<PAGE>


                                                                    Exhibit 10.1


                       SECOND AMENDMENT TO LOAN AGREEMENT

         THIS SECOND AMENDMENT TO LOAN AGREEMENT (this "Amendment"), dated as of
January 15, 2002, is between CONCORD TECHNOLOGIES, LP, a Texas limited
partnership ("Concord"), GEOSPACE ENGINEERING RESOURCES INTERNATIONAL, LP, a
Texas limited partnership ("Engineering"), GEO SPACE, LP, a Texas limited
partnership ("Geo Space"), OYO INSTRUMENTS, LP, a Texas limited partnership
("Instruments"), and OYOG OPERATIONS, LP, a Texas limited partnership
("Operations", and together with Concord, Engineering, Geo Space and
Instruments, the "Borrowers"), jointly and severally, and SOUTHWEST BANK OF
TEXAS, N.A., a national banking association ("Lender").

                                    RECITALS:

         A. Borrowers and Lender entered into that certain Loan Agreement dated
as of February 16, 2001, as amended by First Amendment to Loan Agreement dated
as of February 17, 2001 (the "Agreement").

         B. Borrowers and Lender now desire to amend the Agreement as herein
set forth.

         NOW, THEREFORE, in consideration of the premises herein contained and
other good and valuable consideration, the receipt and sufficiency of which are
hereby acknowledged, the parties hereto agree as follows:

                                   ARTICLE I.

                                   Definitions

         Section I.1.   Definitions. Capitalized terms used in this Amendment,
to the extent not otherwise defined herein, shall have the meanings given to
such terms in the Agreement, as amended hereby.

<PAGE>


                                   ARTICLE II.

                                   Amendments

         Section II.1.  Amendment to Certain Definitions. (a) The following
definitions shall be added to Section 1.1 of the Agreement in proper
alphabetical order:

                  "Advance-A" means an advance of funds by Lender to Borrowers
         pursuant to Article II.

                  "Advance-B" means an advance of funds by Lender to Borrowers
         pursuant to Article XII.

                  "Advance Request Form-A" means a certificate, in substantially
         the form of Exhibit "K" hereto, properly completed and signed by an
         Authorized Representative requesting an Advance-A.

                  "Advance Request Form-B" means a certificate, in substantially
         the form of Exhibit "P" hereto, properly completed and signed by an
         Authorized Representative requesting an Advance-B.

                  "Commitment-A" means the obligation of Lender to make
         Advances-A hereunder in an aggregate principal amount at any time
         outstanding up to but not exceeding $10,000,000.00.

                  "Commitment-B" means the obligation of Lender to make
         Advances-B hereunder in an aggregate principal amount at any time
         outstanding up to but not exceeding $2,500,000.00.

                  "Commitments" means Commitment-A and Commitment-B.

                  "Lockbox" means the post office box designated in any
         agreement executed by Borrowers and Lender with respect to lockbox
         services, which may be comprised of Lender's Corporate Treasury
         Management Services Agreement and the Lockbox Service Exhibit thereto,
         as the same may be amended, supplemented or modified.

                  "Note-A" means the promissory note executed by Borrowers
         payable to the order of Lender, in substantially the form of Exhibit
         "A" hereto, and all extensions, renewals, and modifications thereof and
         all substitutions therefor.

<PAGE>


                  "Note-B" means the promissory note executed by Borrowers
         payable to the order of Lender, in substantially the form of Exhibit
         "O" hereto, and all extensions, renewals, and modifications thereof and
         all substitutions therefor.

                  "Notes" means Note-A and Note-B.

                  "Payment Account" means the depository account of Borrowers
         designated in the Lockbox Agreement as the account into which proceeds
         of the Lockbox collections are to be deposited.

                  "Termination Date-A" means 11:00 a.m., Houston, Texas time on
         January 14, 2003, or such earlier date on which the Commitment-A
         terminates as provided in this Agreement.

                  "Termination Date-B" means 11:00 a.m., Houston, Texas time on
         July 15, 2002, or such earlier date on which the Commitment-B
         terminates as provided in this Agreement.

