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A method and apparatus for measuring dry density and gravimetric water content of soil includes the steps of providing a plurality of spikes adapted to be driven into the soil and driving the spikes into the soil in spaced relationship. An electrical signal is applied to the spikes and a reflected s

A method and apparatus for measuring dry density and gravimetric water content of soil includes the steps of providing a plurality of spikes adapted to be driven into the soil and driving the spikes into the soil in spaced relationship. An electrical signal is applied to the spikes and a reflected signal is analyzed using time domain reflectometry to determine an apparent dielectric constant and the bulk electrical conductivity of the soil. With these parameters, the dry density and gravimetric water content of the soil can be calculated using a predetermined relationship between apparent dielectric constant, bulk electrical conductivity, dry density and gravimetric water content. The predetermined relationship includes experimentally determined soil specific calibration constants. The calculated value of the bulk electrical conductivity as determined by time domain reflectometry is adjusted to correspond to a value for which values of the constants are known. The value of the apparent dielectric constant is adjusted to compensate for temperature.

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We claim: 1. A method for measuring dry density and gravimetric water content of soil, comprising the steps of: providing a plurality of spikes adapted to be driven into the soil; driving said plurality of spikes into the soil in spaced relationship; applying to said plurality of spikes an electric

We claim: 1. A method for measuring dry density and gravimetric water content of soil, comprising the steps of: providing a plurality of spikes adapted to be driven into the soil; driving said plurality of spikes into the soil in spaced relationship; applying to said plurality of spikes an electrical signal suitable for time domain reflectometry; analyzing a reflected signal using time domain reflectometry to determine an apparent dielectric constant Ka of the soil and bulk electrical conductivity ECb of the soil; calculating dry density Pd of the soil using a predetermined relationship between Ka, ECb and ρ d; and calculating gravimetric water content w of the soil using a predetermined relationship between Ka, ECb, and w. 2. The method of claim 1, wherein the soil has a surface and the plurality of spikes have a lower end, and the step of analyzing a reflected signal includes measuring the apparent distance between a signal reflected from the surface of the soil and a signal reflected from the lower end of said plurality of spikes to determine an apparent length La. 3. The method of claim 2, wherein said plurality of spikes have a probe length Lp and the apparent dielectric constant Ka =(La/Lp)2. 4. The method of claim 1, wherein the step of analyzing a reflected signal includes measuring a source voltage Vs of the applied signal and a long term voltage Vf of the reflected signal. 5. The method of claim 4, wherein the bulk electrical conductivity ECb=(1/C)(Vs/Vf-1) where C is a constant related to probe length Lp. 6. The method of claim 1, wherein the predetermined relationship between Ka, ECb and ρd is where a, b, c and d are soil specific calibration constants. 7. The method of claim 6, wherein calibration constants a and b are predetermined experimentally for a given soil using the relationship where ρw is the density of water, Σd is the dry density of the soil, and w is the gravimetric water content of the soil. 8. The method of claim 7, wherein ECb is replaced with an adjusted value ECb, adj for which calibration constants c and d are known. 9. The method of claim 1, wherein the predetermined relationship between Ka, ECb and w is where a, b, c and dare soil specific calibration constants. 10. The method of claim 9, wherein calibration constants c and d are predetermined experimentally for a given soil using the relationship where ρw is the density of water, ρd is the dry density of the soil, and w is the gravimetric water content of the soil. 11. The method of claim 10, wherein ECb is replaced with an adjusted value ECb, adj for which calibration constants c and d are known. 12. The method of claim 11, wherein the calculated value of Ka at a given temperature is adjusted to a value Ka,20째 C. at a standard temperature of 20째 C., where description="In-line Formulae" end="lead"K a,20째 C.=Ka,T횞TCFdescription="In-line Formulae" end="tail" and where 13. A method for measuring dry density of soil, comprising the steps of: providing a plurality of spikes adapted to be driven into the soil; driving said plurality of spikes into the soil in spaced relationship; applying to said plurality of spikes an electrical signal suitable for time domain reflectometry; analyzing a reflected signal using time domain reflectometry to determine an apparent dielectric constant Ka of the soil and bulk electrical conductivity ECb of the soil; and calculating dry density ρd of the soil using a predetermined relationship between Ka, ECb and ρ d. 14. The method of claim 13, wherein the predetermined relationship between Ka, ECb and ρd is where a, b, c and dare soil specific calibration constants. 