Combined Electromagnetic Induction and Magnetic Susceptibility Surveys for the Detection of Near Surface Geochemical Anomalies

James M. Fausnaugh
Geotech.org, Littleton, CO

As surface geochemical exploration for oil and gas gains greater acceptance in the exploration community, the number of methods that can detect alteration phenomena has also increased. Many geophysical methods have been applied to the detection of seepage anomalies with varying success. Electrical and magnetic techniques are at the forefront due to ease of use, rapid data acquisition, and immediate data interpretation. Several of the electrical and magnetic instruments available can be operated by a single user.

The literature states that most hydrocarbon accumulations are associated with magnetic anomalies. The chemical alteration of near-surface sediments by carbon dioxide and hydrogen sulfide produced by microbial degradation of hydrocarbons could account for the observed electrical and magnetic anomalies over petroleum deposits. Magnetic Susceptibility (MS) anomalies associated with oil/gas fields are caused by secondary magnetite, low-temperature oxidation iron products such as maghemite, or the iron sulfide, pyrrhotite. Electromagnetic Induction (EM) surveys measure soil electrical conductivity of salts associated with near surface hydrocarbon alteration.

A combination of EM and MS has been used to identify hydrocarbon alteration in near surface sediments over oil fields. In general, the higher MS values lie within areas of lower conductivity. High conductivity readings occur as geochemical halos and appear at the edge of seepage anomalies.

AAPG Search and Discovery Article #90092©2009 AAPG Rocky Mountain Section, July 9-11, 2008, Denver, Colorado