                  "Termination Dates" means Termination Date-A and Termination
         Date-B.

         (b)      The definition of the term "Default Rate" shall be amended to
read in its entirety as follows:

                  "Default Rate" means a per annum rate of interest equal to the
         lesser of (a) the sum of the Prime Rate then in effect from day to day
         plus two percent (2.0%), but not less than seven percent (7.0%) per
         annum, or (b) the Maximum Rate; provided, however, that with respect to
         the Advances-B and Note-B only, the term "Default Rate" shall mean a
         per annum rate of interest equal to ten percent (10%) per annum.

         (c)      Clauses (a) and (b) contained in the definition of the term
"Eligible Accounts" are amended to read in their entirety as follows:

                  (a) are due and payable within thirty (30) days; (b) have been
                  outstanding less than one hundred twenty (120) days past the
                  original date of invoice;

         (d)      The following terms shall be deleted from Section 1.1 of the
Agreement:

                  "Advance"
                  "Advance Request Form"

                                       -3-

<PAGE>


                  "Annualized"
                  "Commitment"
                  "Note"
                  "Termination Date"

         Section II.2. Amendment to Article II. Each reference in Article II of
the Agreement to "Advance", "Advances", "Advance Request Form", "Commitment",
"the Note" and "Termination Date" in Article II shall be revised to read
"Advance-A", "Advances-A", "Advance Request Form-A", "Commitment-A", "Note-A"
and "Termination Date-A", respectively.

         Section II.3. Amendment to Section 2.4. Clause (b) contained in Section
2.4 of the Agreement is amended to read in its entirety as follows:

         (b) the greater of (i) the Prime Rate in effect from day to day or (ii)
         five percent (5.0%) per annum, and each change in the rate of interest
         charged on the Advances shall become effective, without notice to any
         Borrower, on the effective date of each change in the Prime Rate or the
         Maximum Rate, as the case may be;

         Section II.4. Amendment to Article III. Each reference to "Note" in
Article III shall be revised to read "Notes".

         Section II.5. Amendment to Section 5.2. Section 5.2 of the Agreement is
amended to read in its entirety as follows:

                  Section 5.2. All Extensions of Credit. The obligation of
         Lender to make any Advance-A, any Advance-B or issue any Letter of
         Credit (including the initial Advance-A, the initial Advance-B and the
         initial Letter of Credit) is subject to receipt by Lender of the items
         required by Section 2.5(b), if applicable, 2.9, and 12.5, and such
         additional documents as Lender may reasonably request.

         Section II.6. Amendment to Section 7.1. Clause (l) shall be added to
Section 7.1 of the Agreement and shall read in its entirety as follows:

                  (l) Inventory Report. As soon as available, and in any event
         within thirty (30) days after the end of each month, an inventory
         report as of the end of such month certified by an officer of each
         Borrower acceptable to Lender.

                                       -4-

<PAGE>


         Section II.7. Amendment to Article VII. Section 7.13 shall be added to
Article VII of the Agreement immediately after Section 7.12 in numerical order,
and shall read in its entirety as follows:

                  Section 7.13. Lockbox. (a) Each Borrower will cause the
         proceeds from the accounts receivable of such Borrower to be remitted
         by check to the Lockbox or by wire transfer to the Payment Account. If
         no Event of Default exists, all collected funds (as determined by
         Lender in accordance with its customary practices with respect to
         similar accounts) with respect to acceptable checks received in the
         Lockbox shall be deposited by Lender into the Payment Account. If an
         Event of Default has occurred and is continuing, all funds with respect
         to checks received in the Lockbox and all amounts received in the
         Payment Account shall be paid, delivered or transferred to Lender and
         applied by Lender to the Obligations.

                  (b) Each Borrower hereby pledges and assigns to Lender, and
         grants to Lender a security interest in, the Collateral Account and in
         all cash, instruments, securities and funds on deposit therein, all
         interest and cash or other property received in connection therewith or
         in exchange therefor, and all proceeds of all of the above, now or
         hereafter existing, as additional collateral security for the
         Obligations. In addition to Lender's common law rights of setoff, each
         Borrower hereby grants to Lender, upon the occurrence and during the
         continuance of an Event of Default, the right to offset all or a
         portion of the funds in the Collateral Account.