15. The method of claim 14, wherein calibration constants a and b are predetermined experimentally for a given soil using the relationship where ρw is the density of water, ρd is the dry density of the soil, and w is the gravimetric water content of the soil. 16. The method of claim 14, wherein calibration constants c and d are predetermined experimentally for a given soil using the relationship where ρw is the density of water, ρd is the dry density of the soil, and w is the gravimetric water content of the soil. 17. The method of claim 14, wherein ECb is replaced with an adjusted value ECb, adj for which calibration constants c and d are known. 18. The method of claim 17, wherein the calculated value of Ka at a given temperature is adjusted to a value Ka,20째 C. at a standard temperature of 20째 C., where and where 19. A method for measuring gravimetric water content of soil, comprising the steps of: providing a plurality of spikes adapted to be driven into the soil; driving said plurality of spikes into the soil in spaced relationship; applying to said plurality of spikes an electrical signal suitable for time domain reflectometry; analyzing a reflected signal using time domain reflectometry to determine an apparent dielectric constant Ka of the soil and bulk electrical conductivity ECb of the soil; and calculating gravimetric water content w of the soil using a predetermined relationship between Ka, ECb, and w. 20. The method of claim 19, wherein the predetermined relationship between Ka, ECb and w is w where a, b, c and dare soil specific calibration constants. 21. The method of claim 20, wherein calibration constants a and b are predetermined experimentally for a given soil using the relationship where ρw is the density of water, ρd is the dry density of the soil, and w is the gravimetric water content of the soil. 22. The method of claim 20, wherein calibration constants c and dare predetermined experimentally for a given soil using the relationship where ρw, is the density of water, ρd is the dry density of the soil, and w is the gravimetric water content of the soil. 23. The method of claim 22, wherein ECb is replaced with an adjusted value ECb,adj for which calibration constants c and d are known. 24. The method of claim 23, wherein the calculated value of Ka at a given temperature is adjusted to a value Ka,20째 C. at a standard temperature of 20째 C., where description="In-line Formulae" end="lead"K a,20째 C.=Ka,T =TCFdescription="In-line Formulae" end="tail" and where 25. An apparatus for measuring dry density of soil, comprising: a plurality of spikes adapted to be driven into the soil in spaced relationship; means for applying to said plurality of spikes an electrical signal suitable for time domain reflectometry; means for analyzing a reflected signal using time domain reflectometry to determine an apparent dielectric constant Ka of the soil and bulk electrical conductivity ECb of the soil; and means for calculating dry density ρd of the soil using a predetermined relationship between Ka, ECb and ρd. 26. The apparatus of claim 25, wherein the predetermined relationship between Ka, ECb and ρd is where a, b, c and dare soil specific calibration constants. 27. The apparatus of claim 26, further comprising means for calculating gravimetric water content w of the soil using a predetermined relationship between Ka, ECb, and w. 28. The apparatus of claim 25, further comprising means for compensating for soil temperature. 29. An apparatus for measuring gravimetric water content of soil, comprising: a plurality of spikes adapted to be driven into the soil in spaced relationship; means for applying to said plurality of spikes an electrical signal suitable for time domain reflectometry; means for analyzing a reflected signal using time domain reflectometry to determine an apparent dielectric constant Ka of the soil and bulk electrical conductivity ECb of the soil; and means for calculating gravimetric water content w of the soil using a predetermined relationship between Ka, ECb, and w. 30. The apparatus of claim 29, wherein the predetermined relationship between Ka, ECb and w is where a, b, c and d are soil specific calibration constants. 31. The apparatus of claim 29, further comprising means for compensating for soil temperature. 32. The method of claim 13, wherein said predetermined relationship includes a difference between a function of Ka and a function of ECb. 33. The method of claim 32, wherein ECb is adjusted to reflect a predetermined soil pore fluid electrical conductivity. 34. The method of claim 13, wherein ECb is adjusted to reflect a predetermined soil pore fluid electrical conductivity. 35. The method of claim 19, wherein said predetermined relationship includes a difference between a function of Ka and a function of ECb. 36. The method of claim 35, wherein said predetermined relationship includes a ratio of said difference and a second difference between a function of Ka and a function of ECb. 37. The method of claim 36, wherein ECb is adjusted to reflect a predetermined soil pore fluid electrical conductivity. 38. The method of claim 19, wherein ECb is adjusted to reflect a predetermined soil pore fluid electrical conductivity.