraction of the core; porosity must be added to each sample in order to account for the total volume fraction of the core samples.

Porosity logs for the boreholes at the BHRS were derived from neutron logs recorded at the site. Porosity values varied at the site from less than 15% to greater than 40%; however research conducted by Barrash and Clemo (2000, 2002) on the porosity logs show the existence of five hydro stratigraphic units at the BHRS. Principal component analysis and transition probability analysis were carried out on the samples both for the deposit as a whole and for the hydro stratigraphic units. One of these units is almost entirely comprised of sand to fine gravel lithotype and is a continuous sand channel in the western half of the BHRS. Two of the units exhibit low porosity values with low variance and contain only the mixed gravel and cobble dominated lithotypes and show random independent vertical transition structure. The other two units include all five of the lithotypes but are mostly comprised of the cobble-dominated lithotypes, have higher porosity values and higher variance than the two low-porosity low porosity variance units described previously, and have vertical lithotype sequences that are statistically different from random independent and show a Markov vertical transition structure.

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