         Section II.8. Amendment to Section 8.7. The first sentence contained in
Section 8.7 of the Agreement is amended to read in its entirety as follows:

         No Borrower will make, nor will it permit any Guarantor or any
         Subsidiary to make, any capital contribution to or investment in any
         Person, except for an investment in Labelon Corporation in an aggregate
         principal amount which does not exceed $2,000,000.00.

         Section II.9. Amendment to Section 9.3. Section 9.3 of the Agreement is
amended to read in its entirety as follows:

                  Section 9.3. Ratio of Senior Debt to EBITDA. Parent will
         maintain a Ratio of Senior Debt to EBITDA of not greater than (a) 2.50
         to 1.00 as of December 31, 2001, (b) 4.00 to 1.00 as of March 31, 2002,
         and (c) 2.50 to 1.00 as of June 30, 2002 and at all times thereafter.
         The Ratio of Senior Debt to EBITDA shall be calculated and tested
         quarterly as of the last day of each fiscal quarter of Parent on

                                       -5-

<PAGE>


         a cumulative basis (rolling four quarter basis) for the four fiscal
         quarters ended as of the date of calculation.

         Section II.10. Amendment to Section 10.2. Each reference to
"Commitment" contained in Section 10.2 shall be revised to read "Commitments".

         Section II.11. Amendment to Section 11.6. Each reference to "Note" and
"Commitment" contained in Section 11.6 shall be revised to read "Notes" and
"Commitments", respectively.

         Section II.12. Amendment to Section 11.15. Each reference to "Note",
"Commitment" and Termination Date" contained in Section 11.15 shall be revised
to read "Notes", "Commitments" and Termination Dates", respectively.

         Section II.13. Amendment to Article XI. Section 11.19 shall be added to
Article XI immediately following Section 11.18 of the Agreement and shall read
in its entirety as follows:

                  Section 11.19. Document Imaging. Borrowers understand and
         agree that (a) Lender's document retention policy involves the imaging
         of executed loan documents and the destruction of the paper originals,
         and (b) Borrowers waive any right that it may have to claim that the
         imaged copies of the Loan Documents are not originals.

         Section II.14. Addition of Article XII. Article XII shall be added to
the Agreement immediately after Article XI and shall read in its entirety as
follows:

                                  ARTICLE XII.

                                   Advances-B
                                   ----------

                  Section 12.1. Advances-B. Subject to the terms and conditions
         of this Agreement, Lender agrees to make one or more Advances-B to
         Borrowers from time to time from the date hereof to and including the
         Termination Date-B in an aggregate principal amount at any time
         outstanding up to but not exceeding the Commitment-B; provided that the
         aggregate amount of all Advances-B at any time outstanding shall not
         exceed the Commitment-B. Lender shall have no obligation to make any
         Advance-B if an Event of Default or an Unmatured Event of Default has
         occurred and is continuing. Subject to the foregoing limitations, and
         the other terms and provisions of this Agreement, Borrowers may borrow,
         repay, and reborrow hereunder.

                                       -6-

<PAGE>


                  Section 12.2. Note-B. The obligation of Borrowers to repay the
         Advances-B shall be evidenced by the Note-B executed by Borrowers,
         payable to the order of Lender, in the principal amount of the
         Commitment-B.

                  Section 12.3. Repayment of Advances-B. Borrowers shall repay
         the unpaid principal amount of all Advances-B on the earlier of (a) the
         Termination Date-B or (b) such other dates on which the Advances-B are
         or may be required to be paid pursuant to this Agreement.

                  Section 12.4. Interest. The unpaid principal amount of the
         Advances-B shall bear interest prior to maturity at a rate equal to
         eight percent (8.0%) per annum. Accrued and unpaid interest on the
         Advances-B shall be payable on the fifteenth (15th) day of each month
         commencing on February 15, 2002, and on the earlier of the Termination
         Date-B or any other date on which the principal amount of the
         Advances-B is paid (whether as a result of optional or mandatory
         prepayment or acceleration). If an Event of Default has occurred and is
         continuing, all principal of the Advances-B and all past due interest
         thereon shall bear interest at the Default Rate.

                  Section 12.5. Requests for Advances-B. Borrowers shall give
         Lender notice of each requested Advance-B by delivery to Lender of an
         Advance Request Form-B executed by an Authorized Representative,
         properly completed and containing the information required therein.
         Assuming that each Advance Request Form-B is in proper form, if Lender
         receives an Advance Request Form-B prior to 1:00 p.m. on any Business
         Day, Lender will make the requested Advance-B on the same Business Day,
         and if Lender receives an Advance Request Form-B after 1:00 p.m.,
         Lender will make the requested Advance-B on the next Business Day.

                  Section 12.6.  Use of Proceeds. The proceeds of the Advances-B
         shall be used for working capital and general corporate purposes.

                  Section 12.7.  Facility Fee. Borrowers agree to pay to Lender
         a facility fee in the amount of $10,000.00 on the Closing Date. Such
         facility fee shall be fully earned when paid.

         Section II.15. Amendment to Exhibits. (a) Exhibit "A" to the Agreement
(Note-A) is amended to conform in its entirety to Annex "A" to this Amendment,
(b) Exhibit "F" to the Agreement (Guaranty-General Partner) is amended to
conform in its entirety to Annex "F" to this Amendment, (c) Exhibit "G" to the
Agreement (Guaranty-Limited Partner) is amended to conform in its entirety to
Annex "G" to this Amendment, (d) Exhibit "H" to the Agreement (Guaranty-Parent)
is amended to

                                       -7-

<PAGE>


conform in its entirety to Annex "H" to this Amendment, (e) Exhibit "J" to the
Agreement (Arbitration Agreement) is amended to conform in its entirety to Annex
"I" to this Amendment, (f) Exhibit "K" to the Agreement (Advance Request Form-A)
is amended to conform in its entirety to Annex "J" to this Amendment, (g)
Exhibit "L" to the Agreement (Borrowing Base Certificate) is amended to conform
in its entirety to Annex "K" to this Amendment, (h) Exhibit "M" to the Agreement
(No Default Certificate) is amended to conform in its entirety to Annex "L" to
this Amendment, (i) Exhibit "O" shall be added to the Agreement (Note-B) in the
form of Annex "N" to this Amendment and (j) Exhibit "P" shall be added to the
Agreement (Advance Request Form-B) in the form of Annex "O" to this Amendment.

                                  ARTICLE III.

                              Conditions Precedent

         Section III.1. Conditions. The effectiveness of this Amendment is
subject to the receipt by Lender of the following in form and substance
satisfactory to Lender:

                  (a) Certificate - Borrowers, Parent and General Partner. A
         certificate of the Secretary or the Assistant Secretary (or another an
         officer acceptable to Lender) of Parent, General Partner and each
         Borrower certifying resolutions of the board of directors of the
         Parent, the sole member of General Partner, and the General Partner as
         general partner of each Borrower which authorize the execution,
         delivery and performance by Parent, General Partner and each Borrower
         of this Amendment (if applicable) and the other Loan Documents to which
         such Person is or is to be a party.

                  (b) Governmental Certificates - Parent, General Partner and
         Each Borrower. Certificates issued by the appropriate government
         official of the state of organization of Parent, General Partner and
         each Borrower as to the existence and good standing of such Person, and
         certificates of existence and good standing of Parent and General
         Partner as a foreign entity in the state of Texas.

                  (c) Certificate - Limited Partner. A certificate of a Manager
         of Limited Partner (or another officer of Limited Partner acceptable to
         Lender) certifying resolutions of the Members of Limited Partner which
         authorize the execution, delivery and performance by Limited Partner of
         the Guaranty-Limited Partner and the other Loan Documents to which
         Limited Partner is or is to be a party.

                                       -8-

<PAGE>


                  (d)   Governmental Certificates - Limited Partner.
         Certificates issued by the appropriate government officials of the
         state of Nevada as to the existence and good standing of Limited
         Partner.

                  (e)   Notes. Note-A and Note-B executed by Borrowers.

                  (f) Amendment to Security Agreements. A First Amendment to
         Security Agreements executed by Borrowers, respectively, substantially
         in the form of Annexes "B", "C", "D", "E" and "M" hereto.

                  (g)   Guaranty Agreements. The Guaranty Agreements executed by
         Guarantors, respectively.

                  (h)   Arbitration Agreement. The Arbitration Agreement
         executed by Borrower and Guarantors.

                  (i)   Facility Fee. The facility fee referred to in Section
         12.7 of the Agreement.

                  (j) UCC Search. A Uniform Commercial Code search showing all
         financing statements and other documents or instruments on file against
         Borrowers in the office of the Secretary of State of Texas.

                  (k)   Additional Information. Such additional documents,
         instruments and information as Lender may request.

         Section III.2. Additional Conditions. The effectiveness of this
Amendment is also subject to the satisfaction of the additional conditions
precedent that (a) the representations and warranties contained herein and in
all other Loan Documents, as amended hereby, shall be true and correct as of the
date hereof as if made on the date hereof, (b) all proceedings, corporate or
otherwise, taken in connection with the transactions contemplated by this
Amendment and all documents, instruments and other legal matters incident
thereto shall be satisfactory to Lender, and (c) no Event of Default or
Unmatured Event of Default shall have occurred and be continuing.

                                       -9-

<PAGE>


                                   ARTICLE IV.

                 Ratifications, Representations, and Warranties

         Section IV.1. Ratifications. The terms and provisions set forth in this
Amendment shall modify and supersede all inconsistent terms and provisions set
forth in the Agreement and except as expressly modified and superseded by this
Amendment, the terms and provisions of the Agreement are ratified and confirmed
and shall continue in full force and effect. Borrowers and Lender agree that the
Agreement as amended hereby shall continue to be the legal, valid and binding
obligation of such Persons enforceable against such Persons in accordance with
its terms.

         Section IV.2. Representations, Warranties and Agreements. Each Borrower
hereby represents and warrants to Lender that (a) the execution, delivery, and
performance of this Amendment and any and all other Loan Documents executed or
delivered in connection herewith have been authorized by all requisite action on
the part of such Borrower and General Partner and will not violate the
Organizational Documents of such Borrower, (b) the representations and
warranties contained in the Agreement as amended hereby, and all other Loan
Documents are true and correct on and as of the date hereof as though made on
and as of the date hereof, (c) no Event of Default or Unmatured Event of Default
has occurred and is continuing, (d) Borrower is in full compliance with all
covenants and agreements contained in the Agreement as amended hereby, (e)
Borrower is indebted to Lender pursuant to the terms of the Notes, as the same
may have been renewed, modified, extended and rearranged, including, without
limitation, renewals, modifications and extensions made pursuant to this
Amendment, (f) the liens, security interests, encumbrances and assignments
created and evidenced by the Loan Documents are, respectively, valid and
subsisting liens, security interests, encumbrances and assignments and secure
the Notes as the same may have been renewed, modified or rearranged, including,
without limitation, renewals, modifications and extensions made pursuant to this
Amendment and (g) Borrower has no claims, credits, offsets, defenses or
counterclaims arising from the Loan Documents or Lender's performance under the
Loan Documents.

                                      -10-

<PAGE>


                                   ARTICLE V.

                                  Miscellaneous

         Section V.1. Survival of Representations and Warranties. All
representations and warranties made in this Amendment or any other Loan
Documents including any Loan Document furnished in connection with this
Amendment shall fully survive the execution and delivery of this Amendment and
the other Loan Documents, and no investigation by Lender or any closing shall
affect the representations and warranties or the right of Lender to rely on
them.

         Section V.2. Reference to Agreement. Each of the Loan Documents,
including the Agreement and any and all other agreements, documents, or
instruments now or hereafter executed and delivered pursuant to the terms hereof
or pursuant to the terms of the Agreement, as amended hereby, are hereby amended
so that any reference in such Loan Documents to the Agreement shall mean a
reference to the Agreement, as amended hereby.

         Section V.3. Expenses of Lender. As provided in the Agreement,
Borrowers agree to pay on demand all reasonable costs and expenses incurred by
Lender in connection with the preparation, negotiation and execution of this
Amendment and the other documents and instruments executed pursuant hereto,
including, without limitation, the reasonable costs and fees of Lender's legal
counsel.

         Section V.4. Severability. Any provision of this Amendment held by a
court of competent jurisdiction to be invalid or unenforceable shall not impair
or invalidate the remainder of this Amendment and the effect thereof shall be
confined to the provision so held to be invalid or unenforceable.

         Section V.5. APPLICABLE LAW. THIS AMENDMENT AND ALL OTHER LOAN
DOCUMENTS EXECUTED PURSUANT HERETO SHALL BE DEEMED TO HAVE BEEN MADE AND TO BE
PERFORMABLE IN HOUSTON, HARRIS COUNTY, TEXAS AND SHALL BE GOVERNED BY AND
CONSTRUED IN ACCORDANCE WITH THE LAWS OF THE STATE OF TEXAS.

         Section V.6. Successors and Assigns. This Amendment is binding upon and
shall inure to the benefit of Lender and Borrowers and their respective
successors and assigns, except no Borrower may assign or transfer any of its
rights or obligations hereunder without the prior written consent of Lender.

         Section V.7.   Counterparts. This Amendment may be executed in one or
more counterparts, each of which when so executed shall be deemed to be an

                                      -11-

<PAGE>


original, but all of which when taken together shall constitute one and the same
instrument.

         Section V.8. Effect of Waiver. No consent or waiver, express or
implied, by Lender to or for any breach of or deviation from any covenant,
condition or duty by Borrowers shall be deemed a consent or waiver to or of any
other breach of the same or any other covenant, condition or duty.

         Section V.9.   Headings. The headings, captions, and arrangements used
in this Amendment are for convenience only and shall not affect the
interpretation of this Amendment.

         Section V.10. ENTIRE AGREEMENT. THIS AMENDMENT AND ALL OTHER
INSTRUMENTS, DOCUMENTS, AND AGREEMENTS EXECUTED AND DELIVERED IN CONNECTION WITH
THIS AMENDMENT EMBODY THE FINAL, ENTIRE AGREEMENT AMONG THE PARTIES HERETO WITH
RESPECT TO THE SUBJECT MATTER HEREOF AND THEREOF AND SUPERSEDE ANY AND ALL PRIOR
COMMITMENTS, AGREEMENTS, REPRESENTATIONS AND UNDERSTANDINGS, WHETHER WRITTEN OR
ORAL, RELATING TO THE SUBJECT MATTER HEREOF AND THEREOF, AND MAY NOT BE
CONTRADICTED OR VARIED BY EVIDENCE OF PRIOR, CONTEMPORANEOUS OR SUBSEQUENT ORAL
AGREEMENTS OR DISCUSSIONS OF THE PARTIES HERETO. THERE ARE NO UNWRITTEN ORAL
AGREEMENTS AMONG THE PARTIES HERETO.

         Executed as of the date first written above.

                                       BORROWERS:

                                       CONCORD TECHNOLOGIES, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer


                                      -12-

<PAGE>


                                       GEOSPACE ENGINEERING RESOURCES
                                       INTERNATIONAL, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                       GEO SPACE, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                       OYO INSTRUMENTS, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer


                                      -13-

<PAGE>


                                       OYOG OPERATIONS, LP

                                       By: OYOG, LLC,
                                           its general partner


                                           By: /s/ THOMAS T. MCENTIRE
                                               ---------------------------------
                                               Thomas T. McEntire
                                               Vice President and
                                               Chief Financial Officer



                                     LENDER:

                                       SOUTHWEST BANK OF TEXAS, N.A.


                                       By: /s/ EDWARD K. BOWDON
                                           -------------------------------------
                                           Edward K. Bowdon
                                           Vice President

                                      -14-

<PAGE>




                                      -15-